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Seismicity associated with the great Colombia-Ecuador earthquakes of 1942, 1958, and 1979: Implications for barrier models of earthquake rupture
... The Colombia-Ecuador subduction zone extends approximately 900 km along a trench axis, ranking among the most active convergent margins globally (Chlieh et al., 2021). Along this subduction segment, three large megathrust earthquakes (Figure 1, white stars) were recorded in the 20th century, occurring in 1942 (M w 7.8; Sennson andBeck, 1996), 1958 (M w 7.6;Mendoza and Dewey, 1984), and 1979 (M w 8.1; Beck and Ruff, 1984) respectively. The rupture areas of these three seismic events were adjacent to one another, and their epicenters moved northward in sequence with the time of the events. ...
... The colors of the circles represent earthquake depths. The white stars indicate the epicenters of the 1942 M w 7.8, 1958 M w 7.6, and 1979 M w 8.1 megathrust earthquakes (Beck and Ruff, 1984;Mendoza and Dewey, 1984;Sennson and Beck, 1996). ...
... Hence, the differing positions of the dehydration front within the slab from Colombia to Ecuador result in a flexure change in the dehydration zone along the trench strike. (Beck and Ruff, 1984;Mendoza and Dewey, 1984;Sennson and Beck, 1996). Red cones indicate active volcanoes (Siebert et al., 2010). ...
Throughout the 20th century, several large megathrust earthquakes were observed in the Colombia–Ecuador subduction
zone which widely ruptured plate interfaces, causing considerable damage and loss of life. The occurrence of earthquakes in subduction
zones is thought to be closely related to the thermal structure of the incoming plate. However, in the case of the subducting Nazca Plate
beneath the Colombia–Ecuador zone, the thermal structure remains unclear, especially its hydraulic distribution. On the basis of 3D
thermal models, we present new insights into the plate interface conditions of Colombia–Ecuador interplate and megathrust
earthquakes. We show that the plate geometry strongly affects the along-strike thermal structure of the slab beneath Colombia and
Ecuador, with the subduction of the Carnegie Ridge playing an important role. Our results further reveal that the unique geometry of the
Nazca Plate is the primary reason for the relatively high temperatures of the slab beneath Colombia. We suggest that the positions of the
100–200 °C and 350–450 °C isotherms on the plate interface determine the updip and downdip limits of the seismogenic zone. For
Colombia–Ecuador interplate earthquakes, the released fluids control the distribution of shallow-depth earthquakes, whereas the age
and geometry of the slab control the distribution of intermediate-depth earthquakes. The average temperature of the plate interface at
the upper limit of large megathrust earthquakes is hotter than previously thought, which is more consistent with our understanding of
the Colombia–Ecuador subduction zone. We predict that the potential location of future large seismic events could be in the rupture
zone of past seismic events or offshore of northern Colombia.
Keywords: thermal structure; interplate earthquakes; plate geometry; Colombia−Ecuador; 3D modeling
... These bands of seismicity may be composed of smaller, focused clusters containing earthquake swarms and large-magnitude aftershocks . The distribution and focal mechanisms of the 2016 aftershocks also roughly correspond to those of the 1942 aftershocks, meaning they are likely linked to permanent subducting structures (Mendoza and Dewey, 1984;Yoshimoto et al., 2017). Several clusters are linked to bathymetric subducting features, like the Carnegie Ridge and the Atacames seamounts Soto-Cordero et al., 2020). ...
... but not yet modelled . White stars and white lines show the epicenters and approximate rupture areas of past megathrust earthquakes (Kanamori and McNally, 1982;Mendoza and Dewey, 1984). The yellow star and yellow line show the epicenter and the 1 m contour of the rupture zone of the 2016 Pedernales earthquake . ...
... Shear stress changes induced by the mainshock may account for some of the observed changes in stress drop after a large earthquake (Allmann and Shearer, 2007). (Kanamori and McNally, 1982;Mendoza and Dewey, 1984) while the black line shows the geodetically derived rupture zone of the 2016 Pedernales earthquake . ...
The Ecuador-Colombian subduction zone has hosted a series of large subduction earthquakes over the course of the 20th century. This earthquake sequence started in 1906 with a Mw 8.4-8.8 earthquake, which ruptured a 200-500 km long segment of the megathrust. It was followed by three large earthquakes that broke, from south to north, portions already contained in the 1906 rupture. These earthquakes occurred in 1942 (Mw=7.8), 1958 (Mw=7.6) and 1979 (Mw=8.2), respectively. In 2016, the Pedernales earthquake re-ruptured the 1942 coseismic region, possibly starting a new cascade of large events.The Pedernales earthquake and its aftershocks, recorded thanks to the international deployment of seismic stations in the months following the mainshock, provide an opportunity to better understand the seismotectonic processes that occur in the region. This thesis will focus primarily on the interactions between seismicity and aseismic slip, and on the influence of the structure of the megathrust on the seismic activity.For this purpose, a catalogue of repeating earthquakes was created by correlating the existing aftershock catalogue. The families of repeating earthquakes were then completed using template-matching to find missing events. Repeating earthquakes were then relocated in a 1D model, first using manual picks and then using differential times from correlations. Finally, source properties were determined for a portion of the aftershock catalogue.Repeating earthquakes in Ecuador occur primarily within larger aftershock clusters situated at the edge of the main afterslip regions. Additionally, the slip associated with individual repeating earthquake families seems to have an indirect link to the slip modelled using GPS data. Indeed, family slip appears heterogeneous, suggesting perhaps a more complex link between afterslip and repeating earthquakes, and likely reflecting the complexity of the megathrust structure.Additionally, the study of source properties of Pedernales aftershocks reveals a segmentation of the subduction zone with distance to the trench. Stress drops near the trench are low, and decrease with time during the postseismic period, as observed within families of repeating earthquakes. This is probably due to a variation in pore fluid pressure, which is likely very high near the trench, and which plays a crucial role in seismogenesis in the region.
... The Colombia-Ecuador subduction zone extends about 900 km, measured along the trench axis, from the Gulf of Guayaquil (3°S) in South Ecuador to Buenavantura (4°N) in Colombia (Figure 1). This subduction segment has experienced the great (Mw > 8.5) 1906 Colombia-Ecuador megathrust earthquake; however, many uncertainties remain on its exact moment magnitude that fluctuates from 8.5 to 9.0 in the scientific literature and on its rupture length that is proposed between 300 and 500 km long (Kanamori and McNally, 1982;Kelleher, 1972;Mendoza and Dewey, 1984). From the analysis of long-period surface wave records, it was suggested that the 1906 earthquake did not exceed Mw 8.5 (Okal, 1992), a magnitude similar from its associated tsunami's height modeling with a rupture length of ∼300 km (Tsuzuki et al., 2017;Yoshimoto et al., 2017). ...
... During the 20th century, the 1906 rupture segment hosted three juxtaposed large (Mw > 7.5) seismic ruptures of about 100-150 km long (Figure 1 and Supplementary Figure S1). This sequence started off the Ecuadorian coast in 1942 with the Mw∼7.8 event (Mendoza and Dewey, 1984;Swenson and Beck, 1996), followed northward by the 1958 Mw 7.6 event and finished off the coast of Colombia in 1979 with the Mw 8.1 event (Beck and Ruff, 1984;Kanamori and McNally, 1982) ( Table 1). The same segment that ruptured in 1942 failed again during the 2016 Mw 7.8 Pedernales earthquake which initiated ∼50 km north of the equatorial line and propagated 120 km southward toward the Pedernales city which was severely impacted ( Figure 2). ...
... This oblique convergence is partitioned between quasi-normal motion accommodated on the subduction megathrust interface at the trench axis and right-lateral transpressional strike-slip motion on the Chingual-Cosanga-Pallatanga-Puná (CCPP) fault system Yepes et al., 2016). The ellipses indicate the rupture and aftershocks areas with associated epicenters (stars) of the great 1906 Mw 8.6 (black dashed lines), of the 1942 Mw 7.8 (light blue), of the 1958 Mw 7.6 (light green), and of the 1979 Mw 8.1 (green) megathrust earthquakes (Beck and Ruff, 1984;Kanamori and McNally, 1982;Mendoza and Dewey, 1984;Swenson and Beck, 1996). segmentation which is fundamental to understand the physical parameters that control the genesis, extent, and arrest of large megathrust earthquakes. ...
The Colombia–Ecuador subduction zone is an exceptional natural laboratory to study the seismic cycle associated with large and great subduction earthquakes. Since the great 1906 Mw = 8.6 Colombia–Ecuador earthquake, four large Mw > 7.5 megathrust earthquakes occurred within the 1906 rupture area, releasing altogether a cumulative seismic moment of ∼35% of the 1906 seismic moment. We take advantage of newly released seismic catalogs and global positioning system (GPS) data at the scale of the Colombia–Ecuador subduction zone to balance the moment deficit that is building up on the megathrust interface during the interseismic period with the seismic and aseismic moments released by transient slip episodes. Assuming a steady-state interseismic loading, we found that the seismic moment released by the 2016 Mw = 7.8 Pedernales earthquake is about half of the moment deficit buildup since 1942, suggesting that the Pedernales segment was mature to host that seismic event and its postseismic afterslip. In the aftermath of the 2016 event, the asperities that broke in 1958 and 1979 both appears to be mature to host a large Mw > 7.5 earthquakes if they break in two individual seismic events, or an Mw∼7.8–8.0 earthquake if they break simultaneously. The analysis of our interseismic-coupling map suggests that the great 1906 Colombia–Ecuador earthquake could have ruptured a segment of 400 km-long bounded by two 80 km wide creeping segments that coincide with the entrance into the subduction of the Carnegie ridge in Ecuador and the Yaquina Graben in Colombia. These creeping segments share similar frictional properties and may both behave as strong seismic barriers able to stop ruptures associated with great events like in 1906. Smaller creeping segments are imaged within the 1906 rupture area and are located at the extremities of the large 1942, 1958, 1979, and 2016 seismic ruptures. Finally, assuming that the frequency–magnitude distribution of megathrust seismicity follows the Gutenberg–Richter law and considering that 50% of the transient slip on the megathrust is aseismic, we found that the maximum magnitude subduction earthquake that can affect this subduction zone has a moment magnitude equivalent to Mw ∼8.8 with a recurrence time of 1,400 years. No similar magnitude event has yet been observed in that region.
... The subduction of oceanic plates at convergent tectonic boundaries is responsible for the release of more than 90% of the total seismic moment, mainly occurring along the subduction interface [1,2] and the development of first-order large elongated cordilleras [3]. Scientific research has shown that the subduction zone along the northwestern edge of America is one of the most active convergent margins in the world [4][5][6][7][8][9][10][11]. ...
... The largest one occurred in 1906, with a magnitude of 8.8 (Mw) and a 500-600 km long break along the coast of Ecuador and Colombia, that caused a large tsunami. It was followed by earthquakes in the same area in 1942 (Mw = 7.8), 1958 (Mw = 7.7), 1979 (Mw = 8.2) and 2016 (Mw = 7.9) [4,6,8,10,11]. Such a concentration of earthquakes suggests the presence of asperities in the plate rupture [8,11,15,27] with especial relevance focused on the subduction of the Carnegie Ridge [8,11,29]. ...
... The main seismic events of Ecuador related to tectonic activity [4,6,8,10,11], with a recurrence between 16 and 37 years, are located in the subduction zone along the Pacific margin where the last major earthquake occurred (16 April 2016; Mw = 7.8; 20 km depth) in the area of Pedernales as a result of the propagation of thrust activity. Moreover, in continental areas the main active structures are located in the boundaries of the main domains: the Pallatanga fault zone, at the southern boundary of the North Andean Sliver and the Chingual-Cosanga fault zone in the eastern Andes Mountain front. ...
GNSS observations constitute the main tool to reveal Earth’s crustal deformations in order to improve the identification of geological hazards. The Ecuadorian Andes were formed by Nazca Plate subduction below the Pacific margin of the South American Plate. Active tectonic-related deformation continues to present, and it is constrained by 135 GPS stations of the RENAGE and REGME deployed by the IGM in Ecuador (1995.4–2011.0). They show a regional ENE displacement, increasing towards the N, of the deformed North Andean Sliver in respect to the South American Plate and Inca Sliver relatively stable areas. The heterogeneous displacements towards the NNE of the North Andean Sliver are interpreted as consequences of the coupling of the Carnegie Ridge in the subduction zone. The Dolores–Guayaquil megashear constitutes its southeastern boundary and includes the dextral to normal transfer Pallatanga fault, that develops the Guayaquil Gulf. This fault extends northeastward along the central part of the Cordillera Real, in relay with the reverse dextral Cosanga–Chingual fault and finally followed by the reverse dextral Sub-Andean fault zone. While the Ecuadorian margin and Andes is affected by ENE–WSW shortening, the easternmost Manabí Basin located in between the Cordillera Costanera and the Cordillera Occidental of the Andes, underwent moderate ENE–WSW extension and constitutes an active fore-arc basin of the Nazca plate subduction. The integration of the GPS and seismic data evidences that highest rates of deformation and the highest tectonic hazards in Ecuador are linked: to the subduction zone located in the coastal area; to the Pallatanga transfer fault; and to the Eastern Andes Sub-Andean faults.
... Coseismic slip contours 1 m, maximum slip 6 m. The red stars indicate the epicentral locations of large historic earthquakes; the red shaded regions indicate the areas of major moment release (Beck & Ruff, 1984;Kanamori & McNally, 1982;Swenson & Beck, 1996;Yepes et al., 2016); the dashed yellow contours indicate the aftershock areas (Mendoza & Dewey, 1984); the red dashed line indicates the partial rupture area the 1906 earthquake . The continuous gray contours (5-cm interval) regions of slow slip events (Rolandone et al., 2018;Vaca et al., 2018;Vallée et al., 2013). ...
... The 1906 rupture stopped at the intersection of the Carnegie Ridge with the trench. Three large historic earthquakes ruptured different sections of the 1906 rupture area: from south to north, the Mw 7. 8 1942, Mw 7.7 1958, and Mw 8.2 1979Beck & Ruff, 1984;Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Swenson & Beck, 1996). Recent analysis of tsunami records from the 1906 earthquake indicate large slip near the trench suggesting that perhaps the source region for the 1906 event does not coincide with these more recent large events (Yoshimoto et al., 2017). ...
... (d) Interseismic seismicity, Pedernales mainshock and coseismic rupture (from Nocquet et al., 2017), and Pedernales sequence with respect to previous megathrust earthquakes and aseismic slip. Epicenters white stars, areas of major moment release red shading (Beck & Ruff, 1984;Kanamori & McNally, 1982;Swenson & Beck, 1996), aftershock areas dashed yellow contours (Mendoza & Dewey, 1984). The red dashed line indicates the 1906 partial rupture area (from Collot et al., 2004). ...
The heterogeneous seafloor topography of the Nazca Plate as it enters the Ecuador subduction zone provides an opportunity to document the influence of seafloor roughness on slip behavior and megathrust rupture. The 2016 Mw 7.8 Pedernales Ecuador earthquake was followed by a rich and active postseismic sequence. An internationally coordinated rapid response effort installed a temporary seismic network to densify coastal stations of the permanent Ecuadorian national seismic network. A combination of 82 onshore short and intermediate period and broadband seismic stations and six ocean bottom seismometers recorded the postseismic Pedernales sequence for over a year after the mainshock. A robust earthquake catalog combined with calibrated relocations for a subset of magnitude ≥4 earthquakes shows pronounced spatial and temporal clustering. A range of slip behavior accommodates postseismic deformation including earthquakes, slow slip events, and earthquake swarms. Models of plate coupling and the consistency of earthquake clustering and slip behavior through multiple seismic cycles reveal a segmented subduction zone primarily controlled by subducted seafloor topography, accreted terranes, and inherited structure. The 2016 Pedernales mainshock triggered moderate to strong earthquakes (5 ≤ M ≤ 7) and earthquake swarms north of the mainshock rupture close to the epicenter of the 1906 Mw 8.8 earthquake and in the segment of the subduction zone that ruptured in 1958 in a Mw 7.7 earthquake.
... earthquake, likely ruptured all or most of the megathrust fault (~500 km; Kanamori & McNally, 1982;Kelleher, 1972). Subsequent smaller events (1942 M W 7.8, 1958 M W 7.7, 1979 M W 8.2, and 1998 M W 7.1) are thought to have ruptured several discrete asperities within the 1906 slip area based primarily on aftershock locations and coseismic slip models (e.g., Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Ye et al., 2016), although a recent slip model of the 1906 event suggests that it did not overlap with the later earthquakes (Yoshimoto et al., 2017; Figure 1). The 2016 Pedernales event occurred on the northern edge of the subducting Carnegie Ridge and the 1942 earthquake. ...
... Main map: Along Ecuador, the Carnegie Ridge and inactive spreading center structures subduct obliquely where the Nazca plate subducts under South America at a rate of~47 mm/year. The magenta indicates historic large earthquakes (magenta diamonds and lines; Mendoza & Dewey, 1984), inferred rupture length of the 1906 earthquake (solid magenta line; Yoshimoto et al., 2017), and alternative maximum rupture lengths from earlier studies (dashed magenta line). Recent earthquakes with Global Centroid Moment Tensor solutions (beachballs) and U.S. Geological Survey reported epicenter (stars) follows color coding: 1998 M W 7.1 (teal), 1979 M W 8.2 (purple), 2016 M W 7.8 mainshock (red), M W 6.7 aftershock (blue), and M W 6.9 aftershock (green). ...
... Both the 1942 epicenter (Mendoza & Dewey, 1984) and the 2016 mainshock are located offshore within asperity A3. The coseismic InSAR deformation indicates that the mainshock ruptured on asperity A3 (Figure 7). ...
The 2016 MW 7.8 Pedernales, Ecuador, megathrust earthquake produced notable crustal deformation and generated an extensive aftershock sequence that included two M6.5+ events. We combine an improved teleseismic earthquake catalog for Ecuador with analysis of coseismic interferometric synthetic aperture radar data derived from the Sentinel‐1A satellite to better delineate the spatial and temporal slip history of the megathrust fault in absolute space. The revised teleseismic catalog spans 1961‐2016 and incorporates catalog phase onset times and waveform correlation derived differential times to locate earthquakes. Using teleseismic double‐difference (DD) tomography to simultaneously solve for an updated regional 3‐D compressional velocity model and locations yields earthquakes shifted ~25 km southwest relative to rapidly available teleseismic catalogs. The DD catalog better compares in absolute space to the Ecuadorian local catalog and better models the measured deformation fields of the 2016 Pedernales mainshock and largest aftershocks. Additionally, the DD mainshock location agrees with local‐scale seismic and geodetic studies that show the 2016 event had concentrated slip on a highly coupled asperity that likely participated in the 1942 Ecuador megathrust earthquake. The two large aftershocks also ruptured on the megathrust where moderate to strong interseismic coupling is observed. The DD catalog contains moderate‐sized aftershocks that concentrate outside high slip regions, primarily in areas that produced earthquakes during the interseismic cycle, and outside areas of aseismic slip. Development of rapid relative location approaches linking new seismicity to better constrained global catalogs could aid with near real‐time hazard assessment in areas lacking local data.
... Les Zone tiretées indiquent les zones de rupture des séismes de 1906, 1942, 1958, et 1979. Les étoiles représentent la localisation des épicentres des séismes, tandis que les cercles celles des épicentres de leurs répliques (Kanamori and McNally, 1982;Beck and Ruff, 1984;Mendoza and Dewey, 1984;Swenson and Beck, 1996). La couleur blanche indique les événements associés au séisme de 1942, la couleur noire ceux au séisme de 1958, et (Collot et al., 2004)..............................................................................................73 'IFREMER .........................................................................................................................................................................79 Figure 3.4: Filtrage des valeurs aberrantes effectué avec le module FiltXY ...........................................................80 Figure 3.5: Nettoyage manuel des sondes aberrantes effectué avec le module Odicce. ...
... Cette zone fut partiellement réactivée au cours des trois événements de 1942 (Mw7,8) (Beck and Ruff, 1984), 1958 (Mw 7,7) (Kanamori andMcNally, 1982), et 1979 (Mw 8,2) (Beck and Ruff, 1984). L'étude de la localisation des ces séismes et de leurs répliques (Mendoza and Dewey, 1984) montre que les zones de rupture de chacun de ces séismes se succèdent sans se superposer. Cette observation laisse supposer que les zones de ruptures de ces séismes ont partiellement réactivé celle de 1906, et qu'elles sont probablement limitées par des différence de couplage interplaques. ...
... de 1906, 1942, 1958, et 1979. Les étoiles représentent la localisation des épicentres des séismes, tandis que les cercles celles des épicentres de leurs répliques (Kanamori and McNally, 1982;Beck and Ruff, 1984;Mendoza and Dewey, 1984;Swenson and Beck, 1996). La couleur blanche indique les événements associés au séisme de 1942, la couleur noire ceux au séisme de 1958, et la couleur bleue ceux au séisme de 1979. ...
... Patchy distributions of large-slip regions during large earthquakes have been affirmed by increasingly well-resolved finite-fault slip models, but whether the underlying cause is material property variations (sediments/rock contacts), boundary roughness (seamounts/horst and graben structures), or hydrologic variations (pore fluids), or some combination of these factors, and their persistence over multiple events is still an active area of research. A somewhat complementary perspective of earthquake ruptures being controlled by portions of the fault that delimit sliding, or "barriers," was also advanced about this time (13). The connection between asperities, barriers, and gaps is intrinsically complex as heterogeneity of stress and strain accumulation and variable frictional properties complicate the notion of a fault "sticking," which is intrinsic to the elastic-rebound theory (14). ...
... An M W 7.1 event in 24 1998 ruptured the southernmost portion of the 1906 zone southwest of the 1942 rupture (2). The 25aftershock zones of these ruptures abut without overlap within the larger 1906 rupture zone(13). Analysis 26 of seismic waveforms (14) and GPS data (2) has identified discreet asperities associated with the 1958 27 and 1979 ruptures.Kanamori and McNally (12) note that the cumulative seismic moment of the 1942, 281958 and 1979 earthquakes based on aftershock zone area is considerably less (~1/5) than the seismic 29 moment of 1906. ...
So far in this century, six very large–magnitude earthquakes ( M W ≥ 7.8) have ruptured separate portions of the subduction zone plate boundary of western South America along Ecuador, Peru, and Chile. Each source region had last experienced a very large earthquake from 74 to 261 y earlier. This history led to their designation in advance as seismic gaps with potential to host future large earthquakes. Deployments of geodetic and seismic monitoring instruments in several of the seismic gaps enhanced resolution of the subsequent faulting processes, revealing preevent patterns of geodetic slip deficit accumulation and heterogeneous coseismic slip on the megathrust fault. Localized regions of large slip, or asperities, appear to have influenced variability in how each source region ruptured relative to prior events, as repeated ruptures have had similar, but not identical slip distributions. We consider updated perspectives of seismic gaps, asperities, and geodetic locking to assess current very large earthquake hazard along the South American subduction zone, noting regions of particular concern in northern Ecuador and Colombia (1958/1906 rupture zone), southeastern Peru (southeasternmost 1868 rupture zone), north Chile (1877 rupture zone), and north-central Chile (1922 rupture zone) that have large geodetic slip deficit measurements and long intervals (from 64 to 154 y) since prior large events have struck those regions. Expanded geophysical measurements onshore and offshore in these seismic gaps may provide critical information about the strain cycle and fault stress buildup late in the seismic cycle in advance of the future great earthquakes that will eventually strike each region.
... During the last century, the Ecuadorian-Colombian margin has experienced several major earthquakes (Ramírez, 1975;Kelleher, 1972;Abe, 1979;Herd et al., 1981;Kanamori and McNally, 1982;Mendoza and Dewey, 1984;Beck and Ruff, 1984;Sennson and Beck, 1996). More recently, slow slip events have been observed Vallée et al., 2013;Segovia et al., 2015;Collot et al., 2017;Rolandone et al., 2018), in addition to seismic swarms (Segovia, 2001;Segovia et al., 2009;Vaca et al., 2009) and repeating earthquakes (Rolandone et al., 2018) (see Figure 3.1). ...
... In 1958, the Mw 7.7 Colombia-Ecuador earthquake (Kanamori and McNally, 1982) broke a small segment of the 1906 megathrust earthquake in the offshore portion of the Esmeraldas province to the North. Like the 1942 earthquake, the 1958 event showed a rupture propagation in the NE direction (Rothé, 1969;Kelleher, 1972;Mendoza and Dewey, 1984). Finally, in 1979, the largest event of this series of major earthquakes occurred close to the Ecuadorian-Colombian border with magnitude of Mw 8.2 also rupturing in a NE direction along ∼230 km of the margin (Herd et al., 1981;Kanamori and Given, 1981;Beck and Ruff, 1984). ...
Based on seismicity recorded by the permanent Ecuadorian seismic network and our large emergency network installed shortly after the 2016 Mw 7.8 Pedernales earthquake, I derive a seismic 3D velocity model for central coastal Ecuador based on local earthquake tomography (LET). I manually analyzed seismic waveforms recorded on our combined amphibious network to determine high quality arrival times of seismic phases. I inverted the seismic travel times simultaneously for earthquake locations and three-dimensional (3D) velocity structure (Vp and Vp/Vs) using a staggered approach that increases complexity from 1D to finally obtaining a detailed 3D model.
While our 1D velocity model highlights the first-order structures of the studied margin through the analysis of the relocated seismicity, station correction terms and regional moment tensors (RMT), the resulting three-dimensional tomographic images show an area that is widely affected by the subducting topography and small- to large-scale faults at the surface. The oceanic Nazca plate is well imaged down to ∼40 km depth by an eastward dipping high Vp velocity feature. I also identify a low Vp (∼5.5 km/s) region along strike in the marine forearc, which I interpret as a thermal anomaly that might be caused either the rocks coming from the Galapagos Hot Spot or by shallow serpentinization. The marine forearc region also shows differences to the North and to the South of the Equator line, with a prominent seamount, imaged by a Vp∼5.0 km/s, coming from the Atacames seamount chain in the northern region and, based on the elevated Vp/Vs ratios (>1.84), a deeply fractured oceanic crust in the South caused by the subduction of the Carnegie Ridge. The elevated Vp/Vs ratios (>1.84) also suggests a large presence of fluids. In the oceanic crust, this is associated to a combination of extensional faults formed prior to subducting and highs in topography that contribute to fracturing within the downgoing plate. In contrast, elevated Vp/Vs ratios (>1.84) in the upper continental crust are expressions of a highly fractured marine forearc and the presence of small- to large-scale faults that help fluids to circulate along the overriding plate.
The relocated seismicity shows several clusters, mostly organized along the plate interface, but also occurring in both, oceanic and continental plates. Clustered aftershocks in the oceanic crust are located on the flanks of a subducting seamount which promotes active faulting. On the other hand, seismicity observed in the upper plate is related to the (re)activation of several crustal faults following the 2016 Pedernales earthquake. The imaged three-dimensional seismic velocity structures were associated with the seismotectonical and geological context of the Ecuadorian margin to give insights about the main features controlling the occurrence of large megathrust earthquakes in the region.
I also explore the relation between the observed physical properties of the rocks along the slab interface and the spatial distribution of the coseismic slip of the Pedernales earthquake. Our findings suggest a strong correlation between domains with normal Vp/Vs ratios (1.78-1.84) and the areas where the rupture propagated. In contrast, the areas with elevated Vp/Vs ratios (>1.84) can be colocated with the up- and down-dip limits of the Pedernales earthquake and might have contributed to stop the rupture. Furthermore, a grid search analysis shows that the 2016 event occurred in the only area of the margin capable to host an earthquake with the observed magnitude.
Finally, this study contributes to a better understanding of the processes occurring in subduction zones. Especially, I remark the importance of having a complete seismic velocity structure that includes both Vp and Vp/Vs ratios which complement with each other in order to give a full interpretation for the features observed along the study margin. Furthermore, the analysis of the seismic velocities of the rocks along the seismic interface, together with information derived from geodetic studies and the rupture area grid search approach designed in this work, can provide valuable data neccesary for the estimation of seismic hazard.
... In 1906, an approximately 500 km-long rupture produced a M w 8.8 event (Kanamori and McNally, 1982;Collot et al., 2005). Subsequently, portions of that region have re-ruptured in megathrust events from south to north in 1942 (M w 7.8), 1958 (M w 7.7), 1979 (M w 8.2), and 2016 (M w 7.8) (Fig. 1a.) (Kanamori and McNally, 1982;Mendoza and Dewey, 1984;Swenson and Beck, 1996;Nocquet et al., 2017). ...
... The Esmeraldas Fault, a major upper plate fault visible offshore as a submarine canyon, is mapped onshore along the Esmeraldas River (Hall and Wood, 1985;Egüez et al., 2003). In 1942, the segment of the subduction zone near Pedernales ruptured and was followed in 1958 with a rupture of the Esmeraldas segment (Kanamori and McNally, 1982;Mendoza and Dewey, 1984;Swenson and Beck, 1996). During the 1958 M w 7.7 megathrust event, the Esmeraldas Fault and tectonic transition likely contributed to the southwest limit of the rupture, marking a segmentation boundary within the subduction zone (Hall and Wood, 1985;Collot et al., 2004;Manchuel et al., 2011). ...
Megathrust ruptures and the ensuing postseismic deformation cause stress changes that may induce seismicity on upper plate crustal faults far from the coseismic rupture area. In this study, we analyze seismic swarms that occurred in the north Ecuador area of Esmeraldas, beginning two months after the 2016 Mw 7.8 Pedernales, Ecuador megathrust earthquake. The Esmeraldas region is 70 km from the Pedernales rupture area in a separate segment of the subduction zone. We characterize the Esmeraldas sequence, relocating the events using manual arrival time picks and a local a-priori 3D velocity model. The earthquake locations from the Esmeraldas sequence outline an upper plate fault or shear zone. The sequence contains one major swarm and several smaller swarms. Moment tensor solutions of several events include normal and strike-slip motion and non-double-couple components. During the main swarm, earthquake hypocenters increase in distance from the first event over time, at a rate of a few hundred meters per day, consistent with fluid diffusion. Events with similar waveforms occur within the sequence, and a transient is seen in time series of nearby GPS stations concurrent with the seismicity. The events with similar waveforms and the transient in GPS time series suggest that slow aseismic slip took place along a crustal normal fault during the sequence. Coulomb stress calculations show a positive Coulomb stress change in the Esmeraldas region, consistent with seismicity being triggered by the Pedernales mainshock and large aftershocks. The characteristics of the seismicity indicate that postseismic deformation involving fluid flow and slow slip activated upper plate faults in the Esmeraldas area. These findings suggest the need for further investigation into the seismic hazard potential of shallow upper plate faults and the potential for megathrust earthquakes to trigger slow-slip and shallow seismicity across separate segments of subduction zones.
... The Piñon Terrane is often mapped as underlaying the majority of the Ecuadorian forearc (Kerr et al. 2002;Jalliard et al. 2009 (Chlieh et al. 2014). Dashed yellow outlines show the approximate aftershock areas of the 1942 and the 1958 earthquakes (Mendoza & Dewey 1984;Swenson & Beck 1996). Red polygon is the high slip (>1 m) area for the 2016 Pedernales earthquake; red star demarks the epicentre (Nocquet et al. 2017). ...
... In Ecuador, previous studies (e.g. Mendoza & Dewey 1984;Collot et al. 2004) have suggested that the offshore extension of the Jama Fault system and heterogeneities along fault planes are responsible for the segmentation and large earthquake behaviour. As shown in A-A , the slow velocities beneath the Manabí Basin shallow near the Jama Fault, further supporting the idea that the offshore extension of the fault system may segment large earthquake behaviour. ...
The Ecuadorian convergent margin has experienced many large mega-thrust earthquakes in the past century, beginning with a 1906 event that propagated along as much as 500 km of the plate interface. Many subsections of the 1906 rupture area have subsequently produced Mw ≥ 7.7 events, culminating in the 16 April 2016, Mw 7.8 Pedernales earthquake. Interestingly, no large historic events Mw ≥ 7.7 appear to have propagated southward of ∼1°S, which coincides with the subduction of the Carnegie Ridge. We combine data from temporary seismic stations deployed following the Pedernales earthquake with data recorded by the permanent stations of the Ecuadorian national seismic network to discern the velocity structure of the Ecuadorian forearc and Cordillera using ambient noise tomography. Ambient noise tomography extracts Vsv information from the ambient noise wavefield and provides detailed constraints on velocity structures in the crust and upper mantle. In the upper 10 km of the Ecuadorian forearc, we see evidence of the deepest portions of the sedimentary basins in the region, the Progreso and Manabí basins. At depths below 30 km, we observe a sharp delineation between accreted fast forearc terranes and the thick crust of the Ecuadorian Andes. At depths ∼20 km, we see a strong fast velocity anomaly that coincides with the subducting Carnegie Ridge as well as the southern boundary of large mega-thrust earthquakes. Our observations raise the possibility that upper-plate structure, in addition to the subducting Carnegie Ridge, plays a role in the large event segmentation seen along the Ecuadorian margin.
... The Piñon Terrane is often mapped as underlaying the majority of the Ecuadorian forearc (Kerr et al. 2002;Jalliard et al. 2009 (Chlieh et al. 2014). Dashed yellow outlines show the approximate aftershock areas of the 1942 and the 1958 earthquakes (Mendoza & Dewey 1984;Swenson & Beck 1996). Red polygon is the high slip (>1 m) area for the 2016 Pedernales earthquake; red star demarks the epicentre (Nocquet et al. 2017). ...
... In Ecuador, previous studies (e.g. Mendoza & Dewey 1984;Collot et al. 2004) have suggested that the offshore extension of the Jama Fault system and heterogeneities along fault planes are responsible for the segmentation and large earthquake behaviour. As shown in A-A , the slow velocities beneath the Manabí Basin shallow near the Jama Fault, further supporting the idea that the offshore extension of the fault system may segment large earthquake behaviour. ...
The Ecuadorian forearc is a complex region of accreted terranes with a history of large megathrust earthquakes. Most recently, a M w 7.8 megathrust earthquake ruptured the plate boundary offshore of Pedernales, Ecuador on 16 April 2016. Following this event, an international collaboration arranged by the Instituto Geofisico at the Escuela Politécnica Nacional mobilized a rapid deployment of 65 seismic instruments along the Ecuadorian forearc. We combine this new seismic data set with 14 permanent stations from the Ecuadorian national network to better understand how variations in crustal structure relate to regional seismic hazards along the margin. Here, we present receiver function adaptive common conversion point stacks and a shear velocity model derived from the joint inversion of receiver functions and surface wave dispersion data obtained through ambient noise cross-correlations for the upper 50 km of the forearc. Beneath the forearc crust, we observe an eastward dipping slow velocity anomaly we interpret as subducting oceanic crust, which shallows near the projected centre of the subducting Carnegie Ridge. We also observe a strong shallow positive conversion in the Ecuadorian forearc near the Borbon Basin indicating a major discontinuity at a depth of ∼7 km. This conversion is not ubiquitous and may be the top of the accreted terranes. We also observe significant north-south changes in shear wave velocity. The velocity changes indicate variations in the accreted terranes and may indicate an increased amount of hydration beneath the Manabí Basin. This change in structure also correlates geographically with the southern rupture limit of multiple high magnitude megathrust earthquakes. The earthquake record along the Ecuadorian trench shows that no event with a M w >7.4 has ruptured south of ∼0.5 • S in southern Ecuador or northern Peru. Our observations, along with previous studies, suggest that variations in the forearc crustal structure and subducting oceanic crust may influence the occurrence and spatial distribution of high magnitude seismicity in the region.
... The South American subduction zone has a long history of large megathrust earthquakes and in the past decades a variety of different types of seismicity have been identified, including repeating earthquakes, seismic swarms and slow slip events. During the last century, the Ecuadorian-Colombian margin has experienced several major earthquakes (Ramirez, 1968;Kelleher, 1972;Abe, 1979;Herd et al., 1981;Kanamori and McNally, 1982;Mendoza and Dewey, 1984;Beck and Ruff, 1984;Sennson and Beck, 1996). More recently, slow slip https://doi.org/10.1016/j.tecto.2019.228165 ...
... In 1958, the Mw 7.7 Colombia-Ecuador earthquake (Kanamori and McNally, 1982) broke a small segment of the 1906 megathrust earthquake in the offshore portion of the Esmeraldas province to the North. Like the 1942 earthquake, the 1958 event showed a rupture propagation in the NE direction (Rothé, 1969;Kelleher, 1972;Mendoza and Dewey, 1984). Finally, in 1979, the largest event of this series of major earthquakes occurred close to the Ecuadorian-Colombian border with magnitude of Mw 8.2 also rupturing in a NE direction along 230 km of the margin (Herd et al., 1981;Given, 1981: Beck andRuff, 1984). ...
On April 16th 2016 a Mw 7.8 earthquake ruptured the central coastal segment of the Ecuadorian subduction zone. Shortly after the earthquake, the Institute Geofisico de la Escuela Politecnica Nacional of Ecuador, together with several international institutions deployed a dense, temporary seismic network to accurately categorize the post-seismic aftershock sequence. Instrumentation included short-period and broadband sensors, along with Ocean Bottom Seismometers. This deployment complemented the permanent Ecuadorian seismic network and recorded the developing aftershock sequence for a period of one year following the main-shock. A subset of 345 events with M-L > 3.5, were manually picked in the period of May to August 2016, providing highly accurate P- and S-onset times. From this catalogue, a high-quality dataset of 227 events, with an azimuthal gap < 200 degrees, are simultaneously inverted for, obtaining the minimum 1D velocity model for the rupture region, along with hypocentral locations and station corrections. We observe an average Vp/Vs of 1.82 throughout the study region, with relatively higher Vp/Vs values of 1.95 and 2.18 observed for the shallowest layers down to 7.5 km. The high relative Vp/Vs ratio (1.93) of the deeper section, between 30 km and 40 km, is attributed to dehydration and serpentinization processes. For the relocated seismicity distribution, clusters of events align perpendicular to the trench, and crustal seismicity is also evidenced, along with earthquakes located close to the trench axis. We also compute Regional Moment Tensors to analyze the different sources of seismicity after the mainshock. Aside from thrust events related to the subduction process, normal and strike-slip mechanisms are detected. We suggest that the presence of subducting seamounts coming from the Camegie Ridge act as erosional agents, helping to create a scenario which promotes locking and allows seismicity to extend up to the trench, along zones of weakness activated after large earthquakes.
... Like other mega-earthquakes in Latin America, cores of turbidites obtained from the Colombian coast suggest that one or two similar mega-earthquakes occurred in the last 600 years (Migeon et al. 2017). After this event, a sequence of earthquakes occurred in the years 1942, Mw 7.8 (Figure 1b (Kanamori & McNally 1982, Mendoza & Dewey 1984, Sennson & Beck 1996; recently, the 2016 Mw 7.8 Pedernales earthquake (Figure 1b, #12) took place in the same area as the 1942 event (Figure 1b, #14) (Ye et al. 2016, Nocquet et al. 2017. These events show a complex sequence of ruptures that partially overlap each other, demonstrating the heterogeneity of the megathrust and the variability of ruptures along the dip (Kanamori & McNally 1982, Ye et al. 2016, Nocquet et al. 2017, Yoshimoto et al. 2017. ...
Most seismicity in Latin America is controlled by the subduction process.
Different zones have hosted earthquakes of magnitudes larger than Mw 8.5
that repeat every several centuries. Events around Mw 8.0 are more frequent;
since the beginning of the twentieth century, some collocated earthquakes
have occurred with differences of decades, which allows for comparison of
old and modern seismological records. The rupture zones that have hosted
mega-earthquakes continue to produce smaller earthquakes after three centuries. Therefore, the process of unlocking in the Latin America subduction
zone occurs by giant (≥Mw 9.0), mega- (9.0 > Mw ≥ 8.5), and large (8.5 >
Mw ≥ 7.5) earthquakes, and interaction between these events is not yet fully
understood. We have less understanding of the earthquakes that occurred
in the oceanic plates, which have not been correctly recorded due to poor
seismological instrumentation and lack of knowledge about subduction during the first half of the twentieth century in Latin America. Slow earthquakes
have been observed in some zones of Latin America, several of them with recurrence periods of a few years, as well as tectonic (nonvolcanic) tremors and
low-frequency and very low-frequency earthquakes. How do these slow slip
manifestations relate to ordinary earthquakes? This question is still difficult
to answer for Latin America given the lack of dense geodetic and seismic networks that allow identification of all the slow earthquakes that likely occur
more frequently than currently reported.
... . The beach ball is the GCMT earthquake focal mechanism. Dashed black lines outline the approximate rupture areas of 1942, 1958, and 1979 megathrust earthquakes, and the gray line shows the rupture extent of the 1906 earthquake (Mendoza & Dewey, 1984;Sennson & Beck, 1996). Blue and cyan triangles with labels show the CGPS sites used in our study and in the study of Rolandone et al. (2018), respectively. ...
Postseismic deformation following subduction earthquakes includes the combined effects of afterslip surrounding the coseismic rupture areas and viscoelastic relaxation in the asthenosphere and provides unique and valuable information for understanding the rheological structure. Because the two postseismic mechanisms are usually spatiotemporally intertwined, we developed an integrated model combining their contributions, based on 5 years of observations following the 2016 Pedernales (Ecuador) earthquake. The results show that the early, near‐field postseismic deformation is dominated by afterslip, both updip and downdip of the coseismic rupture, and requires heterogeneous interface frictional properties. Viscoelastic relaxation contributes more to far‐field displacements at later time periods. The best‐fit integrated model favors a 45‐km thick lithosphere overlying a Burgers body viscoelastic asthenosphere with a Maxwell viscosity of 3 × 10¹⁹ Pa s (0.9–5 × 10¹⁹ Pa s at 95% confidence), assuming the Kelvin viscosity equal to 10% of that value. In addition to the postseismic afterslip, the coastal displacement of sites north and south of the rupture clearly require extra slip in the plate motion direction due to slow slip events that may be triggered by the coseismic stress changes (CSC) but are not purely driven by the CSC. Spatially variable afterslip following the Pedernales event, combined with the SSEs during the interseismic period, demonstrate that spatial frictional variability persists throughout the whole earthquake cycle. The interaction of adjacent fault patches with heterogeneous properties may contribute to the clustered large earthquakes in this area.
... Stars show the epicenters of the Pedernales earthquake (in white) and previous megathrust earthquakes (in green). The green circles show the rough outlines of past megathrust earthquakes (Kanamori & McNally, 1982;Mendoza & Dewey, 1984) while the black line shows the geodetically derived rupture zone of the 2016 Pedernales earthquake (Nocquet et al., 2017). The orange lines show the 20 cm edges of the Pedernales afterslip during the first month (Rolandone et al., 2018). ...
Subduction zones are highly heterogeneous regions capable of hosting large earthquakes. To better constrain the processes at depth, we analyze the source properties of 1514 aftershocks of the 16th April 2016 Mw 7.8 Pedernales earthquake (Ecuador) using spectral ratios. We are able to retrieve accurate seismic moments, stress drops, and P and S corner frequencies for 341 aftershocks, including 136 events belonging to families of repeating earthquakes. We find that, for the studied magnitude range (Mw 2–4), stress drops appear to increase as a function of seismic moment. They are also found to depend on their distance to the trench. This is in part explained by the increase in depth, and therefore normal stress, away from the trench. However, even accounting for the shallow depths of earthquakes, stress drops appear to be anomalously low near the trench, which can be explained by a high pore fluid pressure or by inherent properties of the medium (low coefficient of friction/low rigidity of the medium) in that region. We are also able to examine the temporal evolution of source properties thanks to the presence of repeating earthquakes. We find that the variations of source properties within repeating earthquake families are not uniform, and are highly spatially variable over most of the study area. This is not the case near the trench, however, where stress drops systematically decrease over time. We suggest that this reflects an increase in pore fluid pressure near the trench over the postseismic period.
... There is the conjunction of the Pacific, Cocos and Nazca oceanic plates as well as the Galapagos microplate besides their interaction with the Caribbean and South American continental plates ( Fig.1) (Lonsdale, 1988;Klein et al., 2005;Pennington, 1981;Gailler et al., 2007). Out of this constellation result seismic movements with the generation of strong earthquakes and severe tsunamis as recorded in past history (Mendoza & Dewey, 1984;Sennson & Beck, 1996;Graindorge et al., 2004;Pararas-Carayannis & Zoll, 2017;Yamanaka et al., 2017;Pulido et al., 2020;Yoshimoto et al., 2017). There are several studies about Ecuador´s past tsunami impacts and associated seismic hazards, paleo-tsunami deposits, economic damages, prevention and mitigation efforts as well as the vulnerabilities of the public and the infrastructure (Chunga and Toulkeridis, 2014;Toulkeridis, 2016;Matheus Medina et al., 2016;Rodríguez Espinosa et al., 2017;Chunga et al., 2017;Toulkeridis et al., 2018;Mato and Toulkeridis, 2018;Navas et al., 2018;Celorio-Saltos et al., 2018;Matheus-Medina et al., 2018;Chunga et al., 2019;Martinez and Toulkeridis, 2020;Edler et al., 2020;Suárez-Acosta et al., 2021;Del-Pino-de-la-Cruz et al., 2021;Aviles-Campoverde et al., 2021). ...
There are a number of unusual tsunamis, which occur within the continents rather than in the oceans, named inland tsunamis. One of these rare occasions occurred in the morning of the Friday the 13th October 2000 in a volcanic glacial lake of the horseshoe shaped extinct El Altar volcano in central Ecuador. A detailed mapping of available air photos and satellite images have been reviewed and evaluated in order to reconstruct the catastrophic event of 2000 and a previous one, evidencing that climate change and the associated subsequent reduction of the glacial caps have been responsible for the disassociation of a huge mass of rock(s), of which separation has resulted to an impact in the volcanic lake by an almost free fall of some 770 meters above lake level ground. This impact generated a tsunami wave capable to reach an altitude of 125 meters about the lake ́s water level and leave to lower grounds killing some ten people and hundreds of animals with a mixture of a secondary lahar and debris avalanche. We tried to explain how the fall has occurred with some theoretical considerations, which resulted to imply that the rock hit at least once, probably twice the caldera wall prior lake impact. Such phenomena, even if rare, need to be better monitored in order to avoid settlements in potential areas in the reach of such devastating waves and subsequent avalanches, even more so, when due to climate change the accumulation of water in such lakes increases and the corresponding subglacial erosion and corresponding disassociation of lose rock material may set free more rocks with substantial volumes.
... Land-based GPS and seismological measurements have revealed that North of CP (Figure 1b), the plate interface shows strong ISC Nocquet et al., 2014;Gombert et al., 2018) and has produced a remarkable seismic sequence that started with the great Mw 8. 6-8.8, 1906 earthquake (Kelleher, 1972;Kanamori & McNally, 1982;Ye et al., 2016). Four Mw 7.7 to 8.2 earthquakes (Beck & Ruff, 1984;Mendoza & Dewey, 1984;Swenson & Beck, 1996), including the Mw 7.8, 2016 Pedernales earthquake, later broke sub-segments of the same area ( Figure 1a). In contrast, south of CP, no events with Mw > 7.0-7.2 ...
We investigate the relationship between the long‐term (Quaternary) interplate coupling and the short‐term geodetically derived interseismic coupling at the Central Ecuador subduction zone. At this nonaccretionary margin, the Cabo Pasado shelf promontory and coastal area are associated with two inter‐plate geodetically locked patches. The deepest patch ruptured co‐seismically during the Mw7.8‐2016 Pedernales earthquake, while the shallowest underwent dominantly after‐slip. Marine geophysical and chronostratigraphic data allow reconstructing the Quaternary tectonic evolution of the shelf promontory and substantiating variation of the long‐term inter‐plate coupling that led to the geodetically locked patches. Prior to ∼1.8 Ma, the outer‐wedge inter‐plate coupling was strong enough to activate trench‐subparallel strike‐slip faults. Then, between ∼1.8 and 0.79 Ma, shortening and uplift affected the shelf promontory, implying a locally increased inter‐plate coupling. After a short, post‐0.79 Ma period of subsidence, shortening and uplift resumed denoting a high inter‐plate coupling that endured up to the present. The synchronicity of the structural evolution of the shelf promontory with the subduction chronology of two reliefs of the Carnegie Ridge crest suggests that the locked patches are caused by a geometrical resistance to subduction that propagates landward causing permanent deformation. In 2016, the deepest subducted relief localized stress accumulation and high seismic slip, while the shallowest relief, which is associated with a weakened outer‐wedge, prevented updip rupture propagation. Thus, at nonaccretionary margins, active outer‐wedge strike‐slip faults might be considered a proxy of near‐trench coupling, and subducted relief a cause of plate coupling but an obstacle to the tsunami genesis when the relief is shallow.
... Along the zone broken by the great 1906 earthquake, four significant events have been recorded: Pedernales in 1942 (M = 7.8), Esmeralda in 1958 (M = 7.7), and Tumaco in 1979 (M = 8.2), and recently the Pedernales earthquake in 2016 (M w = 7.8). These events are in the southern area of the 1906 shock ( Fig. 1 by Nocquet et al., 2016, Gutscher et al., 1999Mendoza and Dewey, 1984). However, no significant interplate earthquakes have been recorded in the northern zone of the 1906 earthquake rupture area until the present. ...
The subduction zone in southern Colombia and northern Ecuador has been the site of significant events in the area that broke the great 1906 earthquake in Ecuador. The 2007 earthquake on Gorgona Island is located within this area, presenting a normal-type focal mechanism and a moment magnitude Mw 6.8. This event is due to the compressive stresses exerted on the continental plate of South America by the Nazca oceanic plate. We performed a kinematic inversion source process of the Gorgona earthquake. Our results show the main slip patch at the hypocenter and a secondary slip release deeper at SW on the fault plane, which agrees with the location of aftershocks; the source time function obtained reflects these energy releases with a total duration of 15 s. We also analyzed the stress state field in this region using the focal mechanism of its early aftershock sequence, obtaining an extensive regime with an azimuth Shmin of 108° and NW-SE direction. The average interseismic coupling in this convergent margin zone is not so high as in the southern areas. Nevertheless, observations of this kind of seismicity and GPS studies in the upper plate of a subduction zone are essential in understanding earthquake cycles and, perhaps, in anticipating slip distributions in future subduction events.
... earthquakes in , 1958Mw 7.8-8.2 earthquakes in , and 1979 (figure 5.1) (Beck and Ruff 1984, Kanamori and McNally 1982, Mendoza and Dewey 1984, Sewnson and Beck 1996. In 2016, the Mw 7.8 earthquake approximately ruptured the same sub-segment previously ruptured by the 1942 Mw 7.8 earthquake (He et al. 2017, Ye et al. 2016 (2021)) and 1998 (Mw 7.1), and ...
The Northern Andes is a continental domain located at the northwestern edge of the South American Plate. This ~2200 km long and 300 to 1000 km wide region defines a natural laboratory for various studies of divers processes, including deformation partitioning, inter-seismic coupling, and continental collision. The oblique and fast convergence of the Nazca plate beneath South America induces (1) elastic deformation induced by spatially variable locking at the subduction interface along the Equatorian-Colombian margin and (2) long-term shear stress, which results in a translation-like motion of the North Andean Sliver (NAS) towards northeast with respect to the South American plate. Furthermore, Nazca plate convergence also produces a diversity of interplate and intraplate seismicity, which has been observed since the 19th century. In the northwestern Andes, eastward collision of the Panama block against the NAS and the Caribbean subduction induce deformation that dominates the kinematics at the northern part of the NAS. Spatial geodesy techniques, in particular GPS/GNSS measurements, make it possible to quantify movements on the earth's surface with millimeter accuracy. The integration of these measurements with elastic models allows us to provide information about the kinematics and the inter-seismic coupling distribution at the subduction interface. This thesis focuses on studying the inter-seismic phase of the seismic cycle with a particular interest in the continental deformation along and within the NAS. The aim is to improve the kinematic models for the Nazca plate and the North Andean Sliver. For that, GPS measurements collected by several research institutes and the Franco-Ecuadorian collaboration (ADN & S5 projects, SVAN International Joint Laboratory), between 1994.0 and 2019.9 are used to derive a new and more refined horizontal velocity field at the continental scale. The analysis and modeling of this velocity field is centered on two main axes allowing to build the first kinematic elastic block model for the NAS and neighboring regions. This model simultaneously solves for rigid block rotations and spatially variable coupling at the subduction interfaces, providing crustal fault slip rates consistent with the derived kinematics. First, we propose a new Euler pole that describes the current motion of the Nazca plate with respect to South America. This pole is estimated from continuous measurements at 5 GPS sites, spatially sampling the entire plate. Our results show that GPS data are compatible with the kinematics of a single rigid plate (wrms = 0.6 mm/yr). Our pole predicts a maximum convergence rate at 65.5 ± 0.8 mm/yr at latitude ~30°S along the Chile trench, decreasing to 50.8 ± 0.7 mm/yr in northern Colombia, and 64.5 ± 0.9 mm/yr in southern Chile. A second-order result for the Nazca plate is that the velocity east component of Robinson Crusoe Island (latitude ~33.6°S) is ~4-5 mm/yr faster than the overall motion of the plate, which is induced by the visco-elastic relaxation following the Maule Mw 8.8 2010 earthquake in Chili. Secondly, our kinematic model for the northern Andes confirms that the Nazca/SOAM and Caribbean/SOAM relative motions are not accommodated inland by a single fault system. We find internal deformation at 2-4 mm/yr accommodated on active secondary faults (the Oca-Ancon, Santa Martha-Bucaramanga, Romeral, and Latacunga-Quito-El Angel faults). These faults bound tectonic blocks and define the rotation of 6 blocks. The NAS eastern boundary is found to be a right-lateral transpressive system accommodating 5 to 17 mm/yr of motion. Our model also quantifies the motion accommodated by the Panama block with respect to the NAS on active structures that we propose as new boundaries for these two continental domains. Relative motions take place at 6 mm/yr along the Uramita fault and 15 mm/yr in the Eastern Panama Deformed Zone. We also note that ~1 cm/yr of the Panama motion is transferred […]
... The poor quality of the soils due to their young age produces unfavorable conditions that correspond to the Holocene [1][2][3][4][5][6][7][8][9][10][11][12]. Historically, older buildings in alluvial valleys filled with Quaternary deposits are the most vulnerable, as was documented in past earthquakes in the province of Manabí, in the central coastal area of Ecuador ( Figure 1) [11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. Some specialists have evaluated the surface coseismic effects related to liquefaction, exposing their criteria in a variety of studies [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48]. ...
The city of Portoviejo in coastal Ecuador was severely affected during the 16 April 2016, Pedernales earthquake (Mw 7.8). Various coseismic liquefaction phenomena occurred, inducing lateral spreading, sand boils, ground subsidence, and sinkholes in soils with poor geotechnical quality in the alluvial and alluvial–colluvial sedimentary environment. Therefore, the main aim of this study was to collect data from standard penetration tests (SPT) and shear velocity and exploratory trenches and to calculate the liquefaction potential index (LPI) by considering a corresponding seismic hazard scenario with an amax = 0.5 g. From these data, a liquefaction hazard map was constructed for the city of Portoviejo, wherein an Fs of 1.169 was obtained. It was determined that strata at a depth of between 8 and 12 m are potentially liquefiable. Our quantitative results demonstrate that the city of Portoviejo’s urban area has a high probability of liquefaction, whereas the area to the southeast of the city is less sensitive to liquefaction phenomena, due to the presence of older sediments. Our results are in accordance with the environmental effects reported in the aftermath of the 2016 earthquake.
... También se muestran las Islas Galápagos y la Cordillera de Carnegie. Adaptado de Toulkeridis, 2013, modificado de Toulkeridis et al., 2017 Entre los desastres naturales destacan tsunamis locales, cuales impactaron Ecuador en los varias ocasiones, de cual la última ocurrió en 1979 Herd et al., 1981;Mendoza and Dewey, 1984;Beck and Ruff, 1984). Muchos ciudadanos en Ecuador no se dieron cuenta de que con el terremoto más fuerte en 2016 en la región costera, también se ha generado un tsunami local, con daños menores (Ye et al., 2016;Toulkeridis et al., 2017). . ...
RESUMEN Se ha propuesto un estudio extenso de vulnerabilidad de la población, de las autoridades de respuesta como de las infraestructuras por tsunamis en Crucita en el centro costero del Ecuador. Crucita tiene una extensión de 13 km de playa, y una extensa zona muy plana, cual podrá ser bastante afectada por un impacto de un tsunami potencial. Una vez analizada las diferentes variables, se evidenciaron los distintos niveles de vulnerabilidad en la población de la parroquia Crucita frente a la amenaza de un tsunami. En este marco entre los principales hallazgos encontrados se pueden mencionar el bajo conocimiento de la población sobre el tema tsunami, la ausencia de planes de evacuación, el alto nivel de exposición de la infraestructura física (viviendas, servicios básicos y telecomunicaciones), y la ausencia de capacidades institucionales para responder a una situación de emergencias y/o desastres en general, y de tsunamis en particular. Palabras claves: vulnerabilidad poblacional, vulnerabilidad económica, vulneravilidad estructural física, capacidad institucional, grado de exposición ABSTRACT An extensive study of the vulnerability of the population, of the response authorities and of the infrastructure for tsunamis in Crucita in the coastal center of Ecuador has been proposed. Crucita has an extension of 13 km of beach, and a large, very flat area, which may be quite affected by an impact of a potential tsunami. Once the different variables have been analyzed, the different levels of vulnerability in the population of the Crucita parish have been evidenced towards tsunami hazards. In this framework, among the main findings we may mention the low knowledge of the population about the tsunami issue, the absence of evacuation plans, the high level of exposure of the physical infrastructure (housing, basic services and telecommunications), and the absence of institutional capacities to respond to a situation of emergencies and / or disasters in general, and of tsunamis in particular.
... Ecuador is situated within the interaction of a variety of continental and oceanic tectonic plates, along the Pacific Rim and therefore generated strong seismic activity and subsequently several tsunamis within recorded history (Pararas-Carayannis, 1980;Herd et al., 1981;Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Pararas-Carayannis, 2012;Chunga & Toulkeridis, 2014). Such tsunamis have produced devastating results within coastal areas and its relatively unprepared population as well as their settlements (Gusiakov, 2005;Ioualalen et al., 2011;2014;Pararas-Carayannis, 2012;Rodriguez et al., 2016;Heidarzadeh et al., 2017). ...
The current study is a pioneer work of an improved technical risk assessment, where alternative solutions are proposed of how lives may be better saved during a potential tsunami impact in the coastal cities of Manta and Salinas in the central coast of Ecuador. As Ecuador has been already the target of several tsunamis during recorded history, further tsunami impacts are rather the rule than the exception. Due to short times between generation and impact of tsunamis and due to long distances to natural elevated safe sites, alternative solutions may be more required such as close-by buildings with certain heights. Those potential shelters as result of vertical evacuation needed to be evaluated for their seismic resistance as well as their resistance towards a tsunami. Both qualifications have been examined by the application of the Modified Italian Methodology in order to calculate the seismic vulnerability index (SVI) and subsequently also in order to determine the tsunami vulnerability index (TVI). In this respect we evaluated 18 buildings of such characteristics in Manta and further 99 in Salinas. Unfortunately, although many buildings stand the applied evaluations, due to the fact that almost all edifices are of private property, both entrance and stairs remain limited for the general public. Therefore, we propose that given regulations need to improve in order to allow the access to the general public during a tsunami emergency within an evacuation plan besides the implementation of an efficient early alert system.
Corresponding author: ttoulkeridis@espe.edu.ec
... The presence of a 2016 SSE near Esmeraldas (E) is suspected but not yet modeled (Hoskins et al., 2021). White stars and white lines show the epicenters and approximate rupture areas of past megathrust earthquakes (Kanamori & McNally, 1982;Mendoza & Dewey, 1984). The yellow star and yellow line show the epicenter and the 1 m contour of the rupture zone of the 2016 Pedernales earthquake . ...
Repeating earthquakes repeatedly rupture the same seismic asperity and are strongly linked to aseismic slip. Here, we study the repeating aftershocks of the April 16, 2016 MW 7.8 Pedernales earthquake in Ecuador, which generated a large amount of afterslip. Using temporary and permanent stations, we correlate waveforms from a one‐year catalog of aftershocks. We sort events with a minimum correlation coefficient of 0.95 into preliminary families, which are then expanded using template‐matching to include events from April 2015 to June 2017. In total, 376 repeaters are classified into 62 families of 4–15 events. They are relocated, first using manual picks, and then using a double difference method. We find repeating earthquakes during the whole period, occurring primarily within large aftershock clusters on the edges of the areas of largest afterslip release. Their recurrence times, shortened by the mainshock, subsequently increase following an Omori‐type law, providing a timeframe for the afterslip's deceleration. Although they are linked temporally to the afterslip, repeater‐derived estimates of slip differ significantly from GPS‐based models. Combined with the fact that repeaters appear more spatially correlated with the afterslip gradient than with the afterslip maxima, we suggest that stress accumulation at the edge of the afterslip may guide repeater behavior.
... Figure 9 presents an overview of the source areas of major historical earthquakes, with the source area assumed in this study. Many studies have assumed that the 1906 earthquake was a consolidated earthquake involving large areas (e.g., Heidarzadeh et al., 2017;Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Sennson & Beck, 1996;Ye et al., 2016). The cycle of large earthquakes in the Colombia-Ecuador subduction zone are discussed in this section. ...
A tsunami that followed the 1906 Colombia‐Ecuador megathrust earthquake was observed and recorded by several tide gauges. In this study, tide gauge records were first digitized from documents comparing the estimated astronomical tide level changes, and the observed tsunami waveforms were extracted. An inverse analysis was conducted using the observed tsunami waveforms, and we successfully developed a slip distribution model that produced improved tsunami waveforms for tide gauge stations compared with previous studies. Based on a comparison of the developed source model and ruptured areas of other significant earthquakes around Colombia and Ecuador, the 1906 earthquake already ruptured an area affected by the 1979 earthquake, resulting in a substantial release of accumulated slip deficits. In contrast, although the ruptured area of the 1906 earthquake likely covered the source areas of the 1958 and 1942 earthquakes, the release in these areas during the earthquake was moderate and insignificant, and large slip deficits remained after the 1906 earthquake. These results demonstrate that the extent of the release of accumulated slip deficits varied greatly in the source area of the 1906 earthquake.
... Tsunamis with some devastating results, originated from the local, regional and far geodynamic environments prone to hit the Ecuadorian coastal areas and its relatively unprepared population as well as their settlements, which are situated within an active continental margin (Pararas-Carayannis, 1980;Herd et al., 1981;Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Pararas-Carayannis, 2012;Ioualalen et al., 2011;2014;Chunga & Toulkeridis, 2014;Heidarzadeh et al., 2017). The coastal continental platform of Ecuador is situated along the Pacific Rim and therefore within an area which is impacted regularly by tsunamis due to a severe earthquake activity (Gusiakov, 2005;Pararas-Carayannis, 2012;Rodriguez et al., 2016). ...
Ecuador has been the target of many tsunamis in its past documented history. Therefore, we performed a detailed assessment of the seismic and tsunami resistance of existing buildings, which may serve as temporary potential shelters in Bahía de Caráquez,in the coast of Ecuador. Prior to this evaluation we used the extensively validated tsunami modelling tool, which allowed to yield tsunami run up and tsunami amplitude based on a modelized 8Mw event, near the studied area. Furthermore, we calculated and elaborated evacuation times and routes based on the assumption of a potential tsunami impact, with a veriety of GIS tools. Based on the short time of reaction for the vulnerable population within the potential flooded area, we opted to suggest a nearby solution with a vertical evacuation in buildings along the coastal line. In order to evaluate the potential of such 26 buildings, we used the Modified Italian Methodology in order to calculate the seismic vulnerability index (SVI) and later also determine the tsunami vulnerability index (TVI).
The results indicate that only one of the 26 assessed buildings fall within acceptable values below 30 for both, SVI and TVI. As all buildings are of private property, both entrance and stairs remains limited for the general public, hence, new regulations should improve access during a tsunami emergency within an evacuation plan as well as an installed early alert system.
... The maximum magnitude of M7.9 was documented in Jama in 1942, and the recent M7.8 Pedernales earthquake in 2016 [1,18,33,34,58]. Finally, the fourth segment is located within the northern coast of the Province of Esmeraldas and southern Colombia with a rupture area over 450 km, reaching from the Galera peninsula to the Buenaventura Colombian villages, where the maximum estimated magnitude is about M 8.6 [18,35]. ...
Seismically induced soil liquefaction has been documented after the M7.8, 2016 Pedernales earthquake. In the city of Jama, the acceleration recorded by soil amplification yielded 1.05 g with an intensity of VIII to IXESI-07. The current study combines geological, geophysical, and geotechnical data in order to establish a geological characterization of the subsoil of the city of Jama in the Manabi province of Ecuador. Then, the liquefaction potential index (LPI) has been evaluated considering the PGA-rock values calculated from deterministic methods applied to nearby geological faults, as well as the soil acceleration records for the city of Jama since the Pedernales megathrust earthquake. The importance of conducting geotechnical evaluations of particular colluvial, alluvial, and floodplain deposits, for which the liquefaction probability profiles have been additionally obtained, may serve as a useful tool for edifices foundations or earthquake resistant designs. Finally, the site response analysis is presented using a linear equivalent analysis, where previously seismic records compatible with the target spectrum have been selected. Hereby, the results of ground surface effects have been compared with the spectra of the Ecuadorian Regulation of Construction (NEC) in the context of local seismic amplification.
... The 1979 M 7.7 earthquake In 1942, a sequence of three adjacent M 7+ earthquakes started offshore of Ecuador and ended with an M 7.7 earthquake on 12 December 1979 off Colombia. The aftershock sequence outlines a 200 km long fault plane (Mendoza and Dewey, 1984). The end points listed in Table 2 define the line source used as a model for this earthquake. ...
The hypothesis that very large earthquakes kill predominantly rural people is tested for the case of Colombia. For models of the eight largest earthquakes that have occurred in Colombia, the hypothetical ratios of rural to urban fatalities are calculated. Choosing these historic ruptures does not mean that they are expected to reoccur soon; instead, they are selected to sample fatalities in realistic models. The fatalities due to the assumed earthquakes are calculated by the tool Quake Loss Alerts for Rescue and Mitigation (QLARM), using the population data from the 2018 Columbian census. The minimum population for a settlement to be classified as urban is 35,000. The results do not depend on this limit. The condition for a model calculation to be accepted is that it matches the maximum intensities of shaking reported in the literature for the earthquake in question.
Of the eight hypothetical earthquakes, today four would predominantly kill rural people, three would be urban, and one is about evenly split. However, the sum of the hypothetical fatalities in the eight test earthquakes is 79% urban and only 21% rural. This means that the observation that worldwide more than 90% of earthquake fatalities are rural does not apply to Colombia. The reasons for this contrast are that on average the Colombian test earthquakes are only about half as long as the ruptures modeled in the worldwide sample. Relatively short ruptures (75 km on average in Colombia) can rip through industrial areas without many villages because Colombia is strongly industrialized.
By implication, this result means that the observation that most large earthquakes kill mostly rural people cannot be applied universally. The likely ratio of rural to urban earthquake fatalities has to be determined for each country separately for the assessment of the ratio of mitigation efforts to be allocated locally.
... The strong tectonic activity of the EC region is supported by (1) the background seismicity (Manchuel et al., 2011), (2) the 1906, 1942, 1958, 1979 to 8.8 megathrust earthquakes (Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Nocquet et al., 2016), (3) the strong seafloor deformation along the Ancon fault system Ratzov et al., 2011), (4) the shelf uplift during the Pleistocene (Michaud et al., 2015), and (5) the uplifted Pleistocene marine terraces along the Ecuador northern coast (Pedoja et al., 2006). ...
... In western Colombia, the oceanic Nazca Plate underthrusts eastward beneath the NAB with a convergence rate of approxi-mately 5.4 cm/yr [5], producing intense crustal deformation. Several large subduction earthquakes have ruptured along this subduction zone between Guayaquil, Ecuador, and Tumaco, Colombia, including the 1906 Colombia-Ecuador earthquake (M w 8. 8-8.4), the 1942 earthquake (M w 7.8) [6], the 1958 earthquake (M w 7.7), the 1979 Tumaco earthquake (M w 8.1) [7], and the 2016 Ecuador earthquake (M w 7.7) [8]. Among them, the Colombia-Ecuador earthquake in 1906 (M w 8.4) and the Tumaco earthquake in 1979 (M w 8.3) generated destructive tsunamis. ...
Colombia is tectonically active, and several large earthquakes have ruptured the Colombia-Ecuador subduction zone (CESZ) during the last century. Among them, the Colombia-Ecuador earthquake in 1906 ( M w 8.4) and the Tumaco earthquake in 1979 ( M w 8.3) generated destructive tsunamis. Therefore, it is important to characterize the seismic rupture processes and their relation with interplate coupling along the CESZ. We searched for repeating earthquakes by performing waveform similarity analysis. Cross correlation (CC) values were computed between earthquake pairs with hypocenter differences of less than 50 km that were located in the northern CESZ (1°–4°N) and that occurred from June 1993 to February 2018. We used broadband and short-period seismic waveform data from the Servicio Geológico Colombiano (SGC) seismic network. A CC threshold value of 0.90 was used to identify the waveform similarity and select repeating earthquakes. We found repeating earthquakes distributed near the trench and the coast. Our estimated repeating earthquakes near the trench suggest that the interplate coupling in this region is low. This is in clear constrast to the occurrence of a large slip in the 1906 Colombia-Ecuador earthquake along the trench in the southern part of the CESZ, and suggests that rupture modes are different between the northern and southern parts of CESZ near the trench.
... However, tsunamis have impacted Ecuador relatively frequently, as documented in the history of the country and in several associated studies (Pararas-Carayannis, G. (1980;Herd et al., 1981;Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Pararas-Carayannis, 2012;Ioualalen et al., 2011;2014;Chunga & Toulkeridis, 2014;Heidarzadeh et al., 2017;Toulkeridis et al., 2017a;b;Pararas-Carayannis, 2018). Tsunamis with some devastating results, originated from the local, regional and far geodynamic environments prone to hit the Ecuadorian coastal areas and its relatively unprepared population as well as their settlements, which are situated in an active continental margin (López, 2013;Matheus Medina et al., 2016;Ye et al., 2016;Rodriguez et al., 2017;Chunga et al. 2017;Toulkeridis et al., 2018;Mato and Toulkeridis, 2018;Matheus-Medina et al., 2018;Chunga et al., 2019;Toulkeridis et al., 2019). ...
Ecuador is a highly vulnerable country in terms of natural hazards, such as volcanic eruptions and tsunami hazards. The education system has a key function to prepare children and adolescents for disaster scenarios. To achieve a nationwide standard of students' knowledge of tsunamis and tsunami hazards, academic research could provide assistance and analyze the current state of students' knowledge and identify possible regional differences. This article introduces to the geodynamic conditions of Ecuador and reports the results of a student questionnaire which was conducted at several Ecuadorian schools at the Pacific coast (Jama, Manta and Puerto Cayo) and in the capital Quito (control condition). It refers to five knowledge-based questions addressing five different topics of tsunami hazards: national regions at risk, locations of safety and danger, formation of a tsunami, risks caused by a tsunami, and protective measures. The statistical results point to significant knowledge differences between school locations at the coast. Comparisons between the coastal schools and Quito additionally indicate nationwide differences.
... We hypothesise that a proportion of aftershocks represent the release of residual coseismically-induced stress on the megathrust interface as well as in the overlying crust (Fig. S13), and are thus possibly controlled by two different processes. Interestingly, the 1942 earthquake has been inferred to have roughly the same rupture area as the 2016 earthquake (Ye et al., 2016;Nocquet et al., 2017), and aftershocks following the 1942 earthquake were mainly located seaward of the hypocentre (Mendoza and Dewey, 1984), implying that these aftershocks may have been driven by afterslip updip of the mainshock rupture area too. If so, these observations may indicate that aseismic slip behaviour, and by inference, frictional properties, persist through at least two seismic cycles. ...
High-Rate (HR) GPS time series following the 2016 M-w 7.8 Pedernales earthquake suggest significant postseismic deformation occurring in the early postseismic period (i.e. first few hours after the earthquake) that is not resolved with daily GPS time series. To understand the characteristics of early postseismic deformation, and its relationship with the mainshock rupture area, aftershocks and longer term postseismic deformation, we estimate the spatio-temporal distribution of early afterslip with HR-GPS time series that span similar to 2.5 min to 72 hr after the earthquake, and compare with afterslip models estimated with daily GPS time series spanning a similar postseismic time period and up to 30 days after the earthquake. Inversion of the HR-GPS time series enables us to image the initiation of afterslip in the initial hours after the earthquake, bringing us closer to the transition between the coseismic and postseismic phases. The spatial signature of early afterslip in the region updip of the mainshock rupture area is consistent with longer-term afterslip that occurs in the 30-day postseismic period, indicating that afterslip initiated updip of and adjacent to peak coseismic slip asperities, in two localised areas, and subsequently continued to grow in amplitude with time in these specific areas. A striking difference, however, is that inversion of the 72-hour HR-GPS time series suggests early afterslip within the mainshock rupture area, but which may have been short-lived. Finally, using the first daily GPS position as the origin of the postseismic displacement (here at 12 hr after the earthquake) biases the postseismic geodetic moment, with similar to 60% missing over the first 72 hr, that corresponds to similar to 10% over the first 30 days. The results of our study demonstrate that imaging the spatio-temporal evolution of afterslip using subdaily GPS time series is important for evaluating postseismic slip budgets, and provides additional insights into the postseismic slip behaviour of faults.
... The strong tectonic activity of the EC region is supported by (1) the background seismicity (Manchuel et al., 2011), (2) the 1906, 1942, 1958, 1979 to 8.8 megathrust earthquakes (Kanamori & McNally, 1982;Mendoza & Dewey, 1984;Nocquet et al., 2016), (3) the strong seafloor deformation along the Ancon fault system Ratzov et al., 2011), (4) the shelf uplift during the Pleistocene (Michaud et al., 2015), and (5) the uplifted Pleistocene marine terraces along the Ecuador northern coast (Pedoja et al., 2006). ...
Deciphering the migration pattern of the Esmeraldas submarine Canyon (EC) and its history of cut‐and‐fill allows constraining the Pliocene‐Pleistocene tectonic evolution of the Ecuador‐Colombia convergent margin. Swath bathymetry, multichannel seismic reflection, and chronological data show that the EC is a 143‐km‐long, shelf‐incising, river‐connected canyon that started incising slope apron deposits in the Manglares fore‐arc basin ~5.3 Ma ago. The EC inception appears contemporaneous with the subduction of the Carnegie Ridge that is believed to have initiated 5–6 Myr ago and is considered an indirect cause of the EC formation. During its two‐stage left‐lateral migration, the EC upper‐half scoured deep incisions providing evidences for uplift episodes in the Manglares Basin that are correlated with mid‐Pliocene and Pleistocene regional tectonic events. Glacioeustatic variations contributed significantly to shape the EC and its upslope tributaries by increasing the rate of canyon incision during rapid sea level falls. Faults, folds, and diapirs have structurally controlled the location of the EC and of its tributary canyons, including the Ancon Canyon, which served as the main spillway of the Manglares Basin prior to be cut from its source ~170 kyr ago by the growth of a fault‐related anticline. The margin wedge that hosts the EC is highly unstable as it is cut by active faults and shaken by large subduction earthquakes. Several mass transport deposits have dammed the EC, one of them between >~65 and ~37 kyr causing an impoverishment of detrital material in the trench sedimentation and a possible interruption of the paleoseismological record.
... Following the Pedernales earthquake, an international effort involving institutions from Ecuador (IG-EPN), France (Géoazur, Cerema, IRD and CNRS), the UK (U. of Liverpool) and the USA and main seismotectonic features. White stars and solid white lines show epicentres and approximate rupture areas of past megathrust earthquakes respectively (Kanamori and McNally, 1982;Mendoza and Dewey, 1984). Yellow star shows epicentre of 2016 mainshock together with its GCMT focal mechanism. ...
We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the aftershock seismicity results from interplate thrust faulting, but we also observe a few normal and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is constrained between the trench and the maximum depth of the coseismic rupture. The most striking feature is the presence of three seismicity bands, perpendicular to the trench, which are also observed during the interseismic period. Additionally, we observe a linear dependency between the temporal evolution of afterslip and aftershocks. We also find a temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the spatial distribution of seismic and aseismic slip processes is correlated to the distribution of bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting that slip in the area could be influenced by the relief of the subducting seafloor. To explain our observations, we propose a conceptual model in which the Ecuadorian margin is subject to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of subduction, relevant for convergent margins with a complex and heterogeneous structure such as the Ecuadorian margin.
... Along the Ecuadorian margin, elastic strain accumulation along the subduction is 49 heterogeneous. In northern Ecuador, the high interseismic locking imaged by GPS ( Fig. 1) is 50 consistent with the large megathrust earthquakes observed during the XX th century [1906, 51 Mw 8.4-8.8 (Kelleher, 1972Kanamori and McNally, 1982;Ye et al., 2016;Yoshimoto et al., 52 2017); 1942, Mw 7.8 ( Mendoza and Dewey, 1984); 1958, Mw 7.7 (Swenson and Beck, 1996); 53 1979, Mw 8.1 (Beck and Ruff, 1984); and the recent Mw 7.8 2016 Pedernales earthquake 54 (Ye et al., 2016;Nocquet et al., 2017;Yoshimoto et al., 2017)]. 55 ...
The recent development of a national seismic broadband network in Ecuador enables us to determine a comprehensive catalog of earthquake focal mechanisms at the country-scale. Using a waveform inversion technique accounting for the spatially variable seismic velocity structure across the country, we provide location, depth, focal mechanism and seismic moment for 282 earthquakes during the 2009-2015 period. Our results are consistent with source parameter determinations at the global scale for the largest events, and increase the number of waveform-based focal mechanism solutions by a factor of two. Our new catalog provides additional constraints on the active deformation processes in Ecuador. Along the Ecuador margin, we find a correlation between the focal mechanisms and the strength of interseismic locking at the subduction interface derived from GPS measurements: thrust earthquakes predominate in Northern Ecuador where interseismic locking is high, while the low-to-moderate locking in Central and Southern Ecuador results in variable fault plane orientations. Focal mechanisms for crustal earthquakes are consistent with the principal axis of strain rate field derived from GPS data and with the location of the main active faults. Our catalog helps to determine the earthquake type to be expected in each of the seismic zones that have recently been proposed for probabilistic seismic hazard assessment.
... Following the Pedernales earthquake, an international effort involving institutions from Ecuador (IG-EPN), France (Géoazur, Cerema, IRD and CNRS), the UK (U. of Liverpool) and the USA and main seismotectonic features. White stars and solid white lines show epicentres and approximate rupture areas of past megathrust earthquakes respectively (Kanamori and McNally, 1982;Mendoza and Dewey, 1984). Yellow star shows epicentre of 2016 mainshock together with its GCMT focal mechanism. ...
We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the aftershock seismicity results from interplate thrust faulting, but we also observe a few normal and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is constrained between the trench and the maximum depth of the coseismic rupture. The most striking feature is the presence of three seismicity bands, perpendicular to the trench, which are also observed during the interseismic period. Additionally, we observe a linear dependency between the temporal evolution of afterslip and aftershocks. We also find a temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the spatial distribution of seismic and aseismic slip processes is correlated to the distribution of bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting that slip in the area could be influenced by the relief of the subducting seafloor. To explain our observations, we propose a conceptual model in which the Ecuadorian margin is subject to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of subduction, relevant for convergent margins with a complex and heterogeneous structure such as the Ecuadorian margin.
... 1). Smaller patches of the segment that ruptured in 1906 slipped from south to north in a series of major earthquakes: 1942M w 7.8, 1958M w 7.7, and 1979M w 8.2 (Kanamori and McNally, 1982Mendoza and Dewey, 1984;Swenson and Beck, 1996). The 2016 Pedernales earthquake ruptured the same portion of the subduction zone that slipped in 1942 (Ye et al., 2016;Nocquet et al., 2017). ...
The April 2016 Pedernales earthquake ruptured a 100 km by 40 km segment of the subduction zone along the coast of Ecuador in an M-w 7.8 megathrust event east of the intersection of the Carnegie ridge with the trench. This portion of the subduction zone has ruptured on decadal time scales in similar size and larger earthquakes, and exhibits a range of slip behaviors, variations in segmentation, and degree of plate coupling along strike. Immediately after the earthquake, an international rapid response effort coordinated by the Instituto Geofisico at the Escuela Politecnica Nacional in Quito deployed 55 seismometers and 10 ocean-bottom seismometers above the rupture zone and adjacent areas to record aftershocks. In this article, we describe the details of the U.S. portion of the rapid response and present an earthquake cata-log from May 2016 to May 2017 produced using data recorded by these stations. Aftershocks focus in distinct clusters within and around the rupture area and match spatial patterns observed in long-term seismicity. For the first two and a half months, aftershocks exhibit a relatively sharp cutoff to the north of the mainshock rupture. In early July, an earthquake swarm occurred similar to 100 km to the northeast of the mainshock in the epicentral region of an M-w 7.8 earthquake in 1958. In December, an increase in seismicity occurred similar to 70 km to the northeast of the mainshock in the epicentral region of the 1906 earthquake. Data from the Pedernales earthquake and aftershock sequence recorded by permanent seismic and geodetic networks in Ecuador and the dense aftershock deployment provide an opportunity to examine the persistence of asperities for large to great earthquakes over multiple seismic cycles, the role of asperities and slow slip in subduction-zone megathrust rupture, and the relationship between locked and creeping parts of the subduction interface.
... As result of both, the collision and subsequent subduction between the Nazca oceanic and the Caribbean with the South American continental plates as well as the movement of the referred Mega-Fault, extreme destruction by a variety of landslides, earthquakes and tsunamis have been manifested in past along the subduction trench and along and aside the fault with high losses of both, life and infrastructure (Barazangi and Isacks 1976;Mendoza and Dewey, 1984;Atakan, 1995;Tibaldi et al., 1995;Pararas-Carayannis, 2012;Chunga, and Toulkeridis, 2014;Parra et al., 2016). Therefore, strong earthquakes with catastrophic results for life and infrastructure occur in regular form in the Ecuadorian territory. ...
This paper presents the latest research development in conceptual cost estimation (CCE). The period is from 1995-2014. The methodology involves compiling all the relevant paper in the construction management related journals. Fifty-six relevant articles obtained from 18 major journals associated with construction management studies were successfully assessed. The four findings are: (1)trend of paper for CCE in building project show an upward trend with some fluctuations during the period; (2)the active contributors of the study in supplying analytical thinking and critical ideas are dominated by researcher from Turkey and Korea; (3) there is a clear positive trend for the papers that have applied quantitative approach which starts from 2005, whereas the papers applied qualitative approach began to steadily decrease; (4) the areas of cost factors that have increasingly higher growth affecting CCE are design and project-specific factor. Two broad recommendations are made to the field of study that researcher must select an appropriate historical data to be used for CCE and also each method is appropriate for certain individual situations. © 2018, Construction Research Institute of Malaysia. All rights reserved.
The Ecuadorian forearc, formed by the accretion of oceanic plateaus, island arcs, subduction of an aseismic ridge, records a history of long-lived subduction. The modern system includes subduction of the Carnegie Ridge and seamounts, young forearc coastal ranges, and translation of a forearc sliver from oblique subduction of the Nazca Plate beneath South America. The margin has experienced large megathrust earthquakes and exhibits slow-slip events and earthquake swarms. We present results from joint tomographic inversion of local earthquakes for 3D velocity structure and earthquake location. Our joint inversion uses seismic arrival-time data from local earthquakes recorded by permanent stations and dense seismic temporary networks deployed near the coast after the 2016 Mw 7.8 Pedernales megathrust rupture and across the entire northern forearc into the foothills of the Andes in 2021-2022. Our results show that seismicity distribution and megathrust rupture are controlled by inherited and modern structures in the upper plate forearc and subducting Nazca Plate. Forearc sedimentary basins observed as low-velocities (Vp < 5.8 km/s, Vs < 3.2 km/s) are dissected by forearc basement highs observed as fast velocities (Vp 6.6-7.2 km/s, Vs. 3.6-4.0 km/s). Localized deep depocenters adjacent to basement highs preserve older sedimentary sections beneath younger forearc deposits. Differences in velocity allow discrimination between oceanic plateau basement associated with the Piñón terrane beneath the forearc and accreted island arc terranes along the eastern forearc boundary with the Andes. Along the coast, basement velocities are consistent with a hydrated upper plate. We observe an apparent transient in Vp/Vs (higher to lower) in the upper plate after the 2016 megathrust rupture, representing a transient flux of fluids from the subducting slab into the upper plate triggered by the earthquake. We observe variable thickness of the subducting Nazca plate from ∼10 km north of the Carnegie Ridge reaching 20-25 km where the Carnegie Ridge subducts beneath the forearc. Lateral velocity variations in the subducting plate indicate heterogeneity along strike and dip associated with magmatic evolution of the ridge. High-velocity domains at depth correlate with seamounts and subducted relief along the Carnegie Ridge. A low-velocity zone marks the boundary between the subducting and overriding plates. The downdip termination of the Pedernales megathrust rupture coincides with structure of the Carnegie Ridge and along strike changes in the plate interface. The downdip edge of the rupture occurs where the low-velocity zone is absent, and the subducting Carnegie Ridge intersects the overlying mantle wedge. Earthquakes located with the joint inversion focus into tight clusters controlled by relief at the top of the subducting slab and basement structure in the overriding plate. Along the coast, seismicity shallows from south to north across the east-west striking Canandé Fault. South of the fault, seismicity locates predominantly within the subducting plate and plate interface. To the north, seismicity concentrates within the plate interface and upper plate. The northward shift in hypocenter depths and an offset in the eastern limit of thick subducting Nazca plate across the Canandé fault marks a significant transition in the forearc across the fault.
Key Points: • An integrated three-dimensional thermal model indicates that the geometry of subducting slabs has a substantial impact on the thermal structure of megathrusts globally. • The model highlights that slab obliquity and depth variations play a crucial role in shaping the thermal regime and earthquake potential, as observed in the Makran Subduction Zone. • Results suggest a strong correlation between zones of slab dehydration and the distribution of large crustal earthquakes, implying that slab metamorphism and fluid accumulation influence earthquake occurrence. Abstract: The dependence of the subduction regime on three-dimensional slab geometry poses a challenge for accurately estimating the evolving thermal structure of megathrusts globally. Although slab dips and ages have gained attention, the specific impacts of oblique subduction remain unmeasured. Here, we present an integrated thermal model that quantifies how slab morphology can shape the thermal state of megathrusts, such as those in the Makran Subduction Zone. The model considers both slab obliquity and depth variations along the trench. We find a considerable match between the slab petrological dehydration zone and the distribution of great crustal earthquakes. We suggest that the accumulation of fluids along megathrusts by slab metamorphism can foster more polarized conditions for decreasing plate coupling and increasing interplate ruptures. It is thus imperative to improve model representation and more realistically represent how drivers of slab geometry affect metamorphic transitions and the occurrence of earthquakes at megathrusts.
Los tsunamis son una amenaza permanente para el Ecuador, y el tiempo disponible desde la generación de un tsunami hasta el arribo de la primera ola a la costa, puede no corresponder al tiempo requerido por los sistemas y la población para realizar una evacuación exitosa hasta el arribo a los sitios seguros.
Para este análisis se consideró el escenario sismo tsunamigénico registrado en Ecuador el 31 de enero de 1906. Se analizó la brecha entre: el tiempo disponible (TD) definido como una función de la amenaza, y el tiempo requerido (TR) definido como una función de la capacidad de respuesta de las instituciones y población. Este análisis se centró en la provincia de Esmeraldas de Ecuador, considerada como la zona de mayor exposición.
Para establecer el TD, se tomó como referencia los eventos Indonesia 2004, Chile 2010 y Japón 2011, y se definió un posible escenario de afectaciones similar al registrado el 31 de enero de 1906. El TR por otro lado, se utilizó las fases estudiadas por Dewi y Yuzal (2013), en la que se contemplan dos (2) fases: el tiempo de los sistemas de alerta temprana (SAT) y el tiempo de evacuación, basados en una aproximación de los tiempos que requieren los SAT para la difusión de una alerta en una situación de crisis durante un desastre, y los estándares promedio establecidos de velocidad de desplazamiento de la población, además de la ubicación actual de los sitios seguros establecidos por la SGR.
Como resultado de este análisis se pudo establecer principalmente que: i) existen poblaciones sin posibilidad física (a mas de 1.2 km, o con barreras naturales) para llegar a los sitios seguros en el tiempo disponible propuesto; ii) el tiempo disponible, actualmente no brinda el margen necesario para que la totalidad de la población de las zonas de riesgo de tsunami arriben a los sitios seguros establecidos; y iii) sin importar la tecnología de los SAT instalados, se debe propender a generalizar el uso de la “auto evacuación” activada por una alerta natural como es el sismo.
The maritime or fluvial Blue Forests, the wetlands, estuaries, mangroves, those spaces where the essential thing is the water that adjectives them, constitute a new hope and strategy against climate change and another auspicious possibility of regeneration, recovery and permanence of a social relationship and individual more harmonious, more respectful, poietic and ethically committed to the vital environment that shelters us and its enormous wealth and beauty. This book brings together the research and experiences of 11 experts in various disciplines involved in the restoration and construction of inhabited territories.
Based on manually analyzed waveforms recorded by the permanent Ecuadorian network and our large aftershock deployment installed after the Pedernales earthquake, we derive three‐dimensional Vp and Vp/Vs structures and earthquake locations for central coastal Ecuador using local earthquake tomography. Images highlight the features in the subducting and overriding plates down to 35 km depth. Vp anomalies (∼4.5–7.5 km/s) show the roughness of the incoming oceanic crust (OC). Vp/Vs varies from ∼1.75 to ∼1.94, averaging a value of 1.82 consistent with terranes of oceanic nature. We identify a low Vp (∼5.5 km/s) region extending along strike, in the marine forearc. To the North, we relate this low Vp and Vp/Vs (<1.80) region to a subducted seamount that might be part of the Carnegie Ridge (CR). To the South, the low Vp region is associated with high Vp/Vs (>1.85) which we interpret as deeply fractured, probably hydrated OC caused by the CR being subducted. These features play an important role in controlling the seismic behavior of the margin. While subducted seamounts might contribute to the nucleation of intermediate megathrust earthquakes in the northern segment, the CR seems to be the main feature controlling the seismicity in the region by promoting creeping and slow slip events offshore that can be linked to the updip limit of large megathrust earthquakes in the northern segment and the absence of them in the southern region over the instrumental period.
The variation in maximum rupture extent of large shallow earthquakes in circum-Pacific subduction zones is interpreted in the context of the asperity model of stress distribution on the fault plane. Comparison of the historic record of large earthquakes in different zones indicates that four fundamental categories of behavior are observed. These are: 1) the Chile-type regular occurrence of great ruptures spanning more than 500 km; 2) the Aleutians-type variation in rupture extent with occasional ruptures up to 500 km long, and temporal clustering of large events; 3) the Kurile-type repeated failure over a limited zone of 100-300 km length in isolated events; and 4) the Marianas-type absence of large earthquakes. -Authors
Magnitude statistics (log N = a − bM) were measured for mine tremors and for the Parkfield earthquakes, listed by McEvilly, Bakun and Casaday (1967), in conditions of varying rates of aseismic ground deformation. Aseismic deformation rates were estimated using a tiltmeter in the ERPM gold mine and using results from small geodetic networks, reported by Smith and Wyss, for the region near Parkfield, California. In the magnitude-frequency relationship, b is independent of the rate of aseismic deformation, ε˙,anda=logε˙+Κ, where K is a constant. Thus, the level of seismicity appears to be determined by the rate of aseismic ground deformation. Changes in shear stress of the order of 100 bars do not have any measurable effect on the magnitude statistics.
New marine geophysical data allow the preparation of revised bathymetric and magnetic anomaly charts of the Panama Basin and demonstrate that the eastern part of the basin, between the fracture zone at long 83°W and the Colombian continental margin, was formed by highly asymmetric sea-floor spreading along the boundary of the Nazca and Cocos plates 27 to 8 m.y. B.P. Lineated magnetic anomalies recording this history are oriented approximately east-west. The oldest set of north-flank anomalies overlaps in age with those adjacent to the Grijalva scarp, south of the western Panama Basin, where they are oriented 065°. Younger anomalies (5C to 5) in the eastern basin are approximately parallel to anomalies of this age identified on the Carnegie platform and the flanks of the Costa Rica rift. The eastern basin now contains a pattern of fossil spreading centers (including the Malpelo rift) and transform faults (including the Yaquina graben) that were abandoned 8 m.y. B.P. by a shift in plate boundaries that transferred a large section of the Cocos plate to the Nazca plate. Cessation of Nazca-Cocos spreading east of long 83°W was heralded by a 3-m.y. deceleration of spreading on the eastern segments, which created rough topography and axial rift valleys typical of slow-spreading ridges. Westward jumping of the Nazca-Cocos-Caribbean triple junction rejuvenated the northern segment of the fracture zone at long 83°W, causing uplift of the adjacent Coiba Ridge. Recently, active transform faulting has jumped farther west, from the foot of the Coiba Ridge to the Panama fracture zone. Apart from changes in plate boundaries, the main event in the tectonic evolution of the region was initiation about 22 to 20 m.y. B.P. of the hot spot that created the Malpelo, Cocos, and Carnegie Ridges. Precursors of effusive ridge-building volcanism included major fracturing of the oceanic crust to the north of the present Malpelo Ridge. Both processes hamper identification of magnetic anomalies in the vicinity of the ridges. Our interpretation of the tectonic history is also incomplete in the easternmost parts of the basin, where data are insufficient; this impairs our interpretation of the adjacent continental geology in terms of changing interaction between oceanic and continental plates. The geologic history of the Isthmus of Panama is compatible with our application of the plate-tectonic model.
Regional variations in the rupture characteristics of large shallow earthquakes in circum-Pacific subduction zones are interpreted in the context of the asperity model of heterogeneous stress distribution on the fault plane. It is assumed that the degree of seismic coupling between the downgoing and overriding plates is reflected in the maximum earthquake rupture dimensions in each region, and that gross features of the regional stress distribution can be inferred from the rupture process of large earthquakes. The results of numerous studies of the historic record and detailed source process of large subduction zone events are summarized for each region. The systematic variation in maximum rupture extent in different zones indicates that four fundamental categories of behavior are observed. -Authors
The arrest of a semi-infinite longitudinal shear crack is caused by either (1) the finiteness of available strain energy, or (2) an increase in fracture energy along the trajectory of the running crack. In the former case the following relationship may be used to evaluate the fracture energy:
where γo is the fracture energy per unit length along the crack edge per unit extension of the cracktip (erg cm⁻²), R is the characteristic radius of the fault (cm), Δσ is the stress drop (dyne cm⁻²) and μ is the rigidity (dyne cm⁻²). This leads to the following relationship:
or from the Keylis-Borok relationship (1959):
where Mo is the seismic moment in dyne cm⁻¹. These two relationships are statistically acceptable for Southern California faults and the Tonga-Kermadec Arc earthquakes. The fracture energy is found to vary from 10³ to 10⁹ erg cm⁻² with fresh fracture being associated with 10⁷-10⁹ erg cm⁻² while frictional rupture with 10³-10⁷ erg cm⁻². These values are in good agreement with other independent estimates
Southwestern Colombia and northern Ecuador were shaken by a shal-low-focus earthquake on 12 December 1979. The magnitude 8
shock, located near Tumaco, Colombia, was the largest in northwestern South America since 1942 and had been forecast to fill
a seismic gap. Thrust faulting occurred on a 280- by 130-kilometer rectangular patch of a subduction zone that dips east beneath
the Pacific coast of Colombia. A 200-kilometer stretch of the coast tectonically subsided as much as 1.6 meters; uplift occurred
offshore on the continental slope. A tsunami swept inland immediately after the earthquake. Ground shaking (intensity VI to
IX) caused many buildings to collapse and generated liquefaction in sand fills and in Holocene beach, lagoonal, and fluvial
deposits.
A fundamental error in the application of the confidence ellipse concept to the estimation of location error is demonstrated. The change in procedure required to be statistically sound and to achieve agreement with observations is explained and illustrated. The resultant procedure is then applied to the estimation of the network detection capability on a world-wide basis of the WWSSN.
Three large earthquakes occurred within the rupture zone of the 1906 Colom- bia-Ecuador earthquake (Mw = 8.8): in 1942 (Ms = 7.9); 1958 (Ms -- 7.8); and 1979 (Ms = 7.7). We compared the size and mechanism of these earthquakes by using long-period surface waves, tsunami data, and macroseismic data. The 1979 event is a thrust event with a seismic moment of 2.9 x 1028dyne-cm, and represents subduction of the Nazca plate beneath South America. The rupture length and direction are 230 km and N40°E, respectively. Examination of old seismograms indicates that the 1906 event is also a thrust event which ruptured in the northeast direction. The seismic moment estimated from the tsunami data and the size of the rupture zone is 2 x 1029 dyne-cm. The 1942 and 1958 events are much smaller (about ~ to ~-~o of the 1979 event in the seismic moment) than the 1979 event. We conclude that the sum of the seismic moments of the 1942, 1958, and 1979 events is only ~ of that of the 1906 event despite the fact that the sequence of the 1942, 1958, and 1979 events ruptured approximately the same segment as the 1906 event. This difference could be explained by an asperity model in which the fault zone is held by a discrete distribution of asperities with weak zones in between. The weak zone normally behaves aseismically, but slips abruptly only when it is driven by failure of the asperities. A small earthquake represents failure of one asperity, and the rupture zone is pinned at both ends by adjacent asperities so that the effective width and the amount of slip are relatively small. A great earthquake represents failure of more than one asperity, and consequently involves much larger width and slip.
The large Mindanao earthquake (Mw=8.1) of August 16, 1976, presents a complex rupture history. The epicenter of this earthquake is located at the southern end of the 160×80 km2 aftershock area, and the thrust mechanism with a shallow NE dipping plane indicates the subduction of the North Celebes Sea beneath Mindanao. We have characterized both temporally and spatially the rupture process of this event by deconvolving the source time functions from long-period P-wave seismograms at 20 azimuthally well-distributed stations.The seismic moment is released in a jagged fashion in two main pulses. The observable directivity associated with these two pulses of moment release defines three segments on the fault: (1) from 0 to 54 km N-NW of the epicenter with ~1/2 of the moment release and an apparent rupture velocity of 2.0 km/s, (2) from 54 to 72 km N-NW of the epicenter with low to no resolvable moment release and an apparent rupture velocity of 0.8 km/s, and (3) from 72 to 157 km N-NW of the epicenter with ~1/2 of the moment release and a rupture velocity of 3.3 km/s. Although the overall moment release is comparable in the first and second pulses, the second pulse has a substantially higher rupture velocity and a higher level of short-period radiation. The sharp truncation of the second pulse generates the largest amplitudes in the P-wave seismograms and is interpreted as the abrupt termination of the rupture front against Mindanao Island, 160 km N-NW of the epicenter. An additional third pulse of moment release after the dominant truncation is identified at stations to the NW and SE. This additional moment release is located on a separate fault to the west which is probably strike-slip in nature and related to a large strike-slip aftershock along the coast of Mindanao. The jagged, multiple event moment release of the Mindanao earthquake is in sharp contrast to the smooth rupture of the 1979 Colombia subduction zone earthquake (Mw=8.2), which resulted from the rupture of a single large asperity (~60 km). Although these two earthquakes are both underthrusting events, they occur in very different tectonic settings. The Mindanao earthquake occurred at a relatively young subduction zone in a region with rapidly evolving plate boundaries, whereas the Colombia earthquake occurred at a well-established plate boundary.
The earthquakes examined in this paper are all within the oceanic lithosphere and are associated with the bending of plates before subduction. Accurate determinations of the depth of these earthquakes are needed to study the stress pattern within a bending plate. Routinely-determined depths of shallow sub-oceanic earthquakes published in bulletins are unreliable. The depths can be accurately determined to within a few kilometers if the original seismograms from these events are studied. In some cases, the reflected phases pP and pwP can be clearly identified. There exists the possibility that the wave reflected at the water-air interface, pwP, may be misidentified as pP, leading to erroneous estimates of depth. Additional methods of analysis, such as surface wave radiation patterns or the apparent frequency-dependence of reflection at the crust-water interface, can remove this possible source of confusion. One of the most powerful techniques for depth analysis is the modelling of long-period waveforms. The pattern of stresses within the bending oceanic lithosphere revealed by the depths and focal mechanisms of these intraplate earthquakes is one of horizontal, deviatoric tension down to a depth of about 25 km, with horizontal compression at greater depths.
Body and surface waves for a shallow shock that occurred on November 25, 1965, in the middle of the Nazca plate indicate a double-couple dip-slip source with a horizontal pressure axis in the east-west direction. This suggests that the Nazca plate is being compressed in the direction of plate motion. A focal depth of 9 km below the ocean bottom was precisely determined from a combination of body- and surface-wave data. For a dip-slip source, the azimuthal radiation pattern for Rayleigh waves is strongly frequency dependent, and this characteristic can be used for an accurate focal depth determination. The maximum effect of the continental margin on surface-wave amplitude was estimated assuming conservation of energy without reflections or changes in mode. Love waves are potentially more affected than Rayleigh waves by the continental margin. The effects on Rayleigh- and Love-wave amplitudes due to the continental margin were measured between Galapagos and Quito.
One or more earthquakes with abnormally large numbers of aftershocks at the beginning of their aftershock sequences are possible long-term precursors of a stronger earthquake. This precursor, named pattern B (burst of aftershocks), was tested together with two other possible premonitory patterns, S and Sigma, described previously. Pattern S (swarm) consists of the spatial clustering of earthquakes at a time when the seismicity of the region is above average. Pattern Sigma consists of an increase in the sun of earthquake energies to the 2/3 (roughly) power, in a sliding time window. These three patterns were tested by retrospective long-term prediction of earthquakes of Southern California, 1932-1977, with magnitudes ?6.5. The total duration of identified periods of elevated probability of occurrence is about 14 years. Five out of six earthquakes occurred during these intervals. However, the patterns do not indicate the exact location of future strong earthquakes with the region. All three patterns seem to represent different projections of the same general pattern, of 'bursts of seismicity,' i.e., an abnormal clustering of earthquakes in the time-space-energy domain. Pattern B also precedes strong earthquakes in New Zealand and Italy; Pattern Sigma precedes strong earthquakes in New Zealand.
Total-moment spectra are computed for 14 large earthquakes recorded by the International Deployment of Accelerometers (IDA) network using the scalar-moment retrieval method proposed by Silver and Jordan (1982). For each event we obtain estimates of MT averaged over the 10 disjunct, 1-mHz intervals in the low- frequency band 1-11mHz; typical IDA record sets from events with MT>approx 0.2A yield standard errors on the 1-mHz averages that are generally <20%. Our multiple-band estimates of MT are usually consistent with comparable single-band values found by other investigators. From the total-moment spectra we derive the zero-frequency (static) moment MT0 = MT(0) and the characteristic source duration.-Authors
Spreading along the Cocos-Nazca plate boundary since the breakup of the Farallon plate in the Miocene has resulted in the formation of the Panama basin and a complex interaction of plates in and near northwestern South America. Current plate boundaries have been defined, and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes, considering only welllocated events, and through focal mechanism determinations. The existence of relict plate boundaries, bathymetric features, and the Panamanian isthmus has affected the subduction process of the Nazca plate beneath Sou_th America and determined the present-day configuration of the subducting lithospheric plate. There is no single triple junction separating the Caribbean, South American, and Nazca plates. Instead, the Panamanian isthmus and surrounding areas are accommodating east-west compression (and a lesser degree of north-south compression) along a series of thrust faults striking NW to NE, and the Andean ranges of Ecuador, Colombia, and Venezeula are moving as a block NNE relative to the rest of the South American plates, along a system of faults following the front of the Eastern Cordillera. The subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study. From north to south, they are (1)a 'Bucaramanga' segment continuous with the Caribbean seafloor northwest of Colombia, (2) a 'Cauca' segment continuous with oceanic crust (Nazca plate) currently being subducted beneath South America at the Colombia-Ecuador trench, and (3) an 'Ecuador' segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern Peru. The segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted. _
Recently, Bouchon (1979a) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1066 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 109 erg cm-2. Using the formulas of Ida (1973b), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.
Foreshocks occur before a large fraction of the world's major (M more than 7.0) earthquakes. Teleseismically located events before major earthquakes from 1914 to 1973 were considered together to examine possible average temporal and spatial patterns of foreshock occurrence. Several days before the main shocks and apparently near the epicenters of them (delta approx. less than 30km) the activity begins to increase, culminating in a final rapid acceleration of activity in the last day. The acceleration continues up to the time of the main shocks, except for a possible temporary decrease about six hours before them. The seismicity increases approximately as the inverse of time before main shock. This relationship is essentially unrelated to the magnitude of the main shock. The magnitude of the largest foreshock is also unrelated to the magnitude of the main shock. In addition, pairs of major events are common. 10% of the world's major events are preceded by other major events within 100km and three months. For foreshocks within each of three sequences studied, the ratio of the amplitudes of the P and S waves were approximately the same, suggesting that the faulting mechanisms are the same for events in each sequence. By assuming an inhomogeneous fault plane on which asperities fail by static fatigue, we derived an equation for accelerating premonitory slip as a function of timme, which agrees with the observed time dependence of foreshocks.-Authors
Shear cracks with finite cohesive forces can propagate by skipping past barriers. The barriers left behind may remain unbroken or may eventually break because of subsequent increase in dynamic stress depending on the ratio of barrier strength to tectonic stress. This model can explain a variety of observations on rupture in the earth, including (1) segmentation of the fault or ruptured zone in earthquakes and rock bursts, (2) ripples in seismograms which cannot be explained by path effect, and (3) departure of the scaling law of the seismic spectrum from that based upon the similarity assumption. The model also explains why the simple uniform dislocation model sometimes works better than the crack model without barriers. It also predicts, contrary to common belief, that an earthquake with low average stress drop may generate relatively greater amounts of high-frequency waves than an earthquake with high average stress drop. One important consequence of of our barrier model is the possibility of predicting the occurence of aftershocks by analyzing the source spectrum of the main shock.
The patterns of seismic activity before large strike slip and thrust-type earthquakes were examined for intervals as great as 45 years before the main shocks. The earthquakes chosen for this investigation occurred along the northwestern, northern, and eastern margins of the Pacific, and most of these events had rupture zones that extended hundreds of kilometers. The observed distribution of prior seismicity for these earthquakes correlates with the configurations of their rupture zones and with the epicentral locations of the main shocks within the zones. An obvious and consistent feature of these spatial distributions is the relatively aseismic character of extensive portions of the rupture zones until the times of the main shocks. These reduced levels of seismic activity extended to events at least several magnitudes smaller than the main shocks and possibly many magnitudes smaller. For most of the large earthquakes examined, rupture initiated in an area of moderate seismic activity and then propagated as much as hundreds of kilometers into adjacent quiet regions. In at least several instances, rupture also terminated in a region of some prior activity. Thus prior seismicity frequently occurred near the epicenters of the main shocks and/or near the edges of the rupture zone. There are some indications that the level of this prior activity, particularly in the vicinity of the epicenter, increased as the time of the main shocks approached. Thus one important conclusion of this study is that gaps in seismicity for great earthquakes along major plate boundaries may also be gaps for smaller-magnitude acticity. Further, such gaps may commonly remain so until the time of the principal shock. That is, if premonitory activity is associated with large earthquakes, such activity should be sought along the boundaries of a seismic gap rather than in the gap itself. Along the great transform fault system near western Canada and southeastern Alaska the nature of the seismicity varies noticeably with distance northwest of the Juan de Fuca spreading center. That is, the area near the spreading center is a region of frequent moderate-size earthquakes (mM=6); further north is a region of occasional large earthquakes (M=7); even further north is a region of infrequent great earthquakes (M=8). This changing seismic regime is similar to that of the San Andreas system proceeding north from the spreading centers in the Gulf of California. Along both transform fault systems, and possibly others, the change in characteristic seismicity may be a function of the changing lithospheric structure with distance from the spreading centers.
A digital computer program has been written to locate seismic events and to determine the standard errors and joint confidence regions for the focal coordinates. Modified Jeffreys-Bullen travel times are used, with surface focus travel times based on Nevada explosions. Results for eleven nuclear explosions in Nevada show a mean computed depth of focus of 39 ± 2 km. For these events, the displacement of the computed epicenter from the known epicenter is less than 20 km for all but three explosions when the Jeffreys-Bullen travel times are used. The epicenter displacement is less than 15 km for all but the same three explosions when surface focus travel times are used. Linearized joint confidence regions for latitude and longitude were computed for all events; for all the Nevada explosions the semimajor axis of the 75% confidence region was less than 14 km. Exact confidence regions can be constructed for the focal coordinates. In a small neighborhood of the computed epicenter (10 to 20 km being sufficiently small), the linearized confidence region has the same statistical behavior as the exact confidence region. The axis ratio and orientation of the confidence region for latitude and longitude depends only on the distribution of the stations in distance and azimuth around the epicenter; the size of the confidence region depends also on the station residuals and the confidence level required. A quality factor for station distribution, varying between zero and unity, can be used to measure the capability of a given station network with respect to a given epicenter. This quality factor is related to the generalized variance of the estimates of focal coordinates. The statistical model used for earthquake location can be modified in several ways to take into account station time corrections, regional time corrections, travel time bias, etc., without basically altering the procedure for computing the confidence regions.
Short-period synthetic seismograms are computed to determine the relative amplitudes and arrival times of P, pP, pwP (water surface reflection), and sP phases. Except along nodal planes of upgoing p, pwP is of greater amplitude than sP. For central Aleutian earthquakes, pwP dominates over sP and pP at North American stations for thrust mechanisms and modelled crustal structures. The pwP phase is clearly identified in three Aleutian events and can be used to constrain focal depths. Complex fracturing processes are identified in two of the events. In simple events, smaller phases which are consistent from station to station can be identified as sub-surface reflections and used for modelling the structure of the forearc.
This study attempts to forecast likely locations for large shallow South
American earthquakes in the near future by examining the past space-time
pattern of occurrence of large (M ≥ 7.7) earthquakes, the lateral
extent of their rupture zones, and, where possible, the direction of
rupture propagation. Rupture zones of large shallow earthquakes
generally abut and do not overlap. Patterns of rupture propagation
appear to follow certain trends. These facts, plus the nonrandom
behavior of the space-time history of seismic activity, present
consistencies that may permit prediction, in a gross sense, of future
events. By mapping the rupture zones of large earthquakes (in contrast
with plotting only epicenters), it is possible to identify segments of
the shallow seismic zone that have not ruptured in many decades. Limited
experience elsewhere indicates that these gaps between rupture zones
tend to be filled by large-magnitude earthquakes. In certain places it
is possible to make approximate estimates of the time of occurrence of
the next large earthquake. For at least 300 or 400 years, the entire
fault segment near the Central Valley province of central and southern
Chile (about 32°-46°S) has fractured about once each century
from a generally N-S progression of several large (M ≥ 8)
earthquakes. Large earthquakes in this region have almost always
occurred to the south of a previous large earthquake. In addition, it is
possible to infer a direction of rupturing for two large earthquakes in
this century (1928 and 1960). Both these earthquakes fractured southward
away from the rupture zone of an earlier earthquake. It would be
consistent with these observations if a new series started about the end
of this century near Valparaiso (33°S) and progressed southward. In
other sections of South America there are several extensive segments of
the active seismic belt that have not ruptured during this century.
Northern Chile and southernmost Peru (about 17°-25°S) have been
relatively aseismic for about 100 years. South of Lima (about
12.5°-14°S), between the rupture zones of the 1940 and 1942
Peruvian earthquakes, there is another significant gap in recent
activity. Both these regions are probably areas of relatively high
earthquake risk. The northern Peru and southern Ecuador region (about
9°-1°S) has also been relatively aseismic during this century.
However, this region differs from the two previously mentioned gaps in
that this coastal zone was a region of moderate seismicity during
historic times. Perhaps aseismic creep is an unusually important factor
in relieving tectonic strain along this particular segment of the
shallow seismic zone. Another possibility is that large shallow
earthquakes in this region have an extremely long recurrence time. Much
of the shallow seismic zone of northern Ecuador and southwestern
Colombia has ruptured twice during this century. During large
earthquakes in this region, the rupturing tends to be directed toward
the north or NE. The data for this region suggest that the area to the
NE of the 1958 Colombian earthquake may be a region of relatively high
earthquake risk.
A tsunami eqrthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep-sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio-temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long-period surface waves and the long-period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep-sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep-sea terrace and a depressional feature of the trench. This consistency implies a causal relationship betwen great earthquake activities and geomorphological features near the trench.
The time—space-magnitude interaction of shallow earthquakes has been investigated for three catalogues: worldwide (M≥ 7.0), Southern and Northern California (M≥ 4.0) and Central California (M≥ 1.5). The earthquake sequences are considered as a multi-dimensional stochastic point process; the estimates of the parameters for a branching model of the seismic process are obtained by a maximum-likelihood procedure. After applying magnitude—time and magnitude—distance scaling, the pattern of relationship among earthquakes of different magnitude ranges is almost identical. The number of foreshocks diminishes as the magnitude difference between the main shock and the foreshocks increases, while the magnitude distribution of aftershocks has the opposite property. The strongest aftershocks are likely to occur at the beginning of the sequence; later they migrate away with velocities of the order of km/day. The sequences which are composed of smaller aftershocks last longer and there are indications that they remain essentially in the focal region. Foreshocks also appear to migrate, but in this case, toward the main shock. The rate of occurrence of dependent shocks increases as t-1 as the origin time of the main shock is approached, effectively making every earthquake a multi-shock event. This interaction of earthquakes was modelled by a Monte-Carlo simulation technique. The statistical inversion of simulated catalogues was undertaken to derive the information we would be able to retrieve from actual data, as well as possible errors of estimates. The possibility of using these results as a tool for seismic risk prediction is discussed and evaluated.
Dynamical rupture process on the fault is investigated in a quasi-three-dimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress. The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress-fracture criterion, using the finite difference method.
If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage.
The stick-slip faulting and irregular rupture propagation radiate high-frequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults.
The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station adjustments computed by the method of joint hypocenter determination.
An area of significant seismic quiescence is found near Oaxaca, southern Mexico. The anomalous area may be the site of a future large earthquake as many cases so far reported were. This conjecture is justified by study of past seismicity changes in the Oaxaca region. An interval of reduced seismicity, followed by a renewal of activity, preceded both the recent large events of 1965 and 1968. Those past earthquakes have ruptured the eastern and western portions of the present seismicity gap, respectively, so that the central part remaining is considered to be of the highest risk of the pending earthquake.
The most probable estimates are: 7 1/2±1/4 for the magnitude and ϕ=16.5°±0.5°N, λ=96.5°±0.5W for the epicenter location. A firm prediction of the occurrence time is not attempted. However, a resumption of seismic activity in the Oaxaca region may precede a main shock.
A method for rapid retrieval of earthquake-source parameters from long-period surface waves is developed. With this method, the fault geometry and seismic moment can be determined immediately after the surface wave records have been retrieved. Hence, it may be utilized for warning of tsunamis in real time. The surface wave spectra are inverted to produce either a seismic moment tensor (linear) or a fault model (nonlinear). The method has been tested by using the IDA (International Deployment of Accelerographs) records. With these records the method works well for the events larger than M_s = 6, and is useful for investigating the nature of slow earthquakes.
For events deeper than 30 km, all of the five moment tensor elements can be determined. For very shallow events (d ⩽ 30 km) the inversion becomes ill-conditioned and two of the five source moment tensor elements become unresolvable. This difficulty is circumvented by a two-step inversion. In the first step, the unresolvable elements are constrained to be zero to yield a first approximation. In the second step, additional geological and geophysical data are incorporated to improve the first approximation. The effect of the source finiteness is also included.
Thesis (Ph. D. in Geophysics)--University of California, Berkeley, Sept. 1971. Includes bibliographical references (leaves 117-124). Microfilm. s
一つの地震の発生は,多かれ少なかれ,地殻応力の解放と地殻の構造状態の変化(破壊)をもたらし,その後の地震活動に何らかの影響を与えるものであり,とくに大きい地震ではその影響が著しいはずである.従って,地震活動の本質は,単に定常的なものとしてではなく,時間的.空間的に変化しつづけるものとして,動的な取扱いによって明らかにされると思われる.このような観点から,前回の報告にひきつづいて,最近の日本及びその周辺の地震活動にみられる時間的・空間的規則性を論じた.その結果を次に要約する.
The rupture process of the 1979 Colombia earthquake
- S Beck
- L Ruff
Beck, S, and L. Ruff (1983). The rupture process of the 1979 Colombia earthquake, Trans. Am. Geophys.
Union 64, p. 264.
Historical survey of U.S. seismograph stations
- B B Poppe
Poppe, B. B. (1979). Historical survey of U.S. seismograph stations, U.S. Geol. Surv. Profess. Paper I096,
389 pp.
Long-term precursory seismicity fluctuations, in Methodology[or Identifying Seismic Gaps and Soon-to-Break Gaps, Conference VI
- M Wyss
- R E Habermann
- A C Johnston
Wyss, M., R. E. Habermann, and A. C. Johnston (1978). Long-term precursory seismicity fluctuations,
in Methodology[or Identifying Seismic Gaps and Soon-to-Break Gaps, Conference VI, Natl. Eq. Haz.
Red. Prog., 869-894.