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Tectonic map of Ecuador. The Nazca plate converges obliquely with respect to the stable South

Tectonic map of Ecuador. The Nazca plate converges obliquely with respect to the stable South

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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 seism...

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... In contrast, as indicated by seismic reflection data, neither the Mw 8. 6-8.8, 1906, and Mw 7.8, 1942megathrust earthquakes nor the Mw ∼ 7, 1896, 1956 thrust earthquakes that occurred astride the creeping barrier (Font et al., 2013;Vaca et al., 2019) were able to reactivate the BJ and Cabuyal fault systems to transfer detectable slip to the seafloor. Rather, post-0.79 ...
... Although multiple mechanisms occurring either deep or shallow along the plate interface could contribute to the permanent deformation as discussed by Melnick (2016) about the central Andean coast, transient aseismic slips and moderate size earthquakes might be the main drivers of the post 0.79 Ma permanent deformation in the study aera. The inter-seismic seismicity (Vaca et al., 2019), the SSEs (Rolandone et al., 2018), the 2016-after-slip and after-shocks sequence Rolandone et al., 2018;Soto-Cordero et al., 2020) contribute to multiple faults activation and potentially to the fold growth. The 2016 after-shocks include earthquakes thought to rupture the same seismic asperity repeatedly. ...
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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.
... However, the authors mention a crustal event in 1587 probably related to the QFS with an estimated magnitude between 6.3 and 6.5. In general, the QFS produces moderate-size earthquakes with predominantly reverse focal mechanisms (Alvarado et al. 2014;Vaca et al. 2019), and since the development of the Ecuadorian seismological network in the early 1990s, seven earthquakes of magnitude larger than 4.0 have been recorded and located in the Quito area (Alvarado et al. 2018). Two of them had magnitudes greater than 5.0, one in 1990 (Mw 5.3 in fig. ...
... Nevertheless, the authors mention a crustal event in 1587 probably related to the QFS with an estimated magnitude between 6.3 and 6.5. In general, the QFS produces moderate-size earthquakes with predominantly reverse focal mechanisms (Alvarado et al. 2014;Vaca et al. 2019), and since the development of the Ecuadorian seismological network in the early 1990s, seven earthquakes of magnitude larger than 4.0 have been recorded and located in the Quito area (Alvarado et al. 2018). Two of them had magnitudes greater than 5.0, one in 1990 (M w 5.3) and the other in 2014 (M w 5.1, Fig. 1b), both associated with the QFS (Beauval et al. 2014). ...
Thesis
The city of Quito (Ecuador's capital) is located in an Andean valley at 2800 meters above sea level. Surrounded by volcanoes, this city of approximately 2 million inhabitants is prone to major earthquakes, and it is particularly vulnerable since no seismic code is formally used for constructions. The study of the hazard and the seismic risk is, therefore, essential. Three types of earthquakes threaten the city: a) a close earthquake of moderate magnitude (M ~ 6.5), which would occur on the Quito fault system, b) a more distant earthquake which could have a higher magnitude (M ~ 7.5) coming from the cordillera, and c) finally a subduction earthquake coming from the coastline more than 170 km away, the magnitude of which could be very high (M> 8.5). This third type of earthquake struck Ecuador on April 16, 2016 (Pedernales earthquake, Mw 7.8). Pedernales earthquake caused very significant damage to the coast region and several hundred victims. It also made the city of Quito tremble but caused no damage. What about a stronger earthquake? Could the seismic wave amplifying effect due to the Quito sedimentary basin, as was the case in 1985 in the Mexico City basin, generate very strong ground motion values causing significant damage? Could smaller magnitude but closer earthquakes threaten the city more seriously? These scientific questions are at the heart of this thesis subject's concerns, which is included in a major study project of the Ecuador-Chile zone carried out jointly by the Institute of Geophysics of Quito and French researchers from GeoAzur, ISTerre, CEREMA, and IFSTTAR laboratories. Objectives of the thesis: Understand, characterize, and simulate ground movements in Quito's city, taking into account the effects of basin resonance (i.e., the geometry of the basin's bedrock, alluvial filling) well as those due to the strong surrounding topography. Carry out simulations of likely future earthquakes. Available data (not yet used): Background noise recordings by 20 broadband stations in and around the basin for six months (the measurement campaign will be carried out from July to December 2017). Small earthquakes recordings on the Quito accelerometric network (RENAQ) since 2010. Recordings of the Pedernales earthquake and its main aftershocks on the RENAQ network Methodology : Seismic noise cross-correlation to estimate inter-station Green's functions, relying on existing geological and geotechnical data. Other inversion techniques (e.g., receiver functions, the study of converted waves on the sediment/bedrock interface) considered depending on the quality of the estimated Green's functions. Low-frequency stimulation using Green's functions obtained by cross-correlation of noise and high-frequency simulations using empirical Green's functions (i.e., recordings of small earthquakes).
... The known historical earthquakes that have damaged Quito were located on the faults of the Cordillera, such as the Guaylabamba 1587, Riobamba 1797, Quito 1859 and Ibarra 1868 earthquakes (Del Pino & Yepes 1990;Beauval et al. 2013;Alvarado et al. 2014). The city is built on the hanging wall of an active reverse fault that generates moderate size earthquakes (Alvarado et al. 2014;Vaca et al. 2019) and is partially creeping (Marinière et al. 2020). The seismic hazard related to the activity of this fault should not be neglected, but it is not the topic of this study. ...
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In 1906, an earthquake with a magnitude estimated between Mw 8.4 and 8.8 occurred in the subduction zone along the coast of Ecuador and Colombia. This earthquake caused extensive damage on the coast but had a rather small impact on the capital city of Quito, situated 180 km away. At that time, the city of Quito extended over a small area with a few thousand inhabitants, while today it stretches over 40 km and has a population of over 3 million, with most of the city built without paraseismic regulations. The aim of this study is to obtain new insights on the impact that large earthquakes from the subduction zone would have on the city today. This question is crucial since we know that the city of Quito is prone to site effects and that the southern part of the city amplifies seismic waves at low frequencies, around 0.3-0.4 Hz (Laurendeau et al., 2017). In April 2016, an Mw 7.8 earthquake occurred on the subduction interface in the Pedernales area. This event was the first large earthquake in the city of Quito to be well recorded by 13 stations of the permanent accelerometric network (RENAC). In this study, we take advantage of this dataset (mainshock and large aftershock recordings) to (1) test an empirical Green's function blind simulation approach where the input stress drop is taken from a global catalog of source time functions, (2) compare the synthetic accelerograms and ground motion values we obtain for an Mw 7.8 earthquake with the actual recordings of the Pedernales earthquake, and then (3) simulate larger earthquakes of Mw 8.2 and Mw 8.5 from the subduction zone. For Mw 7.8 simulations, our approach allows a good reproduction of the ground motions in the whole frequency bands and properly takes into account site effects. For Mw 8.2 and Mw 8.5 simulations, we obtain for the stations in the southern part of the basin, larger values at low frequencies than the predicted motion given by Ground Motion Models (GMMs). These values, although high, should be supported by new or recent buildings if they are constructed respecting the building code that applies in Quito. Therefore, for this type of strong but distant earthquake, the seismic standards appear to be well suited and it is imperative to ensure that they are well considered in the design of the new buildings to be constructed, especially in the southern part of the expanding city.
... We use focal mechanism solutions from global (Ekström et al. 2012, Trabant et al. 2012 and local (SGC 2020a, Vaca et al. 2019) catalogs for the NAS and neighboring regions. As these catalogs have solutions of different quality, we exclude solutions with Mw < 5.0 and at depths greater than 30 km for crustal faults. ...
... However, few large crustal earthquakes as the 1868 Mw ∼7.2 Ibarra earthquake, the 1766 Mw 6.5 Cali earthquake, and the 1885 Mw ∼ 6.4El Tambo earthquake have happened well inside the NAS(Beauval et al. 2010, Dimaté et al. 2005, SGC 2021). Focal mechanisms(Figure 4.1) also attest for regular Mw>5 occurring on active faults within the NAS(Ekström et al. 2012, Vaca et al. 2019). How much of the deformation is taken up by these faults, knowing whether they are significantly less active than the one delimiting the eastern boundary of the NAS remains largely unknown.Understanding and modeling the current motion and internal deformation of the NAS requires to define the kinematics conditions acting along its edges. ...
... It also includes more sites within the Amazon basin allowing us to test how far deformation spreads inside the South America plate east of the Andes. Many Slow Slip Events (SSE) have been documented all along the Ecuadorian coast and northern Peru, Segovia et al. 2015, Vaca et al. 2019, Vallée et al. 2013, Villegas-Lanza et al. 2016a), with recurrence time of a few (2-3) years only at certain locations. Compared with the Nocquet et al. (2014) solution, (eventually used in Chlieh et al. (2014), Villegas-Lanza et al. (2016b) and Mora-Páez et al. (2019)), our updated solution is more robust to potential biases induced by undetected SSE because: (1) several survey sites from Nocquet et al. (2014) have been progressively equipped with cGPS from 2011 onwards, allowing better detection of SSEs, (2) the survey sites used in our study have additional epochs of measurements. ...
Thesis
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 […]
... Nevertheless, the authors mention a crustal event in 1587 probably related to the QFS with an estimated magnitude between 6.3 and 6.5. In general, the QFS produces moderate-size earthquakes with predominantly reverse focal mechanisms Vaca et al. 2019), and since the development of the Ecuadorian seismological network in the early 1990s, seven earthquakes of magnitude larger than 4.0 have been recorded and located in the Quito area (Alvarado et al. 2018). Two of them had magnitudes greater than 5.0, one in 1990 (M w 5.3) and the other in 2014 (M w 5.1, Fig. 1b), both associated with the QFS (Beauval et al. 2014). ...
Article
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Quito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin’s seismic response and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito’s urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve inter-stations surface-wave Green’s functions in the frequency range of 0.1 - 2 Hz. We find that Love waves traveling in the basin’s longitudinal direction (NNE-SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them in conjunction with the HVSR curves to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin’s structure between its northern and southern parts. In the center and northern areas, the estimated basin depth and mean shear-wave velocity are about 200 m and 1800 ms−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement’s depth and shear-wave velocity in the southern part are about 900 m and 2500 ms−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the center-north and the south of the basin explains the spatial distribution of low-frequency seismic amplifications observed during the Mw 7.8 Pedernales earthquake in April 2016 in Quito.
... Whereas the Huyarapungo fault section located further south in continuation of the BFS highlights morphological and microtectonic evidence of active strike-slip faulting (Egüez et al., 2003), the morphological signature of the BFS led Ego et al. (1996a) to propose a normal kinematics for this fault system and, together with an abnormal throw-to-length ratio, an origin due to local processes different from far-field tectonics. Such a non-tectonic origin of the BFS have the advantage to clarify the occurrence of apparent normal faulting within the actual stress regime affecting the wider area of the El Angel seismoctectonic zone (Yepes et al., 2016), highlighting right-lateral strike-slip motion and E-W shortening from earthquake focal mechanisms (Vaca et al., 2019). This stress regime is also coherent with active fault systems mapped within the IAV, showing both reverse and right lateral strike slip evidences (e.g. ...
... Purely tectonic processes. As pointed out in section 3, the actual stress regime expected in the area is related to E-W shortening (Vaca et al., 2019), leading to reverse and right lateral strike slip motions along regional faults (Alvarado, 2012). The wider Billecocha area is unfortunately not covered by GPS measurements, but this expected E-W shortening is coherent with what is observed further south in the Quito region (Alvarado et al., 2014;Marinière et al., 2020). ...
... In this context, a purely normal stress regime is hardly compatible with regional seismotectonic data (García-Villarruel, 2018;Vaca et al., 2019) and may also not explain the occurrence of reverse faulting in the area. On the contrary, a purely reverse stress regime, more compatible with seismotectonic data, hardly explain the occurrence of primary vertical ruptures along straight and near vertical fault segments. ...
Article
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The Billecocha plateau (4000 m a.s.l.) lies in the high elevation Ecuadorian Andes volcanic arc. It overhangs by 2000 m above the interandean valley. Both the plateau and surrounding volcanoes are heavily affected by active faulting characterized by straight, sharp and discontinuous scarps within a 6 km wide and 24 km long corridor. Contrasting interpretations have been proposed to explain the expression at surface of the so-called Billecocha fault system (BFS), from normal faulting related to postglacial elastic rebound or gravitational processes, to right-lateral faulting compatible with the North-Andean Sliver tectonic regime. The instrumental seismicity recorded around the BFS is low, however, a M ≈ 7 earthquake heavily struck the region in 1868. With the aim to discuss the kinematic and coseismic nature of the encountered deformations as well as the seismogenic character of the BFS, we performed (1) morphological analysis to map and quantify evidence of active faulting and (2) paleoseismological investigations across the longer segment of the fault system. In three trenches, we show that surface deformations are at least partly coseismic in origin during the Holocene with a minor lateral component, the last paleoseismic event being compatible in date with the 1868 earthquake. In addition, some of the enlightened paleoseismic events could have occurred in relationship with volcanic eruptions of the surrounding volcanoes. While field evidence of reverse and strike slip faulting suggests that regional tectonics could be involved, the geomorphological signature of the BFS at the mountain scale, as seen on the digital surface model, can partly be related to the development of deep seated gravitational deformations, hence suggesting an interaction between boundary (i.e. tectonic, volcanic) and body forces (i.e. gravity, post-glacial rebound). Further studies are however mandatory to better address the influence of each process at the BFS, in particular geodetic and seismological surveys. Given the available data, we suggest that the BFS could actually correspond to the distributed surface expression of the tectonic reactivation of the inherited Pujilí suture, enhanced by gravitational phenomenon. In this light, paleoearthquakes identified along the BFS may help evidencing the recurrence of major events in the region. However, it also imply that surface deformations along the BFS shall not be used without a careful and more detailed field work to derive fault slip rates for seismic hazard calculations.
... Seismic data recorded by permanent instruments deployed by the Instituto Geofísico at the Escuela Politécnica Nacional (IG-EPN) along with other preexisting stations in Ecuador offer an opportunity to better resolve the crustal thickness, volcanism, and tectonics in this region. Previous studies in the region have estimated crustal thickness using gravity (Feininger and Seguin, 1983;Tamay et al., 2018) or local seismicity (Vaca et al., 2019). Additionally, seismic imaging has shown broad variations in the seismic velocities of the crust (Araujo 2016;Lynner et al., 2020). ...
... Previous crustal thickness estimates in Ecuador have come primarily from gravity observations (Feininger and Seguin, 1983;Tamay et al., 2018), the joint determination of earthquake locations and crustal structure (Vaca et al., 2019), and local tomography (Araujo, 2016). Condori et al. (2017) and Poveda et al. (2015) used receiver functions and H-k stacking in Northern Peru and Colombia respectively. ...
... The P m s conversion throughout our study region has a large amplitude, is continuous in nature, and is consistent with previous studies (Feininger and Seguin, 1983;Poveda et al., 2015;Araujo, 2016;Condori et al., 2017;Tamay et al., 2018;and Vaca et al., 2019). We, therefore, interpret this P m s conversion as the crust-mantle boundary, where crustal rocks transition into the peridotites that comprise the lithospheric mantle (Moho; Fig. 3). ...
Article
The Northern Andes of Ecuador contain some of the most active volcanic systems in the Andes and extend over a broad region from the Western Cordillera to the Subandean Zone. While it is known that the arc straddles a range of basement compositions, from accreted mafic oceanic terranes in the west to silicic continental terranes in the east, the details of the crustal structure beneath the arc is unclear despite being critical for understanding magmatic and tectonic processes in this portion of the Andes. To gain insight into these processes, we create two 3D models of crustal and upper mantle seismic properties throughout the region. The first highlights the discontinuity structure using receiver functions, which allows for the recovery of crustal thickness beneath the Ecuadorian Andes. We observe a range from ∼50 to 65 km under the high elevations, with thicker crust beneath the lower elevation Western Cordillera compared to the higher elevation Eastern Cordillera. This can largely be explained by density variations within the crust that are consistent with observed terranes at the surface, implying these terranes extend to depth. The second model combines our receiver functions with Rayleigh wave dispersion data from ambient noise measurements in a joint inversion to construct a 3-D shear wave velocity model. This model shows several mid-crustal (5–20 km below sea-level) low velocity zones beneath Ecuadorian arc volcanoes that contain a maximum of ∼14% melt. These low velocity zones likely represent zones of long-term magma storage in predominantly crystalline reservoirs, consistent with “mush zones”. Furthermore, the depth of the inferred reservoirs below several of the volcanic centers (e.g., Chiles-Cerro Negro and Tungurahua) are in broad agreement with previous geobarometry and geodetic modeling. Our results provide new observations of possible long-term magma reservoirs below other less-studied volcanic systems in the Ecuadorian arc as well, and further contributes to a mounting number of observations indicating long-term magma storage at low melt percentages in the mid-crust beneath active arc systems.
... On the upper left corner, arrows indicate the subduction direction of the Nazca and Cocos tectonic plates, overlapped on 50-m resolution DEM map. C: Average dip angle of ~20 • for Ecuadorian front arc (Yepes et al., 2016), and crustal thickness data (~50 km) is the average value proposed for the Ecuador segment (Vaca et al., 2019). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ...
Article
Trace volatile elements like He are key for understanding the mantle source signature of magmas and to better constrain the relative roles of subduction and crustal processes to the variability of along-arc chemical and isotopic signatures of magmatic fluids. Here we report on noble gas abundances and isotopic data of Fluid Inclusions (FIs) in eruptive products and/or fumarolic gases from the Colombia-Ecuador segment of Andean Northern Volcanic Zone (NVZ). FIs in olivine phenocrysts from Ecuador (El Reventador, Cotopaxi and Tungurahua) yield air-normalized corrected ³He/⁴He ratios of 7.0–7.4 RA, within the MORB range (8 ± 1 RA). With exception of the Cotopaxi lavas (opx < <oliv.), these are indistinguishable of those obtained for their cogenetic orthopyroxene pairs and of gas emissions previously reported in literature. Olivine phenocrysts from Nevado del Ruiz fissure lavas also yield the highest ³He/⁴He (8.5 ± 0.3 RA) for this volcanic system, which is in the range of fumarolic gases for Galeras (previously reported as high as 8.8 RA and here measured to a maximum of 8.3 ± 0.1 RA). Our dataset highlights disparities between isotope signatures of eruptive products from Ecuador (avg. ~7.2 RA) and those reported for the Colombian portion of the NVZ (avg. ~8.5 RA). Previous studies on the geochemistry of erupted products put in evidence significant along-arc variations ascribed either to the involvement of different slab components, or to variable depths of evolution of arc magmas within the continental crust. However, the same variation is not discernible in the signature of noble gases, especially helium, from FIs and gas emissions analyzed in this study, with little inter-variation between Cotopaxi, Reventador and Tungurahua (all within 0.2 RA from the Ecuador average of 7.2) and Galeras and Nevado del Ruiz, whose maximum values differ by ~0.3 RA. We therefore suggest a homogenous MORB-like ³He/⁴He signature for the mantle wedge beneath this arc segment, whereby along-arc variations in crustal thickness (from <35 km at the northernmost part of the segment to ≥50 km at the Ecuadorian arc segment) may factor largely into the variability recorded on our data set. The first CO2/³He ratios obtained in FIs from Andean rocks support the hypothesis of increasing crustal contamination from Colombia to Ecuador, concomitant with increasing crustal thicknesses under the respective arc regions.
... https://earthquake.usgs.gov/learn/today/index.php?month=8day=5) (Figure 3.1a). 2016)), large historical earthquakes (Beauval et al. (2010(Beauval et al. ( , 2013), seismologically recorded moderate size earthquakes (Vaca et al. (2019)) and geomorphic markers of neotectonics faulting ) witness active deformation distributed within the North Andean Sliver. In central Ecuador, Alvarado et al. (2016) proposed the existence of an additional Latacunga-Quito block encompassing the Inter-Andean valley and the eastern cordillera from latitude 1.5 • S to the strike-slip Guayllabamba fault (lat. ...
... We select solutions from several published catalogs: the ISC-GEM v7.0, the global CMT catalog (1976( -2017( , Dziewonski et al. (1981; Ekström et al. (2012)) and the ISC event reviewed catalog (1906( -2017( , ?Storchak et al. (2017). Besides we have included the solutions from Vaca et al. (2019), moment magnitudes estimated for earthquakes between 2009 and 2015, in the spatial window from -6 • to 2 • in latitude and from -83 • to -76 • in longitude. ...
... Magnitudes ISC-GEM are always associated with the locations from the same catalog. Magnitudes from Vaca et al. (2019) are also associated with the locations from the same author. The ISC locations are used for the other events. ...
Thesis
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Probabilistic Seismic Hazard Assessment (PSHA) relies on long-term earthquake forecasts, and ground-motion models. Up to now, geodetic data has been rather under-used in PSHA, although it provides unique and unprecedented information on the deformation rates of tectonic structures from local to regional scales. The aim of this PhD thesis is to improve earthquake recurrence models by quantitatively including the information derived from geodetic measurements, with an application to Ecuador, a country exposed both to shallow crustal earthquakes and megathrust subduction events. The second chapter presents the building of a probabilistic seismic hazard model for Ecuador, using historical and contemporary seismicity, recent knowledges about active tectonics, geodynamics, and geodesy. I contributed to this collective effort in two ways: 1) the building of earthquake catalogs from global seismic datasets; 2) the establishment of average slip rates on a set of simplified crustal faults, from GPS velocities. The hazard calculations led at the country scale indicate that uncertainties are largest for sites on the northern coast and along the faults in the Cordillera. The second chapter of this PhD focuses on the determination of the seismic potential of the Quito fault system. Quito city lies on the hanging wall of this ∼60-km-long reverse active fault, representing significant risks due to the high population density. We constrain the present-day strain accumulation associated with the fault system with GPS data and Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) analysis. 3-D spatially variable locking models show that a large part of the fault is presently experiencing shallow creep, hence reducing the energy available for future earthquakes, which has a significant impact for hazard calculation. In the third part of this PhD, we evaluate the ability of geodetic data to constrain earthquake recurrence models for the subduction zone in northern Ecuador. We quantify the annual rate of moment deficit accumulation at the interface using interseismic coupling models, and identify the uncertainties related to the conversion in terms of total seismic moment release. Based on a newly-developed earthquake catalog, we propose to establish recurrence models that match both the catalog-based seismicity rates and the geodetic moment budget. We set up a logic tree for exploring the uncertainties on the seismic rates and on the geodetic moment budget to be released in earthquakes. The exploration of the logic tree leads to a distribution of possible maximal magnitudes Mmax bounding the earthquake recurrence model; we extract only those models that lead to Mmax values compatible with the extent of the interface segment according to earthquake scaling laws. This new method allows 1) to identify which magnitude-frequency form is adapted for the Ecuadorian subduction; 2) to generate a distribution of moment-balanced recurrence models representative of uncertainties and propagate this uncertainty up to the uniform hazard spectra; and 3) to evaluate a range for the aseismic component of the slip on the interface. Considering the recent availability of massive quantity of geodetic data, this new approach could be used in other regions of the world to develop recurrence models consistent both with past seismicity and measured tectonic deformation.
... Imaging the crust in Ecuador has been challenging for years. A new model has been recently proposed for the whole country Araujo, 2016;Vaca et al., 2019), and allowed relocation of earthquake events, including at depth. Based on an inversion of arrival times of more than 45,000 seismic events spanning more than 20 years of seismicity, Araujo (2016) proposes a new seismic tomography imaging the subducted slab and the overriding crust. ...
... Besides, very few events of M > 4 were recorded along the CCPP and, according to Araujo (2016), the Pisayambo cluster is actually the major source of seismic energy release, with its strongest event being the Mw∼5.0 (26/3/2010) which caused a surface rupture on a CCPP segment (Champenois et al., 2017). Vaca et al. (2019) provide new focal mechanisms of earthquakes within the 2009-2015 time span, based on seismic velocity structure and data from the Ecuadorian broadband seismic network. In the area comprised between Pallatanga (Pa) and the Pisayambo (Pi) area (Figure 1), focal mechanisms of low-moderate magnitude (Mw 3.3-5.4) ...
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Based on new geological data and the analysis of a 4 m spatial resolution Digital Elevation Model (DEM), we provide a detailed and comprehensive description of section of the Chingual Cosanga Pallatanga Puna Fault System, a major active fault system in Ecuador. This work allows estimating new slip rates and large earthquakes parameters (displacement, recurrence) along a ∼100 km-long section of the continental-scale dextral shear zone that accommodates the extrusion of the North Andean Sliver with respect to the South America continental Plate. We focus on the NE-SW Pallatanga strike-slip fault zone and related contractional and transcurrent features that extend to the north in the Inter-Andean valley and the Cordillera Real, respectively. The detailed analysis of the available DEM allowed mapping a series of lineaments at the regional scale and along the entire fault system. Field studies on key areas show valley deflections, aligned and elongated hills of Tertiary or Quaternary sediments, as well as faulted Holocene deposits and even preserved coseismic free-face ruptures in some places. Such morphological anomalies strongly suggest that those landscape scars represent long-living (Holocene to historical times) earthquake faults. Altogether, these new data confirm that very large crustal earthquakes (M∼7.5) have been generated along the fault system, probably during multiple segment ruptures. This conclusion agrees with reports of large earthquakes during historical times (post-1532 CE) in 1698, 1797, and 1949. They all occurred in the vicinity of the Pallatanga fault, causing catastrophic effects on environmental and cultural features. Based on new sample dating of both soils and volcanic series, we infer that the NE-SW dextral Pallatanga fault slips at rates ranging from ∼2 to 6 mm/yr for southern and central strands of the studied area, respectively. Further north, surface faulting is distributed and the deformation appears to be partitioned between sub-meridian fault-related folds (∼2 mm/yr) and NE-SW strike-slip fault(s), like the ∼1 mm/yr Pisayambo Fault that ruptured the surface in 2010. All this information offers the opportunity to size the earthquake sources for further seismic hazard analyses.