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Source characteristics of historic earthquakes along the central Chile subduction Askew et alzone
Abstract
We have analyzed four large to great historic earthquakes that occurred along the central Chile subduction zone from north to south on November 11, 1922 (M-s = 8.3), April, 1943 (M-s = 7.9), December 1, 1928 (M-s = 8.0) and January 25, 1939 (M-s = 7.8). Waveform modeling and P-wave first motions indicate that the 1922, 1928 and 1943 earthquakes are shallow and consistent with underthrusting of the Nazca Plate beneath the South American plate. In contrast, the 1939 earthquake is not an underthrusting event but rather a normal fault event within the down-going slab. The 1922 earthquake is by far the largest event with a complex source time function showing three pulses of moment release and a duration of 75 a. The 1943 earthquake has a simple source time function with one pulse of moment release and a duration of 24 s. This event had a local tsunami of 4 m and a far-field tsunami height in Japan of 10-30 cm. The 1928 earthquake also has a simple source time function with a duration of 28 s. The aftershocks and highest intensities are south of the epicenter indicating a southward rupture with most of the seismic moment release occurring 50-80 km south of the 1928 epicenter but still north of the adjacent 1939 earthquake region. The 1939 Chillan earthquake was not an underthrusting but rather a complex normal fault earthquake. Our preferred model is a normal fault mechanism at a depth of 80 to 100 km with two pulses of moment release and a total duration of approximately 60 s. The high intensities, lack of a tsunami, and inland location associated with the 1939 event are all consistent with an intraplate event within the down-going slab. The 1939 earthquake was clearly more destructive than the other similar size or larger events. This may in part be due to the intraplate nature of the event but also due to high amplification of the sites in the Central Valley of south central Chile.
... Coastal areas can also be submerged and flooded by tsunamis. The tsunami events that potentially affected the Coquimbo region occurred in 2015,1960,1922,1849, and 1751, after large fault line ruptures (Fig. 3) (Lomnitz 1970(Lomnitz , 2004Kellehier 1972;Nishenko 1985;Dunbar et al. 1992;DeMets et al. 1994;Beck et al. 1998;Campos et al. 2002;Saillard et al. 2009;Udías et al. 2012;May et al. 2013;Métois et al. 2013;Soto et al. 2014;Carvajal et al. 2017;Ruiz and Madariaga 2018;Kanamori et al. 2019;Salazar et al. 2022;Leon et al. 2023). Of these, the most destructive, according to historical records, occurred in 1922, affecting 400 km of the coastline from the Atacama to Coquimbo regions (27-30° S) (Lomnitz 1970;DeMets et al. 1994;Beck et al. 1998;Udías et al. 2012;Metois et al. 2013;Carvajal et al. 2017;Kanamori et al. 2019), causing extensive damage to the area. ...
... The tsunami events that potentially affected the Coquimbo region occurred in 2015,1960,1922,1849, and 1751, after large fault line ruptures (Fig. 3) (Lomnitz 1970(Lomnitz , 2004Kellehier 1972;Nishenko 1985;Dunbar et al. 1992;DeMets et al. 1994;Beck et al. 1998;Campos et al. 2002;Saillard et al. 2009;Udías et al. 2012;May et al. 2013;Métois et al. 2013;Soto et al. 2014;Carvajal et al. 2017;Ruiz and Madariaga 2018;Kanamori et al. 2019;Salazar et al. 2022;Leon et al. 2023). Of these, the most destructive, according to historical records, occurred in 1922, affecting 400 km of the coastline from the Atacama to Coquimbo regions (27-30° S) (Lomnitz 1970;DeMets et al. 1994;Beck et al. 1998;Udías et al. 2012;Metois et al. 2013;Carvajal et al. 2017;Kanamori et al. 2019), causing extensive damage to the area. More than 1000 deaths resulted from the earthquake, while the tsunami killed hundreds of people in coastal areas, mainly in Coquimbo. ...
... Location of the study area in north-central Chile. The white line represents the rupture lengths and extent of the main historical earthquakes between 28°S and 32°S, Chile (Lomnitz 1970(Lomnitz , 2004Kellehier 1972;Nishenko 1985;Dunbar et al. 1992;DeMetz 1994;Beck et al. 1998;Campos et al. 2002;Vigny et al. 2009;Saillard et al. 2009, Udias et al. 2012May et al. 2013;Métois et al. 2013;Carvajal et al. 2017;Ruiz and Madariaga 2018;DePaolis et al. 2021); tsunami historical data. Symbols (*) represent historical accounts of tsunami damage (Lomnitz 1970;NCEI/WDS 1993;Udias et al. 2012;Cisternas et al. 2012;DePaolis et al. 2021; NOAA, 23/04/2022; https:// www. ...
This study used sedimentary archives obtained from a high-standing terrace along the coastal zone of the Coquimbo region to reconstruct past extreme events (floods and tsunamis). Using sedimentological (grain size), geochemical (XRF), and geochronological (¹⁴C and ¹³⁷Cs) analyses, we identified nine flood deposits (1957, 1965, 1984, 1987, 1991, 1997, 2002, 2015, and 2017) and one tsunami deposit (1922). The sedimentary flood record appears complete for the last 70 years and is useful for reconstructing paleoflood events, highlighting the relevance of coastal high-standing terraces to improve our understanding of the frequency of floods in Chilean coastal areas.
... The map of the study region. (a) The map of regional morphotectonic elements of the Chilean Andes (according to [24,25]), (b) the regions of the epicenters of aftershocks of strong earthquakes on the west coast of the South American Plate (Chilean sector) and gravity anomalies after the Maule earthquake for 2015 (according to [14,30]). (1) The Chile Trench; (2) volcanoes; ...
... Activation of the aftershock process in the oceanic lithosphere is characteristic of strong M = 8.0 and σ xx τ xy σ xx τ xy M = 7.7 earthquakes on July 30, 1995 and July 14,2007. The aftershocks of the first phase are located in the region of the oceanic lithosphere in the form of a cloud that is not reflected in the subduction model of the submerged oceanic crust (Fig. 7a). ...
This article presents seismotectonic consequences of the strong Mauli earthquake in Chile, which occurred on the February 27, 2010, Mw = 8.8. The consequences are considered as a manifestation of a large-scale fragment of the general seismotectonic process on the western edge of the South American plate (Chilean sector). Our study shows that manifestations of postseismic processes of the Maule earthquake cover a much larger area compared to the epicentral zone of the aftershocks. Based on the comparison of the results of numerical modeling of the stress-strain state before and after the earthquake, seismological, geodetic, and satellite data, an alternative model of the development of the seismotectonic process in the Chilean sector of the South American plate was proposed. The stress-strain modeling was performed by the finite element method. The source of the Mauli earthquake, at a depth of 33 km, falls into the region of relatively high values of compression stresses and positive maximum shear stresses. It was shown, that other strong earthquakes of the Chilean sector in the interval of depths from 20 to 50 km are caused by high concentration of tectonic stresses in the region of transition from oceanic to continental lithosphere. Within the framework of the proposed model of the seismotectonic process, ruptures weaken the contact between the oceanic and continental lithosphere after strong earthquakes. Abrupt sinking of the continental lithosphere into the mantle causes an increase in viscous melt pressure, promotes penetration into mega-cracks, and rises to the surface, causing subsequent volcanic eruptions. It is shown that the results obtained in comparison with the coseismic consequences of earthquakes do not contradict these results of numerical modeling and give new insights into the structure of the lithosphere in the continent‒ocean transition zone and the development of the seismotectonic process.
... Only two megathrust earthquakes were experienced in North-Central Chile through the previous century: the 1906 Valparaiso earthquake (M w 8.4; 32-34.5°S; Beck et al., 1998) and the 1922 Copiap'o earthquake (M w 8.4; 26-30°S; Lomnitz, 2004). Since then, many additional significant earthquakes have happened: in 1943, a M w 7.9 occurrence cracked south of La Serena; in 1985, a M w 8.0 event cracked over in front of Valparaiso; and in 1946 and 1983, two M w 7.8 earthquakes cracked the subduction boundary from 26°S to 27.5°S. ...
... In the same region, several notable earthquakes have taken place, including a magnitude 7.6 event on 19 January 1958 and a magnitude 7.7 event on 12 December 1979, as documented by Pararas Carayannis in 1980, drawing from the US Geological Survey [USGS] and the Global Earthquake Model catalogues (please refer to Data and Resources). Within the central segment of this area, additional megathrust earthquakes associated with subduction have occurred: a magnitude 7.1 earthquake on 3 May 1896, a magnitude 7.4 event on 1 June 1907, a magnitude 7.9 event on 14 May 1942, a magnitude 7.4 event on 16 January 1956 and a magnitude 7.1 event on 4 August 1998, as reported by Beck et al. in 1998 andYepes et al. in 2016. Analyzing the relatively short seismic history over the past 120 years suggests an approximately 20-year recurrence period for events with magnitudes greater than 7.0 in the central sector. ...
The book presents earthquake source, wave propagation, site amplification, and other seismological studies including earthquake simulation, application of Artificial Neural Network (ANN) in seismology, earthquake early warning system, waveform inversion, moment tensor analysis, receiver function analysis, earthquake prediction, and earthquake early warning system applications. To minimize the losses due to an earthquake, it is better to understand the source properties, medium characteristics, site condition, and amplitude of a probable earthquake at a particular site. The evolutions of earthquake source models make it possible to understand the source dynamics. However, analysis of the source using a single-domain method does not provide a better understanding of the source dynamics. Therefore, this book combines methods from the earthquake spectrum to waveform inversion and joint inversion. The book also discusses earthquake prediction methods and their reliability around the globe, and techniques of simulation viz. stochastic, empirical, semi-empirical, and hybrid, along with their limitations and application. Seismology is an interdisciplinary subject. Therefore, the information presented in the book will appeal to a wider readership from students, teachers, researchers, planners engaged in developmental work, and people concerned with earthquake awareness.
... Although there is no instrumental record of great intraplate earthquakes within the downgoing slab in our study area, these types of earthquakes have been widely reported further north and have been accompanied by strong yet localized earthquake shaking. The most remarkable example is the M ~ 7.8 1939 Chillan intermediate-depth earthquake (Beck et al. 1998;Ruiz and Madariaga 2018) which caused ~ 20,000 deaths (1939 event in Fig. 1a, b). ...
... ). The brown line indicates the approximate location of intermediate-depth earthquakes such as the 1939 intermediate-depth earthquake rupture within the subducted slab(Beck et al. 1998) ...
Damming rivers by landslides and ensuing outburst flooding is a common and potentially hazardous phenomenon worldwide, especially in tectonically active regions. Remarkable examples are the damming of the upper course of the San Pedro River (SPR) in south Chile during the 1960 Chile earthquake (M9.5) and its predecessor in 1575. Outburst floods following both events had tragic consequences for downstream communities. Here, we study both events from multiple sources of information, including previously published and newly found historical records, satellite imagery, LiDAR topography, and sedimentological and geomorphological field observations. We present the first detailed geomorphic map of the region. Morphological similarities between ancient deposits at the SPR and those associated with the 1960 earthquake suggest that the SPR has been dammed repeatedly in the past. The steep incision of the SPR and the sediments of glacio-lacustrine origin in the surrounding slopes facilitate the initiation of large landslides. The knowledge gained from studying these past events provides important implications for future risk assessments. We propose that besides large earthquakes, smaller and more frequent earthquakes as well as changes in land use, can also result in river-damming events.
... The map of the study region. (a) The map of regional morphotectonic elements of the Chilean Andes (according to [24,25]), (b) the regions of the epicenters of aftershocks of strong earthquakes on the west coast of the South American Plate (Chilean sector) and gravity anomalies after the Maule earthquake for 2015 (according to [14,30]). (1) The Chile Trench; (2) volcanoes; ...
... Activation of the aftershock process in the oceanic lithosphere is characteristic of strong M = 8.0 and σ xx τ xy σ xx τ xy M = 7.7 earthquakes on July 30, 1995 and July 14,2007. The aftershocks of the first phase are located in the region of the oceanic lithosphere in the form of a cloud that is not reflected in the subduction model of the submerged oceanic crust (Fig. 7a). ...
This article presents the seismotectonic consequences of the strong Maule earthquake in Chile, which occurred on February 27, 2010 at Mw = 8.8. The consequences are considered as a manifestation of a large-scale fragment of the general seismotectonic process on the western edge of the South American plate (Chilean sector). Our study shows that manifestations of postseismic processes of the Maule earthquake cover a much larger area compared to the epicentral zone of the aftershocks. Based on the comparison of the results of numerical modeling of the stress–strain state before and after the earthquake, seismological, geodetic, and satellite data, an alternative model of the development of the seismotectonic process in the Chilean sector of the South American plate was proposed. Stress–strain modeling was performed by the finite-element method. The source of the Maule earthquake, at a depth of 33 km, falls into the region of relatively high values of compression stresses and positive maximum shear stresses. It was shown that other strong earthquakes of the Chilean sector in the interval of depths from 20 to 50 km are caused by high concentration of tectonic stresses in the region of transition from oceanic to continental lithosphere. Within the framework of the proposed model of the seismotectonic process ruptures weaken the contact between the oceanic and continental lithosphere after strong earthquakes. Abrupt sinking of the continental lithosphere into the mantle causes an increase in viscous melt pressure, promotes penetration into mega-cracks, and rises to the surface, causing subsequent volcanic eruptions. The results obtained in comparison with the coseismic consequences of earthquakes do not contradict these results of numerical modeling and give new insights into the structure of the lithosphere in the continent‒ocean transition zone and the development of the seismotectonic process.
... The hazard curve, which represents an estimate of the mean annual frequency of exceedance for each value of Sa(T*), was determined by a probabilistic seismic hazard analysis carried out for the central zone of Chile using the software R-CRISIS v20.0 [27]. It was taken into consideration that most of the large earthquakes in Chile originate from subduction between the Nazca plate and the South American plate [28] and present a tectonic setting defined by a series of earthquakes with magnitudes of Mw~8.0-8.5, where the Nazca plate subducts northeastward at 68-80 mm/year every approximately 80 years [29]. The source parameters were obtained from Martin [30], and the study used the Ground Motion Prediction Equations (GMPE) proposed by Montalva et al. [31] and Abrahamson et al. [32]. ...
... ysis carried out for the central zone of Chile using the software R-CRISIS v20.0 [27]. It was taken into consideration that most of the large earthquakes in Chile originate from subduction between the Nazca plate and the South American plate [28] and present a tectonic setting defined by a series of earthquakes with magnitudes of Mw ~8.0-8.5, where the Nazca plate subducts northeastward at 68-80 mm/year every approximately 80 years [29]. The source parameters were obtained from Martin [30], and the study used the Ground Motion Prediction Equations (GMPE) proposed by Montalva et al. [31] and Abrahamson et al. [32]. ...
Chile’s reinforced concrete (RC) design is based on ACI 318-08, where high-strength reinforcement is not allowed in seismic force-resistant members. In 2019, new requirements adopted by ACI 318 permitted the incorporation of high-strength reinforcement in walls. This study compared the seismic performance of two Chilean 20-story residential buildings on soft soil, one designed with traditional Grade 60 and the other with high-strength Grade 80 reinforcement. The performance was assessed in terms of the probability of exceeding the ASCE 41 limit states during a 50-year lifecycle. Analyses showed that both buildings had similar seismic performance. However, the reduction in reinforcement in the Grade 80 building was close to 18%. It is concluded that using high-strength reinforcement in a typical wall building implies a significant reduction in the reinforcement used without affecting the seismic performance.
... The epicenters of the subduction seismicity are located from 10 km east to the trench to 160 km eastward in the littoral zones and hypocenters between 10 and 55 km being deeper eastward [1,9]. Other Nazca intraplate earthquakes are due to slab pull with normal mechanism [10][11][12][13]. Epicenters of these earthquakes are located 160-270 km from the trench eastward and hypocenters between 100 and 200 km. ...
... Epicenters of these earthquakes are located 160-270 km from the trench eastward and hypocenters between 100 and 200 km. The Chillán Mw = 8 earthquake of January 25, 1939 [10] was the deadliest earthquake that occurred in Chile with more than 25,000 persons killed. Other significant slab pull earthquakes are the Punitaqui 1997 Mw = 7.1 [12] and Tarapaca 2005 Mw = 7.8 [14]. ...
On February 27th 2010 occurred the Mw = 8.8 Maule subduction earthquake, filling a seismic gap of south Chile. The uplift trend is mostly typical for subduction earthquakes with decreasing uplift trend from trench to arc in Andes Cordillera. However local perturbations occurred due to the reactivations of crustal faults occurred such as Pichilemu fault (normal), Santa María fault (normal) and Tirua-Mocha fault (reverse). Different kind of faults and seismic behavior evidence complex stress distribution at the overriding South American Plate. In this paper, the activity and seismicity linked of some crustal faults at Maule earthquake rupture área are considered, and the related seismic potential that can increase the seismic hazard. Some questions are the bigger magnitude that can generate these faults and if their activity is related to the interseismic or coseismic phases of the subduction seismic cycle.
... The latest tsunami events to have affected the Coquimbo region occurred in 1730,1880,1922,1943 and 2015, after large ruptures of the fault line [2,[10][11][12][13][14][15][16][17][18][19][20]. Of these, the most destructive according to historical records occurred in 1922, which affected 400 km of coastline from the Atacama to the Coquimbo regions (27-30° S) [16,[20][21][22] and caused extensive damage in an area stretching approximately from Caldera to Coquimbo (27-30° S). ...
... The latest tsunami events to have affected the Coquimbo region occurred in 1730,1880,1922,1943 and 2015, after large ruptures of the fault line [2,[10][11][12][13][14][15][16][17][18][19][20]. Of these, the most destructive according to historical records occurred in 1922, which affected 400 km of coastline from the Atacama to the Coquimbo regions (27-30° S) [16,[20][21][22] and caused extensive damage in an area stretching approximately from Caldera to Coquimbo (27-30° S). More than 1000 deaths resulted from the earthquake, and the tsunami killed hundreds of people in coastal areas, mostly in Coquimbo. ...
The coast of Chile has been exposed to marine submersion events from storm surges, tsunamis and flooding due to heavy rains. We present evidence of these events using sedimentary records that cover the last 1000 years in the Pachingo wetland. Two sediment cores were analyzed for granulometry, XRF, pollen, diatoms and TOC. Three extreme events produced by marine submersion and three by pluvial flooding during El Niño episodes were identified. Geochronology was determined using a conventional dating method using ¹⁴C, ²¹⁰Pbxs and ¹³⁷Cs). The older marine event (E1) was heavier, identified by a coarser grain size, high content of seashells, greater amount of gravel and the presence of two rip-up clasts, which seems to fit with the tsunami of 1420 Cal AD. The other two events (E3 and E5) may correspond to the 1922 (E3) tsunami and the 1984 (E5) storm waves, corroborated with a nearshore wave simulation model for this period (SWAM). On the other hand, the three flood events (E2, E4, E6) all occurred during episodes of El Niño in 1997 (E6), 1957 (E4) and 1600 (E6), represented by layers of fine-grain sands and wood charcoal remains.
... by the M w ∼ 8:8 1877 C.E. and M w ∼ 8:5 1922 C.E. megathrust earthquakes nucleated to the north (Fig. 1a,b). These events caused observed run-ups on the order of 5-7 m in the area (SHOA CITSU, 2012, 2014, 2015, 2017, 2018 and run-ups of up to 20-22 and 10-12 m in front of the corresponding rupture regions, respectively (Abe, 1979;Kausel, 1986;Comte and Pardo, 1991;Beck et al., 1998;Ruiz and Madariaga, 2018;Kanamori et al., 2019;León et al., 2019). ...
... In the Illapel-Coquimbo region, previous historic megathrust earthquake ruptures were M w ∼ 7:5 1880 C.E. and M w 7.9 1943 C.E. earthquakes (Lomnitz, 1970(Lomnitz, , 2004Beck et al., 1998;Ruiz and Madariaga, 2018;Fig. 1a,b). ...
On 16 September 2015, the Mw 8.3 Illapel megathrust earthquake broke the subduction contact of the Nazca plate beneath the South American plate, causing unexpectedly high tsunami waves that strongly impacted the coast along the Coquimbo region in central–northern Chile. Here, we report results from a postearthquake and tsunami survey, evidencing a complex coastal geological response driven by the near coast and near trench seismic patches that ruptured during this event. Systematically, high tsunami run-ups, up to 10–11 m a.s.l., were measured on the western coast of the Punta Lengua de Vaca Peninsula in front of the near trench rupture patch, whereas tsunami run-ups up to 5–6 m were measured close to the near coast rupture patch. Detailed measurements conducted in the Coquimbo area revealed tsunami run-ups systematically higher than 6–6.5 m and up to 7.8 m. Field observations of bleached coralline algae supported by ulterior laboratory experiences were compared with geodetic measurements that evidenced complex and variable alongshore uplift-subsidence responses of the coast, suggesting a relative consistency with respect to its modern geomorphological configuration. From the analysis of pit dug in the Tongoy area, which was strongly impacted by this last tsunami, we evaluated distinctive sandy layers with benthic foraminifera tests interbedded between alluvial fine sediments, which we interpreted as paleotsunami events dated before 1108 ± 77 C.E., shortly before 1346 ± 50 C.E., and approximately 1473 ± 37 C.E. In addition to the historical massive tsunamis that occurred in 1730, 1877, and 1922 C.E. in central and northern Chile, we interpreted prehistoric tsunamis revealed here as events triggered by large megathrust earthquake ruptures, evidencing the high exposition of these coasts to near fields as well as to distant tsunamis produced along the Chilean subduction margin but also in the pan-Pacific region.
... Although less frequent and deeper underground than shallow events, intermediate-depth earthquakes can still lead to major disasters. For example, the~80-90 km deep 1939 M S 7.8 earthquake in central Chile took~28,000 lives 16 , exceeding the death tolls of the M W 9.5 shallow southern Chile earthquake in 1960 or the 2011 Tohoku-Oki earthquake and subsequent tsunami (<23,000). The 2017 M W 8.2 Chiapas earthquake at 58 km depth occurred within the slab bend and broke the entire underthrust Cocos lithosphere 14 , causing about 100 fatalities and damaging 41,000 homes in Chiapas 17 . ...
The distribution and rupture properties of intermediate-depth earthquakes (70–300 km) provide insights into the interior stress state of the subducting plate, its petrological composition, and the subduction history. Here we evaluate the kinematic spatio-temporal rupture evolutions of four M7+ intermediate-depth earthquakes beneath Peru between 1997 and 2021 using teleseismic waveforms, which reveal similar rupture behavior over a limited depth range within the flat-slab segment. The intermediate-depth seismogenic zone is confined within a narrow band ~20 km thick near the slab surface, as also found in Pampean and Mexican flat slabs, contrasting to the broader seismicity distributions in the adjacent normally-dipping segments. The elongated earthquake ruptures in flat slabs with few aftershocks and the narrow, sparse seismicity bands are attributed to flexure from along-dip slab steepening and limited dehydration reactions within relatively dry flat-slab segments. Conversely, the diffuse seismicity in adjacent normally-dipping segments is likely associated with more widespread dehydration reactions.
... Although subduction occurs along the entire Chilean margin, some areas have remained unaffected by large earthquakes for more than a hundred years. The Atacama region was last struck by a M W 8.5 earthquake in 1922 and a similar event in 1819 (Figure 1a) (Beck et al., 1998;Vigny et al., 2024;Yáñez-Cuadra et al., 2022). In the region around 27°S, the inter-seismic phase is characterized by elevated seismic activity including earthquakes up to M W 7 and slow slip events (Figure 1b) (e.g., Klein et al., 2018Klein et al., , 2023Ojeda et al., 2023). ...
The Chilean margin is one of the Earth's tectonically most active plate boundaries, and yet, some of its segments are still underexplored. Here, we present amphibious data from the Copiapó region at ∼27°S located within the mature Atacama seismic gap. Combined 2D seismic refraction, multibeam bathymetry, and local seismicity data show a typical oceanic crust thickness of 6–7 km and seismic P‐wave velocities between 3.0 and 7.3 km/s with slightly lower velocities and increased thicknesses underneath the Copiapó Ridge seamounts. The latter is most likely due to predominantly extrusive formation. Elevated velocities underneath one of the seamounts indicate a local region of magmatic underplating, while bending‐related faults visible in the bathymetry and reduced mantle velocities near the trench suggest mantle hydration. The subduction angle of the down‐going Nazca plate smoothly increases from 12° below the marine forearc to 22° at greater depths (40–60 km) with no abrupt change in the dip angle as observed at ∼22°S. The local seismicity off‐ and onshore Copiapó shows three separated bands of earthquakes sub‐parallel to the down‐going plate, and are most likely related to the plate interface, the oceanic Moho and the Double Benioff Zone. The largest event (MW 5.9) during our observation period (December 2022–June 2023) and its aftershocks occurred in the deepest band ∼20 km below the subduction interface. Along the interface, seismicity is most pronounced in areas of high locking offshore, whereas areas of low locking are characterized by previously observed slow slip events and sparse seismicity.
... Another work that highlights the interplay between earthquakes and art is "Terremoto" by Nemesio Antunez (1958). Terremoto is a 30 m 2 painting inspired by the M 8.3 Chillán earthquake in central Chile, one of the most destructive events in Chile's history, which killed ∼28,000 people (Beck et al., 1998;Ruiz and Madariaga, 2018 Similarly, the cultural value of the 1973 seismograms can be understood in terms of the impact of scientific records on the arts. Seismograms have long served as inspiration or production material for musical and visual works. ...
This study reveals the seismic signature of the 1973 Chilean coup d'état by analyzing historical paper seismograms from September 1973. The continuous traces of the seismic activity preceding, during, and after the military coup were recorded by a Teledyne Geotech seismometer installed near (∼2 km) the presidential palace La Moneda. Our analysis revealed that distinct seismic patterns corresponded to specific societal activities , such as the morning commute and nighttime quietness. The seismic records captured the upheaval caused by the coup, including the military intervention, the bombing of La Moneda palace, and the subsequent imposition of a national curfew. The latter led to a significant reduction in anthropogenic noise making it easier to detect seismic events with enhanced clarity. To reconstruct the events of September 1973, we combined the seismic data with the historical archives of the Museo de la Memoria y los Derechos Humanos (Museum of Memory and Human Rights Foundation). This research offers a unique perspective on the recent history of Chile. It also highlights the importance of seismic and scientific records as elements of cultural heritage worthy of preservation and recognition. In addition, it underlines the close link between Chile's seismological characteristics and the profound influence they have had on the shaping of the country's identity serving as an invaluable source of inspiration for artistic representations of natural hazards and disasters. Finally, the article emphasizes the imperative of preserving historical scientific records, not only as invaluable resources for advancing scientific understanding, but also as foundational elements for fostering transdisciplinary collaborations. The safeguarding of these records facilitates the interplay between science and art, creating new pathways for the dissemination of complex scientific knowledge, while inspiring creative expressions that engage with and reflect on Chile's cultural and natural history.
... Another work that highlights the interplay between earthquakes and art is "Terremoto" by Nemesio Antunez (1958). Terremoto is a 30 m 2 painting inspired by the M 8.3 Chillán earthquake in central Chile, one of the most destructive events in Chile's history, which killed ∼28,000 people (Beck et al., 1998;Ruiz and Madariaga, 2018 Similarly, the cultural value of the 1973 seismograms can be understood in terms of the impact of scientific records on the arts. Seismograms have long served as inspiration or production material for musical and visual works. ...
This study reveals the seismic signature of the 1973 Chilean coup d’état by analyzing historical paper seismograms from September 1973. The continuous traces of the seismic activity preceding, during, and after the military coup were recorded by a Teledyne Geotech seismometer installed near (∼2 km) the presidential palace La Moneda. Our analysis revealed that distinct seismic patterns corresponded to specific societal activities, such as the morning commute and nighttime quietness. The seismic records captured the upheaval caused by the coup, including the military intervention, the bombing of La Moneda palace, and the subsequent imposition of a national curfew. The latter led to a significant reduction in anthropogenic noise making it easier to detect seismic events with enhanced clarity. To reconstruct the events of September 1973, we combined the seismic data with the historical archives of the Museo de la Memoria y los Derechos Humanos (Museum of Memory and Human Rights Foundation). This research offers a unique perspective on the recent history of Chile. It also highlights the importance of seismic and scientific records as elements of cultural heritage worthy of preservation and recognition. In addition, it underlines the close link between Chile’s seismological characteristics and the profound influence they have had on the shaping of the country’s identity serving as an invaluable source of inspiration for artistic representations of natural hazards and disasters. Finally, the article emphasizes the imperative of preserving historical scientific records, not only as invaluable resources for advancing scientific understanding, but also as foundational elements for fostering transdisciplinary collaborations. The safeguarding of these records facilitates the interplay between science and art, creating new pathways for the dissemination of complex scientific knowledge, while inspiring creative expressions that engage with and reflect on Chile’s cultural and natural history.
... This suggests that, on average, a megathrust earthquake occurs in this region approximately every 110 years, a relatively frequent seismic activity rate for this type of earthquake. Earthquakes of M < 8 have also been recorded in the northern part of this segment, such as the 1928 M w 7.8/M S 8.0 earthquake with an underthrusting focal mechanism and a depth of 0-25 km; the 1939 M S 7.8 earthquake which had a normal faulting mechanism and a depth of 80-100 km; and the 1953 M S 7.6 earthquake, with a reverse faulting mechanism and a depth of approximately 60-80 km (Beck et al. 1998;Ruiz and Madariaga 2018). ...
... On 16 September 2015, the Mw 8.3 Illapel, Chile, earthquake initiated at 22.4 km depth on the Nazca megathrust, near the coast of the Coquimbo region. This earthquake ruptured a zone extending ∼250 km along strike between the Challenger Fracture Zone and the Juan Fernández Ridge (Figure 1a, Beck et al., 1998;Contreras-Reyes & Carrizo, 2011;Métois et al., 2016). Numerous Global Navigation Satellite System (GNSS)-based studies of the co-and postseismic slip distributions on the megathrust interface show moderate to high overlap between these two modes of slip (e.g., Barnhart et al., 2016;Klein et al., 2017;Shrivastava et al., 2016), contrary to expectations from rate-and state-dependent friction laws (Dieterich, 1979;Ruina, 1983;Scholz, 1998). ...
Plain Language Summary
Between consecutive earthquakes in the Central and South‐Central Andes, during what is known as the interseismic phase, the traditional Nazca‐South America two‐plate model underpredicts the surface Global Navigation Satellite System (GNSS) velocities on the east side of the mountain belt. Previous studies show that adding an “Andean microplate,” thus forming a three‐plate model, kinematically explains the observed velocity field on both sides of the Andes. We analyzed the GNSS coseismic displacements caused by the Mw 8.3 2015 Illapel, Chile, earthquake, and found that the traditional two‐plate model also underpredicts GNSS observations in the eastern side of the Andes. We show that adding the Andean microplate to the model, in the same manner as for the interseismic phase, and allowing aseismic slip on the detachment interface beneath it significantly reduces the observed surface displacements misfit. We also show that this holds for the displacements observed 50 days after the main shock, evidencing that the detachment geometry slips before, during, and after a large magnitude earthquake. Using the three‐plate model produces fault slip distributions that decrease overlap between local maxima in coseismic and aseismic slip, consistent with expectations from rate‐ and state‐dependent friction laws.
... Its seismicity map (Fig. 2) indicates that the epicenter distributions of stronger earthquakes are located at a small distance from the erent active faults and subduction line. For instance, Illapel 1943, 7.9 Mw (Beck et al., 1998);Illapel 2015, 8.3 Mw (Ruiz & Madariaga, 2018); Tongoy 2019, 6.7 Mw and Punitaqui 1997, 7.1 Mw (Lemoine, 2001). Even though rural infrastructure predominates in the region, cities such as La Serena, Ovalle, Coquimbo, home to a combined 400,000 residents, are located not more than 200 km from the epicenters. ...
In order to seismically characterize Chile’s northern Coquimbo Region, data from 2003 to 2020 were considered. The region was divided into 30 zones of 0.5∘ latitude and 0.5∘ longitude and non-extensive statistical physics was used. Both, the Sotolongo–Costa–Posadas (SCP) and Mathai models were deployed to analyze the magnitude-frequency distribution. A sub-division into cells of the catalog allowed to demonstrate that systems with value of q∼1 present exponential behavior, while it was expected to obtain q>1, by superimposing sub-systems supporting the superstatistical model. Thus, by subdividing the Coquimbo region into south and north, we found that in both zones the entropic index is greater than 1, q>1, However, in the southern zone the long-range effects are greater than in the north, according to the value obtained, which means both sectors are well described under a nonextensive statistical model, be it the SCP model or the Mathai one. The entropic index is q>1 and in both cases R2>0.99. As the region is considered as a whole, the nonextensive statistical distribution is the more adequate one. With respect to probabilistic seismic hazard assessment, Mathai’s model proved to have the better fit. Thus, the frequency-interevent time distribution was used for different limit magnitude values. Our analysis showed that the probability occurrence of a seismic event in the region’s north is lower than in the south considering the same period. In the north the behavior is of Poissonian type.
... This suggests that, on average, a megathrust earthquake occurs in this region approximately every 110 years, a relatively frequent seismic activity rate for this type of earthquake. Earthquakes of M < 8 have also been recorded in the northern part of this segment, such as the 1928 M w 7.8/M S 8.0 earthquake with an underthrusting focal mechanism and a depth of 0-25 km; the 1939 M S 7.8 earthquake which had a normal faulting mechanism and a depth of 80-100 km; and the 1953 M S 7.6 earthquake, with a reverse faulting mechanism and a depth of approximately 60-80 km (Beck et al. 1998;Ruiz and Madariaga 2018). ...
Characterizing the spatial distribution of ruptures from historical and recent earthquakes is key to understanding the seismic cycle of large earthquakes in subduction zones, and thus to assessing the potential risks associated with future earthquakes. Central Chile (35∘S–38∘S) has been repeatedly affected by large earthquakes, such as the 2010 Maule (Mw 8.8) and the 1835 earthquakes witnessed by Robert Fitzroy (HMS Beagle captain). Here, we identify the rupture pattern and tsunami propagation of the 1751, 1835, and 2010 mega-earthquakes, events that overlapped in central Chile, by compiling historical records and applying robust statistical tools. We used an adaptation of a logic tree methodology to generate random sources of slip distribution for each event, constrained by tsunami and vertical deformation data. We find that the three events studied have different slip peaks. The 1751 earthquake has the largest slip with a maximum patch of ∼ 26 m, while the 2010 and 1835 earthquakes reach slips of ∼ 16 m and ∼ 10 m, respectively. Our results show that a part of the margin between 36∘S and 37∘S was consistently affected by large earthquakes, but with different slip and depth. The shallower depths of the megathrust north of 36∘S accumulated energy for at least 300 years, which was released by the 2010 earthquake. Our results provide important constraint for rupture patterns and spatial relationships between historical and recent earthquakes, thus extending the time scale for seismic slip distribution analyses over multiple cycles and contributing to a more comprehensive understanding of seismic hazards.
... Most source inversions suggested that the rupture lasted around 100 s (Heidarzadeh et al. 2016 ;Melgar et al. 2016 ;Tilmann et al. 2016 ) but some studies report much larger rupture durations (e.g. Lee et al. 2016 ). The previous ear thquake to r upture this section of the megathrust occurred in 1943, with a smaller magnitude between M w = 7.9 −8.3, and a duration of approximately 30 s (Beck et al. 1998 ;Lomnitz 2004 ;Ruiz & Madariaga 2018 ). The hypocentral depth of the 1943 event is unfortunately not well resolved and is estimated between 10 and 30 km. ...
The 2015 moment magnitude Mw = 8.3 Illapel earthquake is the largest mega-thrust earthquake that has been recorded along the Chilean subduction zone since the 2010 Mw = 8.8 Maule earthquake. Previous studies indicate a rupture propagation from the hypocentre to shallower parts of the fault, with a maximum slip varying from 10 to 16 m. The amount of shallow slip differs dramatically between rupture models with some results showing almost no slip at the trench and other models with significant slip at shallow depth. In this work, we revisit this event by combining a comprehensive data set including continuous and survey GNSS data corrected for post-seismic and aftershock signals, ascending and descending InSAR images of the Sentinel-1A satellite, tsunami data along with high-rate GPS, and doubly integrated strong-motion waveforms. We follow a Bayesian approach, in which the solution is an ensemble of models. The kinematic inversion is done using the cascading capability of the AlTar algorithm, allowing us to first get a static solution before integrating seismic data in a joint model. In addition, we explore a new approach to account for forward problem uncertainties using a second-order perturbation approach. Results show a rupture with two main slip patches, with significant slip at shallow depth. During the rupture propagation, we observe two regions that are encircled by the rupture, with no significant slip, westward of the hypocentre. These encircling effects have been previously suggested by back-projection results but have not been observed in finite-fault slip models. We propose that the encircled regions correspond to zones where the yield stress largely exceeds the initial stress or where fracture energy is too large to be ruptured during the Illapel earthquake. These asperities may potentially break in the future and probably already broke in the past.
... Most source inversions suggested that the rupture lasted around 100 s (Heidarzadeh et al. 2016 ;Melgar et al. 2016 ;Tilmann et al. 2016 ) but some studies report much larger rupture durations (e.g. Lee et al. 2016 ). The previous ear thquake to r upture this section of the megathrust occurred in 1943, with a smaller magnitude between M w = 7.9 −8.3, and a duration of approximately 30 s (Beck et al. 1998 ;Lomnitz 2004 ;Ruiz & Madariaga 2018 ). The hypocentral depth of the 1943 event is unfortunately not well resolved and is estimated between 10 and 30 km. ...
The 2015 moment magnitude Mw=8.3 Illapel earthquake is the largest mega-thrust earthquake that has been recorded along the Chilean subduction zone since the 2010 M_W=8.8 Maule earthquake. Previous studies indicate a rupture propagation from the hypocenter to shallower parts of the fault, with a maximum slip varying from 10 to 16 meters. The amount of shallow slip differs dramatically between rupture models with some results showing almost no slip at the trench and other models with significant slip at shallow depth. In this work, we revisit this event by combining a comprehensive data set including continuous and survey GNSS data corrected for post-seismic and aftershock signals, ascending and descending InSAR images of the Sentinel-1A satellite, tsunami data along with high-rate GPS, and doubly integrated strong-motion waveforms. We follow a Bayesian approach, in which the solution is an ensemble of models. The kinematic inversion is done using the cascading capability of the AlTar algorithm, allowing us to first get a static solution before integrating seismic data in a joint model. In addition, we explore a new approach to account for forward problem uncertainties using a second-order perturbation approach. Results show a rupture with two main slip patches, with significant slip at shallow depth. During the rupture propagation, we observe two regions that are encircled by the rupture, with no significant slip, westward of the hypocenter. These encircling effects have been previously suggested by back-projection results but have not been observed in finite-fault slip models. We propose that the encircled regions correspond to regions where the yield stress largely exceeds the initial stress or regions where fracture energy is too large to be ruptured during earthquakes such as the Illapel one. These asperities may potentially break in the future and probably already broke in the past.
... Most great (≥M W 7.5) earthquakes along the tectonically active western margin of South America are associated with the subduction of the Nazca Plate beneath the South American continent, although some correspond to normal-faulting events within the down-going slab (Beck et al., 1998). Figure 1d shows the spatial and temporal distribution of those earthquakes, which display patterns of recurrent great events in distinct seismotectonic segments of the forearc (Lomnitz, 2004;Molina et al., 2021;Saillard et al., 2017). ...
The tectonically active South American margin is characterized by the accumulation of deformation contributing to uplift of the Andean forearc at millennial time scales. However, the mechanisms responsible for permanent coastal uplift are debated, mainly because methodologically consistent, continental‐scale analyses of uplifted terraces have not yet been carried out for South America. Uplifted marine terraces are generally used to infer permanent coastal deformation and uplift; we used almost 2,000 measurements of last‐interglacial marine terraces to calculate an uplift‐rate signal on which we performed a wavelength analysis. The same spectral analysis was applied to tectonic and subduction parameters associated with accumulation of permanent deformation to detect possible links with the uplift‐rate signal. The uplift‐rate signal displays a constant background‐uplift rate along the margin, perturbed by changes at variable wavelengths. Similarities between its wavelength spectrum and the spectra of tectonic parameters suggest potential correlations pointing toward underlying processes. For example, crustal faulting is mainly responsible for short‐wavelength deformation; intermediate‐wavelength to long‐wavelength tectonic features indicate various extents of locked areas on the megathrust that relate to its long‐term seismotectonic segmentation. We suggest that moderate, long‐term background uplift is caused by major, deep earthquakes near the Moho, although records of such events are sparse. Due to their disparate occurrence, we infer accumulation of permanent deformation over millennial time scales through multiple, distinct uplift phases that are spatially and temporally distributed. Our study highlights the application and utility of a signal‐analysis approach to elucidate the mechanisms driving surface deformation in subduction zones at a continental scale.
... Moderate-and large-magnitude intraplate intermediate-depth (IID) events frequently occur throughout the territory of Chile (e.g., Herrera et al., 2017Herrera et al., , 2023Ruiz et al., 2019;Delouis and Legrand, 2007;Peyrat et al., 2006). The most destructive events of the last century were in Chillán (1939), Calama (1950), andTarapacá (2005) of magnitudes around M 8.0 (Beck et al., 1998;Ruiz and Madariaga, 2018). Before the 20th century, few intraplate events were recorded by historians and seismologists, probably because many were poorly recorded in the seismic catalogs built during the 1970s (Ruiz and Madariaga, 2018). ...
Large intraplate intermediate-depth (IID) events are the most destructive to Chilean structures and they occurred throughout the country. Cities located above the hypocenters of these large earthquakes have been completely destroyed. In recent years, only small- and moderate-magnitude IID events have been reported below Santiago, the most populated Chilean city; however, the occurrence of large-magnitude IID events cannot be ruled out. In this study, we investigated the strong-ground motion generated by large-magnitude earthquakes occurring below the Santiago metropolitan region. We used a stochastic methodology to simulate synthetic records, considering IID events of magnitude Mw 7.8. To validate this method, we simulated intermediate-depth events of magnitudes Mw~5-6 that occurred near Santiago. We further calibrate our results by reproducing the strong-ground motion data recorded during the 2005 Tarapacá Mw 7.8 Northern Chile IID earthquake. We observe that in some areas of the Santiago Basin, high PGA values can reach values close to 1 g, in which, in addition to directivity effects and takeoff angles, the rupture distances (~100 km) and soil type (C) play a key role in amplifying strong-ground motion. Finally, our findings highlight the importance of reproducing high-frequency time histories as a proxy to re-evaluate the seismic hazard due to large IID earthquakes.
... The seismicity map of Coquimbo (Fig. 2) indicates that the epicenter distributions of some of the strong earthquakes are located at a small distance from the different active faults and subduction line. For instance, Illapel 1943, 7.9 Mw (Beck et al. 1998 (Lemoine 2001;Brown 2002). Even though rural infrastructure predominates in the Coquimbo region, very close to the epicenters, at most 200 km away, there are large cities such as La Serena, Ovalle, Coquimbo (together a population of over 400,000 inhabitants). ...
In order to characterize seismically the Coquimbo Region, Chile, data from 1943 to 2020 were considered. The region was divided into 30 zones of 0.5 • latitude and 0.5 • longitude and the non-extensive statistical physics was used. Both, the Sotolongo-Costa-Posadas and Mathai models were proposed to analyze the magnitude-frequency distribution. The subdivision into cells of the catalog allowed to demonstrate that systems with value of q ∼ 1 present exponential behavior, while with the superposition of subsystems it was expected to obtain q > 1, supporting the su-perstatistical model. Thus, by subdividing the Coquimbo region into the South and North regions, we find that in both the entropic index is greater than one, q > 1, however , according to the value obtained, in the southern sector the long-range effects are greater than in the northern sector. This means that both sectors ae well described by the nonextensive statistical model, either by SCP model or Mathai model; the entropic index is q > 1 and in both cases R 2 > 0.99. As the Coquimbo region is considered as a all, the nonextensive statistical distribution is the more adequated. With respect to the probabilistic seismic hazard assessment, we showed that the Mathai’s model has the better fit. Thus, in the frequency-interevent time distribution was used for different limite magnitudes values. Our analysis shows that the probability occurrence of an seismic event in the northern sector of the region is lower than in the southern sector for the same period; in the former the behavior is of Poissonian type. Thus, the region’s northern and southern zones show different behaviors in terms of earthquake occurrence probability.
... In the last 400 years, at least 5 earthquakes M ≥ 8 have been recorded in the South Central segment, these events corresponds to the years 1570, 1657, 1751, 1835, and 2010 ( Fig. 1), indicating that here, a megathrust earthquake occurs in average every 110 years, which is a frequent seismic activity rate for this type of earthquake. Earthquakes of M < 8 have also been recorded in the northern part of the segment in 1928, 1939and 1953(Beck et al. 1998). ...
Characterizing the spatial distribution of ruptures from historical and recent earthquakes is key to understanding the seismic cycle of large earthquakes in subduction zones, and thus to assessing the potential risks associated with future earthquakes. Central Chile (35S-38S) has been continuously affected by large earthquakes, such as the 2010 Maule (Mw 8.8) and the 1835 earthquakes witnessed by Robert Fitzroy (HMS Beagle captain). Here, we identify the rupture pattern and tsunami propagation of the 1751, 1835, and 2010 mega-earthquakes, events that overlapped in central Chile, by compiling historical records and applying robust statistical tools. We used an adaptation of a logic tree methodology to generate random sources of slip distribution for each event, constrained by tsunami and deformation data. We find that the three events studied have different slip peaks. The 1751 earthquake has the largest slip with a maximum patch of 26 m, while the 2010 and 1835 earthquakes reach slips of 16 m and 10 m, respectively. Our results show that a part of the segment between 36S and 37S was consistently affected by large earthquakes, but with different slip and depth. The northern part of the segment accumulated energy for at least 300 years and was released by the 2010 earthquake. This work provides important information for identifying rupture patterns between historical and recent earthquakes, and highlights the importance of extending the time scale of earthquake slip distribution analyses to multiple cycles to describe both earthquake characteristics and their spatial relationship, and thus gain a better understanding of seismic hazard.
... Besides, the 1420 Oei orphan tsunami could possibly be attributed to a great earthquake magnitude M8.8-9.4 (Abad et al., 2020) along the Atacama segment. Other large earthquakes occurred in 1819 (Ms 8.3) and 1922 ) (Beck et al., 1998;Kanamori et al., 2019;Carvajal et al., 2017), while several earthquakes magnitude M∼7 or similar have been reported in 1796, 1859, 1909, 1918, 1946, and 1983(Comte et al., 2002Ruiz and Madariaga, 2018). Recent studies have shown that the Atacama segment experiences both seismic and aseismic slips. ...
Earthquake swarms commonly occur along the Chilean subduction zone, witnessing fast seismic and slow aseismic slip behavior at the plate interface. However, the largest seismic swarms observed in Chile, particularly in the Copiapó-Atacama region, remain poorly documented, and the underlying processes have yet to be understood. Here, we perform seismological and geodetic analyses to investigate the 2006 Copiapó swarm, which developed in April and May 2006. The swarm began on April 19, with a magnitude Ml 5.3 earthquake. During the nine following days, we observe a migration of seismicity along the plate interface, the occurrence of doublets events, and a potential slow slip event in the GPS time series at site Copiapó. Then, on April 30, a first earthquake with Mw 6.6 occurred at 15 km depth at the plate contact. It likely triggered a second earthquake of magnitude Mw 6.5, which occurred 144 min later, 10 km northwest of the first earthquake. Using InSAR, we determined the slip distribution associated with these two earthquakes and detailed the postseismic slip they triggered in the next days and weeks. This “postseismic” phase appears to be predominantly aseismic, while the moment released during the “coseismic” phase is comparable to other seismic crises that occurred in Atacama. Although we did not find a larger seismic and aseismic ratio than in other swarms in South America, we suggest a similar mechanism of slow deformation as a driver of seismicity during seismic swarms. Finally, we propose that the slow and fast behavior of the 2006 Copiapó swarm is a consequence of the subduction of the Copiapó Ridge seamounts, which affects both the plate interface and the overriding plate by inducing complex interactions between seismic and aseismic processes.
... Chile is one of the seismically most active regions on Earth, where the Nazca plate subducts under the South American plate with a convergence rate of approximately 67 mm/yr [Angermann et al., 1999, Vigny et al., 2009 [Lomnitz, 2004, Ruiz andMadariaga, 2018] [Li et al., 2016, Ruiz andMadariaga, 2018]. The same area was previously hit by an earthquake of similar magnitude in 1943 [Beck et al., 1998]. ...
The general objective of this thesis is to conduct a thorough source study covering different aspects of earthquake characterization. First, I focused on the initiation phase of earthquakes. Specifically, I analyzed the initiation phase of the 2017 Valparaiso earthquake (magnitude Mw=6.9). This earth-quake is an interesting case study because it was preceded by a transient displacement accompanied by many pre-shock earthquakes. We show that this earthquake was probably preceded by an asismic slip on the fault. In a second part of my thesis, I am interested in the detailed characterization of the co-seismic slip distribution associated with large earthquakes. In particular, I examine the 2014 Illa-pel earthquake (Mw=8.3). The kinematic characterization of the source of this earthquake is there-fore performed via a Bayesian sampling approach, which has proven useful in characterizing the a posteriori uncertainty. Our kinematic models indicate a high complexity in the rupture process, asso-ciated with "encircling asperities" that have been previously suggested by previous studies.
... This study aims to better understand the underlying physical processes responsible for strain accumulation along the Chilean margin. Presently, there are three major longstanding seismic gaps along the Chilean megathrust where no large subduction earthquake occurred for more than a century: 1877 (M s ∼ 8.5) Northern Chile (20-23°S), 1922 (M w ∼ 8.5) Atacama (25-30°S) and 1730 (M s ∼ 8.7) Valparaiso (32-34°S) (e.g., Beck et al., 1998;Comte & Pardo, 1991;Lomnitz, 2004;Ruiz & Madariaga, 2018). We focus our efforts in the Atacama seismic gap. ...
Geodetically constrained interseismic interplate coupling has been widely used to assess seismic potential in subduction zones. Modeling interseismic deformation is challenging, as it involves interplate coupling and often ignores continental internal deformation processes. We present a novel methodology to jointly estimate interplate coupling along with upper plate rigid motion and surface strain, constrained by GNSS‐derived velocities. We use a least squares inversion with a spatially variable Equal Posterior Information Condition Tikhonov regularization, accounting for observational and elastic structure uncertainties. Our modeling reveals three megathrust regions with high tsunamigenic earthquake potential located within the Atacama Seismic Gap (Chile). This study indicates the presence of a downdip segmentation located just above the 1995 (Mw8.0) Antofagasta earthquake rupture, raising concerns for the potential of tsunamigenic earthquake occurrence at shallower depths. Additionally, we show that surface motion is dominated by strain, with rather negligible rigid motion, dismissing the rigid Andean microplate model typically assumed in previous studies.
... Here, the Nazca plate subducts beneath the South American plate at a rate of ∼68 mm/yr (Altamimi et al., 2007), resulting in frequent seismic events in the region. The historical earthquakes in this area include 1880 Ms 7.5 (Lomnitz, 2004), 1943 Ms 7.9 (Beck et al., 1998) and 2015 Mw 8.3 (Comte et al., 2019) events, which recurred every 60-70 years approximately, during the past 150 years. The largest event in the history of this area occurred in 1730 (Ms > 8.5), and its impact range was up to 500 km (Lomnitz, 2004;Métois et al., 2012;Udías et al., 2012). ...
Plain Language Summary
A large Mw 8.3 megathrust earthquake ruptured on the plate boundary between the Nazca Plate and South American Plates along Chile on 16 September 2015. Global Positioning System (GPS) stations in this area clearly recorded significant ground displacements. This work reveals the rapid ground displacements in the first few hours since completion of the rupture. From kinematic GPS, we find that the postseismic deformation in the first 2 hr accounts for about 50% of the total amount accumulated in the first 12 hr, and the deformation in the first half of the day accounts for about 60% of the total during the first 3 days following the mainshock. The very early rapid ground displacements do not strictly follow the logarithmic or exponential decay, but can be well described by the power function model. Besides, the distribution of coseismic slip and the early afterslip are spatially complementary. We also find that the spatial distribution of the early aftershocks is highly consistent with the spatiotemporal evolution of the early afterslip in the first few hours since the mainshock, showing that early aftershocks may be dominated by early afterslip.
... As a case study to test the methodology outlined in the previous sentences and in Fig. 1, this research focuses on Central Chile where a 500 year of historic record of seismicity is available, as seen in Fig. 2 (Beck et al. 1998;Scholz and Campos 2011;Ruiz and Madariaga 2018;Bravo et al. 2019). In this area, large earthquakes tend to repeat with a recurrence time that varies between 30 and 300 years. ...
The central area of Chile’s Valparaiso Region has been classified as a seismic gap for a major earthquake, which makes it very important to understand the seismic hazard of the zone. Generally, seismic codes consider a qualitative classification of sites to estimate the possible damage in the case of an earthquake scenario. Estimating the values of acceleration could be very important to prevent damages and increase preparedness for these rare events. In this research, a qualitative and quantitative estimation of seismic hazard is performed in the study area (Valparaiso region between Papudo and San Antonio 32°–34° S). This is achieved through an integrated and relatively economical approach which considers the information from Geology, Geophysical experiments (Gravity and seismic methods), and Geotechnical analyses. The results of the geophysical survey and geology information allow dividing the zone into five site types through a new proposal of site classification that depends not only on the Vs30, but also on the sites predominant period (T0), which is an innovation of this work for the Chilean code. The Peak Ground Acceleration (PGA) values in the study zone were estimated using a Ground Motion Predictive Equation developed for the Chilean subduction zone. Additionally, we consider three different seismic scenarios according to the history of events in Central Chile. The results of this quantitative analysis show PGA values up to 0.52 g for the median and 1.2 g for the 84th percentile of the scenarios. Overall, the highest accelerations (PGA) are in zones with low shear wave velocities (< 500 m/s), a long predominant period (> 0.4 s) and where geology establishes the presence of low stiffness soils. The comparison of response spectra from the model against records from 2010 Maule and 1985 Valparaiso earthquakes shows available models tend to overpredict the intensities.
... In addition, marked stations (*) were not used in the waveform modeling. previous studies to obtain the source characterization of other historical earthquakes such us the 1944 (M W 7.0) and 1952 (M W 6.8) events in San Juan, Argentina (Alvarado and Beck, 2006), the 1958 (M W 6.3) Las Melosas earthquake (Alvarado et al., 2009) and many other historical earthquakes in Chile (Beck et al., 1998). Unfortunately, local or regional seismic records were not available for the 1948 event. ...
The region of the Andean back-arc of northwestern Argentina has been struck by several magnitude ≥6 crustal earthquakes since the first historically recorded event in 1692. One of these events corresponds to the Anta earthquake on 25 August 1948, with epicenter in the Santa Bárbara System causing three deaths and severe damage in Salta and Jujuy provinces with maximum Modified Mercalli seismic intensities (MMI) of IX. We collected and digitized analog seismograms of this earthquake from worldwide seismic observatories in order to perform first-motion analysis and modeling of long-period teleseismic P-waveforms. Our results indicate a simple seismic source of M0 = 2.85 × 1019 N m consistent with a moment magnitude Mw = 6.9. We have also tested for the focal depth determining a shallow source at 8 km with a reverse focal mechanism solution with a minor dextral strike-slip component (strike 20°, dip 30°, rake 120°) from the best fit of waveforms. Using magnitude-size empirical relationships, the comparison of the obtained Mw 6.9 magnitude value and the ca. 10,000 km2 area of MMI ≥ IX from our seismic intensity map, which was obtained from newspaper and many historical reports, indicates a rupture length of 42 ± 8 km for the Anta earthquake. We show our results in a 3D geological model around the epicentral area, which integrates modern seismicity, geological data, and information of a previously studied east-west cross section located a few kilometers south of the 1948 epicenter. The integration of all available information provides evidence of the re-activation of the Pie de la Sierra del Gallo fault during the 1948 Mw 6.9 shallow earthquake; this thrust fault bounds the Santa Bárbara System along its western foothill.
... The Chilean subduction zone is extremely seismically active with on average one Mw 8 earthquake every ten years and at least one mega earthquake of magnitude 9 and over per century (Beck et al., 1998, Lomnitz, 2004, Ruiz and Madariaga, 2018. The largest earthquake ever recorded by seismometers, the 22 nd May 1960 Valdivia earthquake of magnitude 9.5, occurred along the southern part of the Chilean subduction zone and generated a giant trans-Pacific tsunami that caused catastrophic damage along the coasts of Hawaii and Japan (Cifuentes, 1989). ...
Over the last 3 decades, GPS measurements have been instrumental in quantifying tectonic plates current motion and deformation. Complex patterns of deformation along the plate boundaries revealed heterogeneous coupling on the plates interface and imaged seismic segments at different stages of their seismic cycle. Along the South-American trench in Chile, where large earthquakes occur frequently, continuous GPS observations (cGPS) captured both the long-term plate motion and the transient deformations associated to the seismic cycle. Over the years, a network of hundreds of cGPS stations has been deployed all across the South-American continent by many different institutions for \m{many ?}all sorts of purposes ranging from geographic reference to Tsunami early warning. We report here on the processing of 20 years (2000-2020) worth of data over a selection of cGPS stations, devoted to the quantification and analysis of the deformation along the Chilean subduction zone between 18°S and 40°S. We use all available data near the trench in Chile and a less dense network inside the continent where the gradient of deformation is lesser. Our database, named SOAM_GNSS_solENS, provides time series of precise daily station position, obtained from double difference (DD) processing and expressed in the International Terrestrial Reference Frame 2014 (ITRF14). These time series allow to quantify, with \m{sub-?}sub-millimetric precision, any kind of ongoing deformation process, either from tectonic origin such as interseismic deformation, co- and post-seismic displacements associated with earthquakes, transient deformation associated to seismic swarms and/or a-seismic slow-slip events, or of other origin such as hydrological loading (for ex, the Amazonian basin load) or any other type of loading affecting the surface of the earth (tides, atmosphere, etc...). We also provide a database of coseismic displacements associated with close to 60 earthquakes of Mw larger than 6.5 that occurred over the last 20 years within the observation area. All time series are directly accessible through a deposit and we also plan make them available through a web interface that will allow any user to perform elementary operations like estimating offsets, detecting outliers, detrending, filtering and stacking. That database will evolve with time, aggregating more data. In the future, we also plan to complement that database with a rapid solution in quasi real time processed in Precise Point Positioning (PPP), and with hourly atmospheric delays associated to water vapor contains of the lower layer of the atmosphere.
... Below, we show that Foamquake succeeds in reproducing the first-order peculiarities of the space-time rupture history over multiple seismic cycles of several megathrust segments. We focus on similarities and differences with three subduction zones that share similar patterns and where a historical record of multi-cycle ruptures is available: the Sanriku-Tohoku segment (e.g., Satake, 2015) and the Nankai Trough of the Japan-Kuril Trench (Fujiwara et al., 2020;Garrett et al., 2018;Kitamura et al., 2018;Kodaira et al., 2006) and the Conception and Valparaiso segments along the Chilean margin (Beck et al., 1998;Carvajal et al., 2017;Melnick et al., 2009). Those three subduction zones are characterized by a similar long term space-time rupture pattern: the superim- (Philibosian & Meltzner, 2020). ...
In the last decades, seismotectonic analog models have been developed to better understand many aspects of the seismic cycle. Differently from other lab‐quake experiments, seismotectonic models mimic the first order characteristics of the seismic cycle in a scaled fashion. Here we introduce Foamquake: A novel seismotectonic model with a granular frictional interface that as a whole behaves elastoplastically. The model experiences cycles of elastic loading and release via spontaneous nucleation of frictional instabilities at the base of an elastic foam wedge, hereafter called foamquakes. These analog earthquakes show source parameters (i.e., moment‐duration and moment‐rupture area) scaling as great interplate earthquakes and a coseismic displacement of few tens of meters when scaled to nature. Models with two asperities separated by a barrier can be performed with Foamquake given the 3D nature of the setup. Such model configuration generates sequences of full and partial ruptures with different recurrence intervals as well as rupture cascades. By tuning the normal load acting on individual asperities, Foamquake reproduces superimposed cycles rupture patterns such as those observed along natural megathrusts. The physical properties of asperities and barriers affect model seismic behavior. Asperities with similar properties and low yield strength fail preferentially in a simultaneous manner. The combination of all those characteristics suggests that Foamquake is a valuable tool for investigating megathrust seismicity and seismic processes that depend on the 3D nature of the subduction environment.
... Large megathrust earthquakes also occurred in the past, the largest one in 1922 (Mw ∼ 8.5-8.8), and several moderate earthquakes have occurred more frequently for example, 1796, 1918, 1983(Beck et al., 1998Kanamori et al., 2019;Ruiz & Madariaga, 2018). The last event, with Mw 6.9, happened on 1 September 2020 and was part of a remarkable seismic and a-seismic sequence (Klein et al., 2021) (Figure 1). ...
We studied the along-dip influence of the Copiapó ridge subduction in the Atacama region, North-Central Chile by building a new seismicity catalog, including similar events and non-volcanic tremors (NVTs). We also obtained a 3-D tomographic model for P- and S-waves velocity (and the implied Vp/Vs ratio). We identify down-dip segmentation involving 4 distinct segments: a locked seismogenic zone hosting ordinary seismicity and clusters of similar events; a transition zone with NVTs and low seismicity; an aseismic zone with slow-slip events; and a deep zone with abundant intraslab seismicity. The velocity models show differences among these zones, with low velocity anomalies of Vp and Vs coinciding with aseismic slip zones, indicating the possible presence of fluids. Due to the spatial distribution along-strike and along-dip of the aseismic zones, we propose that these differences in seismogenic behavior are generated by subduction of the heterogeneous seamounts associated with the Copiapó ridge.
... Large megathrust earthquakes also occurred in the past, the largest one in 1922 (Mw ∼ 8.5-8.8), and several moderate earthquakes have occurred more frequently for example, 1796, 1918, 1983(Beck et al., 1998Kanamori et al., 2019;Ruiz & Madariaga, 2018). The last event, with Mw 6.9, happened on 1 September 2020 and was part of a remarkable seismic and a-seismic sequence (Klein et al., 2021) (Figure 1). ...
We studied the along-dip influence of the Copiapó ridge subduction in the Atacama region, North-Central Chile by building a new seismicity catalog, including similar events and non-volcanic tremors (NVTs). We also obtained a 3-D tomographic model for P- and S-waves velocity (and the implied Vp/Vs ratio). We identify down-dip segmentation involving 4 distinct segments: a locked seismogenic zone hosting ordinary seismicity and clusters of similar events; a transition zone with NVTs and low seismicity; an aseismic zone with slow-slip events; and a deep zone with abundant intraslab seismicity. The velocity models show differences among these zones, with low velocity anomalies of Vp and Vs coinciding with aseismic slip zones, indicating the possible presence of fluids. Due to the spatial distribution along-strike and along-dip of the aseismic zones, we propose that these differences in seismogenic behavior are generated by subduction of the heterogeneous seamounts associated with the Copiapó ridge.
Located in the heart of the Bolivian orocline, the Cochabamba department and its two million inhabitants are exposed to frequent seismic activity. However, the tectonic structures causing these earthquakes remain poorly identified. Indeed, Bolivia’s national seismological network does not optimally cover the area and the hypocentral locations of local earthquakes are therefore subject to large uncertainties which hinder their association with specific faults. We established a regional network consisting of 11 broadband and short-period seismic stations, spaced approximately 20 km apart. This study highlights the initial 6-month seismic bulletin made by manual and automated deep-neural-network based seismic phase picking. We also test the network's ability to resolve focal mechanisms of moderate to small events with a combined inversion of waveforms and polarities. Our preliminary results document midcrustal microseismicity located in the Main Thrust fault shear zone, and in its hangingwall, in a region affected by tectonic slivers and transverse faults impacting the sedimentary cover. These outcomes provide fresh insights into the fault system’s seismogenic behavior and potential across the Bolivian orocline.
Recordings of earthquakes by a temporary deployment of 84 short period seismometers in northern Chile were used to derive regional 3D seismic velocity models for the Taltal segment. We used the Regressive ESTimator (REST) package for event detection and automatic onset estimation of P‐ and S‐wave arrival times to create an earthquake catalog with 23,985 hypocenters. We followed standard acceptability criteria (i.e., azimuthal gap and residual cutoff) to create a high‐quality data set and inverted for 3D Vp, Vs and Vp/Vs models using local earthquake tomography. Plots of hypocenters from the catalog, comprising 16,349 earthquakes, reveal active structures in the upper crust, dip changes along the slab and fracturing within the oceanic crust. Moreover, the wavespeed models illuminate anomalies in both the Nazca and South American plates that correlate with the observed seismicity distribution, including variations from low (1.75) to high (>1.80) Vp/Vs near the Atacama fault system on the coastline and the Domeyko Fault System in the forearc. The seismic velocity models also provide evidence for fluid circulation caused by the subducting Taltal ridge on the coast and partial melting feeding a volcanic complex close to the Andes. Finally, the observed low Vp/Vs ratios (∼1.75) are associated with copper mining operations in the area, suggesting that this kind of imaging can be used to characterize the distribution of potential ore deposits in the area.
In this study, Rfpy software is utilized to compute the receiver functions to map Moho in the Northern Chile region. To obtain the teleseismic waveforms within an epicentral distance of 30° to 90°, the Package makes use of the IRIS station database. The receiver functions over stations AC04 and AC05 indicate a low-velocity layer possible area at a shallower depth. Additionally, the delay periods of the Moho Ps phase at various different back azimuths are used to infer the anisotropy or dipping Moho. With the aid of H-K Stacking the receiver functions for Poisson’s ratio and Moho depth were also inverted. Under the AC07 station, which is situated in Caldera, Atacama, a higher Moho depth of 46 km is discovered. This results in a Poisson’s ratio of 0.24. Below the CO10 station, which is in Coquimbo, an extraordinary poisson ratio is found. Accordingly, a Moho depth of 25 km is discovered. Inverting the acquired P-wave receiver functions may effectively determine the S-wave velocity structure below each station. This would aid in enhancing the region’s crustal imaging. Additionally, harmonic decompositions of the receiver functions might be performed to examine the behavior of anisotropy.
In subduction zones, the forearc crust transitions from a highly fractured wedge near the trench to a less fractured basement near the volcanic arc. Here we study the role of wedge integrity on the frequency content of strong ground motion produced by subduction earthquakes in Northern Chile. Our data includes aftershocks from the 2014 Mw 8.1 Iquique earthquake and intermediate-depth seismicity, including the 2005 Mw 7.6 Tarapacá earthquake. We focus on the S-wave spectral decay parameter k obtained from strong ground motion in the frequency band 2-40 Hz, paying particular attention to how k varies with hypocentral distance. We report two distinct trends of k versus hypocentral distance. One trend applies to events near the trench, where we observe a rapid increase of k with hypocentral distance. Another trend applies to events near the bottom of the megathrust and greater depths, where we observe a relatively slow increase of k with hypocentral distance. We interpret this difference as resulting from a heterogeneous tectonic structure with lateral variations in anelastic attenuation (i.e., attenuation seems stronger in the fractured wedge near the trench). Our results improve our understanding of the role of high-frequency ground motion in seismic hazard analysis near the Chilean coastline. In addition, our results stress the need for a better characterization of anelastic attenuation in the Chilean subduction zone, specially close to the highly fractured wedge near the trench.
Intraslab earthquakes do not produce primary paleoseismic evidence at the Earth’s surface, making efforts to develop an event chronology challenging. However, the strong ground motion from intraslab events may initiate gravity-driven turbidity flows in subaqueous basins; the resulting deposits (turbidites) can provide a paleoseismic proxy if the conditions that initiate these flows are known. To better constrain the initiating conditions, we use two recent intraslab earthquakes in southcentral Alaska, the Mw 7.1 November 30, 2018, Anchorage and the Mw 7.1 January 24, 2016, Iniskin earthquakes, as calibration events. Through a multi-lake investigation spanning a range of shaking intensities and based on a combined geological and geophysical dataset, we document the occurrence, or absence, of earthquake-generated turbidity flows from these two earthquakes. The 2018 and 2016 earthquakes are recorded by centimeter-scale turbidites that can be differentiated from climatically generated deposits, as well as other seismic sources (i.e., the 1964 Alaska megathrust earthquake) based on deposit thickness, sedimentological properties, and deposit age. We show that a Modified Mercalli Intensity (MMI) of ~V-V1/2 is the minimum shaking intensity required to generate localized sediment remobilization from deltaic slopes, and a MMI of ~V1/2 is required to produce a deposit of sufficient thickness that a seismic origin can be confidently assigned. Deltaic slopes are the major source of remobilized sediment that record the 2018 and 2016 events, however sediment from non-tributary sourced basin slopes may become remobilized in steep-sloped, high sedimentation areas, and under elevated shaking intensity. The documentation of seismically generated deposits in quick succession (~2 years) with diagnostic features that can be assigned to the seismic source highlights the utility of using recent earthquakes as calibration events to investigate the subaqueous response to strong ground motion.
El gran terremoto de 1922 de Atacama es uno de los mayores eventos sísmicos del siglo XX en el límite de subducción entre las placas de Nazca y Sudamérica. Los principales daños provocados por el tsunami asociado ocurrieron en el litoral del norte de Chile. En la ciudad de Chañaral, una de la más afectadas, se alcanzó una cota máxima de inundación de 9 m sobre el nivel del mar y una inundación horizontal de 200 m. En este trabajo se aborda el estudio del registro geológico de este evento en el Parque Nacional Pan de Azúcar, 10 km al norte de esta ciudad portuaria, donde se describen formas erosivas y campos de bloques a cotas topográficas por encima de la zona de acción de temporales de invierno. Este trabajo resalta la necesidad de profundizar en el análisis de depósitos de grano grueso en sistemas costeros áridos para la identificación del registro de tsunamis, donde su potencial de preservación es bajo y sus evidencias tienden a aparecer enmascaradas en el paisaje litoral.
“Sea shock” (also “Seaquake”) is a label used to describe blows and/or vibrations experienced on board of vessels, triggered by earthquakes. This phenomenon received little attention in scientific literature, and its physics is still largely unknown. Indeed, the physical models proposed so far were rarely built on actual testimonies and fail to explain the variety of phenomena reported by witnesses. This paper gathers a never-assembled-before list of sea shocks events, for each listing description and relevant characteristics, and presents the first statistical analysis. It is confirmed that several different mechanisms comprise the phenomenon labelled “sea shock”, and it is found that the four physical models proposed so far account only for of the phenomena experienced on board ships. Moreover, it has been found that, contrary to what is commonly believed and usually stated, sea surface perturbations are rarely coupled with sea shocks, and that these phenomena cannot be felt far from the epicentre of the triggering earthquake. Although rare, sea shocks can be quite dangerous for vessels and the lack of a solid quantitative insight into their physics is concerning for the safety of ships and floating structures.
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.
We discussed the peculiarities of the seismic cycle in Aleutian subduction zone, characterized by an oblique subduction setting. It was shown that the orientation of the plate convergence vector relative to the subduction zone axis can have a significant impact on the preparation and occurrence of the largest earthquakes in subduction zones. In particular, from the analysis of the seismic activity occurring in the western part of the Aleutian island arc, it was found that the seismic cycles here are shorter than in the eastern part of the arc. It was revealed that the strongest earthquakes, repeating in the same areas of the western part of the Aleutian subduction zone, differ both in magnitude and length of the fault zone. Taking into account the oblique subduction setting, we proposed the keyboard model of the largest megathrust earthquakes generation as a mechanism potentially capable of explaining the reduction in the seismic cycle duration and noticeable differences in the spatial extent and localization of the fault zones of events with similar magnitudes occurring in the same segment of the western half of the Aleutian subduction zone.
Seismic hazards in subduction settings typically arise from megathrust, intraslab and crustal earthquake sources. Despite the frequent occurrence of intraslab earthquakes in subduction zones and their potential threat to communities, their long‐term recurrence behaviour is barely studied. Sedimentary sequences in lakes may register ground shaking from different seismic sources. This study investigates two long sediment cores (13 m and 16 m) from Skilak Lake, a proglacial lake in south‐central Alaska, to evaluate whether different seismic sources leave a distinct imprint. The sedimentary record shows a continuously varved sediment sequence, occasionally interrupted by turbidites, slump deposits and tephra beds. Turbidites and slump deposits were objectively identified using a statistical outlier analysis on varve thickness. The earthquake origin of these deposits was ascertained by resemblance with deposits induced by instrumentally recorded earthquakes (for example, 1964 CE Mw 9.2 megathrust and 1954 CE Mw 6.4 intraslab earthquakes) and correlation with multiple coeval landslide deposits on sub‐bottom profiles. The Skilak Lake record chronicles 19 earthquakes with moderate to very high confidence level in the past 1350 years. The sedimentary evidence of instrumentally‐recorded intraslab and megathrust earthquakes within the past 70 years demonstrates that not only megathrust earthquakes, but also past intraslab events are recorded. Although reported seismic intensities at Skilak Lake are comparable for the 1964 CE megathrust and the 1954 CE intraslab earthquakes, the long duration and low frequency content of seismic ground motion during megathrust earthquakes facilitate the triggering of multiple, voluminous landslides and the generation of megaturbidites. In contrast, the shorter duration and higher frequency source spectrum of intraslab earthquakes may only induce surficial slope remobilization and the generation of thinner turbidites. This study demonstrates that the sedimentary record of Skilak Lake has the potential to decipher multiple seismic sources, which opens possibilities for a comprehensive seismic hazard analysis for south‐central Alaska.
The 1922 Atacama Earthquake is one of the largest megathrust earthquakes of the 20th century in the limit between the Nazca and South American plates. The main damages of the associated tsunami occurred in the coast of northern Chile. In the city of Chañaral, one of the most affected, a maximum flood height of 9 m above sea level and a horizontal inundation of 200 m were reached. The aim of this study is to analyze the geological record of this event in Pan de Azúcar National Park, 10 km north of this port city, where erosive forms and boulder fields have been described evidencing the wave action on the coast at heights above the winter storms. This work highlights the need to deepen in the study of the coarse-grain deposits in arid coastal systems to identify the record of tsunamis where their preservation potential is low, and their evidence is masked on the littoral landscape.
A great earthquake (surface-wave magnitude, 7.8) occurred along the coast of central Chile on 3 March 1985, causing heavy
damage to coastal towns. Intense foreshock activity near the epicenter of the main shock occurred for 11 days before the earthquake.
The aftershocks of the 1985 earthquake define a rupture area of 170 by 110 square kilometers. The earthquake was forecast
on the basis of the nearly constant repeat time (83 ± 9 years) of great earthquakes in this region. An analysis of previous
earthquakes suggests that the rupture lengths of great shocks in the region vary by a factor of about 3. The nearly constant
repeat time and variable rupture lengths cannot be reconciled with time- or slip-predictable models of earthquake recurrence.
The great earthquakes in the region seem to involve a variable rupture mode and yet, for unknown reasons, remain periodic.
Historical data suggest that the region south of the 1985 rupture zone should now be considered a gap of high seismic potential
that may rupture in a great earthquake in the next few tens of years.
Making use of data of tsunamis which occurred in the region of South America, frequencies of tsunamis generations and of tsunami which propagated to Japan are investigated. Refraction diagrams and the distributions of tsunami height for seven tsunamis which propagated across the Pacific Ocean are shown. Especially, these phenomena along the coast of Japan are shown in detail with the aid of mareograms. The spectral analysis of records obtained in Japan is made for five tsunamis.
Focal depth determination for shallow ( less than equivalent to 60 km) intermediate-size M//s approximately equals 6. 5 to 7. 5) earthquakes continues to be problematic. For these events, the depth phases (i. e. , pP and sP) are not distinctly separated in time, and thus synthetic modeling of wave shapes becomes necessary. Unfortunately, the uniqueness of the solution is not easily determined because of the trade-off between various focal parameters. We have investigated the trade-off between depth and source function both theoretically and experimentally. The changes in the Green's function caused by an incorrect assumed depth can almost always be compensated by changes in the source function, producing the same 'observed' seismogram. Thus, there is considerable trade-off between depth and the source function.
The March 3, 1985 central Chile earthquake (Ms = 7.8) ruptured a well studied seismic gap along the Chilean subduction zone. The epicenter of this event is located near the center of an approximately 300 km long region which ruptured in a great event in 1906 (Mw = 8.2). The northern portion of the 1906 zone has since ruptured in 1971 (Ms = 7.5) and 1973 (Ms = 6.7). The rupture history of the 1985 central Chile earthquake is determined from the deconvolved source functions of body waves (both P and PP). The source functions show one major pulse of moment release, the onset of which occurs about 16 seconds after an initial small pulse. The azimuthal directivity in the P waves indicates that the rupture front propagated from the epicenter southward and that the moment release of the major pulse is concentrated in a region between the epicenter and approximately 75 km south of the epicenter. It is concluded that the region of high moment release is the dominant asperity and was loaded by slip in the relatively weaker northern region.
Seismic source and wave propagation theories allow seismologists to compute long period synthetic seismograms that commonly match event subtle details of observed waveshapes, but the overal trace amplitudes must be scaled by arbitrary factors to emphasize the excellent waveshape correspondence. This unexplained amplitude scatter causes considerable difficulties when formally inverting observed waveforms for seismic source parameters. Omnilinear inversion simultaneously determines the linear model parameters and trace scaling factors to minimize the mismatch between observed and synthetic seismograms. Omnilinear inversion is applied to source time function deconvolution from long period P and SH waves. A simulation with synthetic data shows that omnilinear inversion finds the proper scaling factors, and the source time function and focal depth are better determined than for ``standard'' linear inversion. Omnilinear inversion is then applied to a data set of seven P waves and one SH wave from the underthrusting earthquake of July 5, 1968 (MS 6.6) in northern Honshu, Japan. Omnilinear inversion produces a best-fit focal depth of 36 km with a single pulse time function of 6 s duration and seismic moment of 4×1018 N m (MW 6.4). This focal depth supports the notion that the seismically coupled plate interface extends no deeper than 40 km in northern Honshu.
The seismic potential of the Chilean and southern Peruvian margins of South America is reevaluated to delineate those areas or segments of the margin that may be expected to experience large or great interplate earthquakes within the next 20 years (1984–2004). Long-term estimates of seismic potential (or the conditional probability of recurrence within a specified period of time) are based on (1) statistical analysis of historic repeat time data using Weibull distributions and (2) deterministic estimates of recurrence times based on the time-predictable model of earthquake recurrence. Both methods emphasize the periodic nature of large and great earthquake recurrence, and are compared with estimates of probability based on the assumption of Poisson-type behavior. The estimates of seismic potential presented in this study are long-term forecasts only, as the temporal resolution (or standard deviation) of both methods is taken to range from ±15% to ±25% of the average or estimated repeat time. At present, the Valparaiso region of central Chile (32°–35°S) has a high potential or probability of recurrence in the next 20 years. Coseismic uplift data associated with previous shocks in 1822 and 1906 suggest that this area may have already started to rerupture in 1971–1973. Average repeat times also suggest this area is due for a great shock within the next 20 years. Flanking segments of the Chilean margin, Coquimbo-Illapel (30°–32°S) and Talca-Concepcion (35°–38°S), presently have poorly constrained but possibly quite high potentials for a series of large or great shocks within the next 20 years. In contrast, the rupture zone of the great 1960 earthquake (37°–46°S) has the lowest potential along the margin and is not expected to rerupture in a great earthquake within the next 100 years. In the north, the seismic potentials of the Mollendo-Arica (17°–18°S) and Arica-Antofagasta (18°–24°S) segments (which last ruptured during great earthquakes in 1868 and 1877) are also high, but poorly constrained.
An updated compilation of earthquake locations and focal mechanism solutions from the International Seismological Centre and Preliminary Determination of Earthquakes is the basis of a comprehensive study of the geometry of the Wadati-Benioff zone beneath western South America. The new data support previous mapping of a sharp flexure rather than a tear in the subducted Nazca plate beneath southern Peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with about 30 deg dip at latitude 33 deg S. In contrast, the transition from 30 deg slab dip beneath Bolivia to a nearly horizontal dip in the region between 28 deg S-32 deg S is more gradual, occurring over several hundred kilometers of along-strike distance between 20 deg S and 32 deg S. This southward flattening corresponds to a broadening of a horizontal, benchlike part of the subducted plate formed between 100 and 125 km depth. The transition in continental tectonic style near 27 deg S-28 deg S, from a wide, volcanically active plateau to a narrow, nonvolcanic cordillera, appears not to be associated with the main slab flattening, which begins to the north of these latitudes, but with a more abrupt change in curvature of the subducted slab, from convex upward to concave upward, immediately below the plate boundary interface. The concept of Gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the South American plate.
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.
The historic record for the central Peru subduction zone suggests significant variations in the earthquake size during the last 400 years. During this century there have been four great underthrusting earthquakes along the central Peru seismic zone. From the north to south these are the 17 October 1966 (M{sub w} = 8.1), 24 May 1940 (M â¼ 8), 3 October 1974 (Mâ = 8.1), and 24 August 1942 (M â¼ 8.2) earthquakes. Modified Mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 October 1746 and 20 October 1687 earthquakes. The 1746 earthquake had maximum intensity values between 9° and 13°S while the 1687 event had maximum values between 12° and 14°S suggesting that the two events failed different segments of the subduction zone. The authors find that the 1746 event occurred along the segment that includes both 1940 and 1966 earthquakes. The size of the 1746 event is estimated to M{sub w} â¼ 8.8 based on the ratio of near-field tsunami heights for the 1746 and 1966 earthquakes. The 1687 earthquake probably ruptured the 1974 segment as well as the adjacent segment to the south where there is at present a gap between the 1942 and 1974 rupture zones. The size of the 1687 event is estimated to be M{sub w} â¼ 8.7 based on both far-field and near-field tsunami height ratios of the 1687 and 1974 events. Both 1746 and 1687 earthquakes appear to be much larger than the events of this century. In contrast to the simple, single asperity nature of the 20th century earthquakes, these older and larger events may represent multiple-asperity ruptures along the Peru subduction zone. Hence, variations in the mode of earthquake rupture from cycle to cycle along the central Peru seismic zone may explain the significant difference in earthquake size during the last 400 years.
The seismic potential for 96 segments of simple plate boundaries around the circum-Pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5, 10, and 20 years (i.e., 1989–1994, 1989–1999 and 1989–2009). This study represents the first probabilistic summary of seismic potential on this scale, and involves the comparison of plate boundary segments that exhibit varying recurrence times, magnitudes, and tectonic regimes. Presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments, as well as accounting for individual variations in recurrence time along a specific fault segment, and uncertainties in the determination of the average recurrence time.
The definition of specific segments along simple plate boundaries relies on the mapping of earthquake rupture zones as defined by the aftershock distributions of prior large and great earthquakes, and historic descriptions of felt intensities and damage areas. The 96 segments are chosen to represent areas likely to be ruptured by “characteristic” earthquakes of a specified size or magnitude. The term characteristic implies repeated breakage of a plate boundary segment by large or great earthquakes whose source dimensions are similar from cycle to cycle. This definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single, larger event. Conversely, a segment may also break in a series of smaller ruptures.
Estimates of recurrence times and conditional probabilities for characteristic earthquakes along segments of simple plate boundaries are based on 1) the historic and instrumental record of large and great earthquake occurrence; 2) paleoseismic evidence of recurrence from radiometric dating of Holocene features produced by earthquakes; 3) direct calculations of recurrence time from the size of the most recent characteristic event and the long-term rates of plate motion assuming the validity of the time-predictable model for earthquake recurrence; and 4) the application of a lognormal distribution for the recurrence times of large and great earthquakes.
Time-dependent estimates of seismic potential are based on a physical model of earthquake occurrence which assumes that the probability for an earthquake is low immediately following the occurrence of a characteristic earthquake and increases with time as the stress on the fault segment recovers the stress drop of the event. This study updates earlier work on seismic gaps by explicitly including both recurrence time information and the temporal proximity to the next event as factors in describing earthquake hazards.
Currently, 11 out of 96 regions have a high (i.e., ≥ 50%) probability of recurrence during the next 10 years and are characterized by either fairly short (i.e., less than 30–40 years) recurrence times or long elapsed times relative to the average recurrence time. The majority of these segments are located in the southwest Pacific (Vanuatu, New Guinea, and Tonga). When a longer time window is considered (e.g., 20 years or 1989–2009), 30 out of 96 regions have a high potential. Many of these regions are located near areas of high population density. These determinations do not preclude rupture of other fault segments, with less than a 50% chance in 10 or 20 years, or large and great earthquakes in areas we have not studied in detail. While this study has summarized the seismic potential for a large number of regions around the circum-Pacific, there are still a number of geographic and seismotectonic regions that need to be considered, including Indonesia, the Philippines, New Zealand, and the countries that surround the Caribbean basin.
Earthquakes under coastal northern Chile, from 18oS to 33oS occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a couple of Ms approx= 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40km depth, while the 1965 March 28 earthquake has a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P- and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 X 1027 dyne cm from both P- and surface waves. The source area for this event is relatively small compared to that of other underthust events with similar moments. -Authors
We present the results of a systematic study of events with Ms > 6 in northern Chile (20–33°S), for the period between 1963 and 1971. Medium to large earthquakes near the coast of this region are of three types: (1) Interplate events at the interface between the downgoing slab and the overriding South American plate. These events can be very large reaching magnitudes greater than 8. (2) Intra-plate earthquakes 20–30 km inside the downgoing slab. They have fault mechanisms indicating extension along the dip of the slab and may have magnitudes up to 7.5. (3) Less frequent, Ms∼ 6 events that occur near the top of the downgoing slab and have thrust mechanisms with an almost horizontal E-W compressional axis. This type of mechanism is very different from that of the events of type 1 which are due to shallow dipping reverse faulting. There is a rotation of about 30° of the compressional axis in the vertical plane between events of types (1) and (3). Three groups of events near 32.5°, 25.5° and 21°s were studied in detail. Depth and mechanisms were redetermined by P-wave modelling and relative locations were obtained by a master event technique. Near 32.5°S, only events of types 1 and 2 were found in the time period of this study. At the two other sites, the three types of events were identified. This shows clearly that there are compressive stresses at the top of the slab and extension at the centre, a situation which is usually found in the areas where a double Benioff-zone has been identified in the seismicity.
—The 12 November 1996 M
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7.7 Peru subduction zone earthquake occurred off the coast of southern Peru, near the intersection of the South American trench and the highest topographical point of the subducting Nazca Ridge. We model the broadband teleseismic P-waveforms from stations in the Global Seismic Network to constrain the source characteristics of this subduction zone earthquake. We have analyzed the vertical component P-waves for this earthquake to constrain the depth, source complexity, seismic moment and rupture characteristics. The seismic moment determined from the nondiffracted P-waves is 3–5 × 1020 N·m, corresponding to a moment magnitude M
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of 7.6–7.7. The source time function for the 1996 Peru event has three pulses of seismic moment release with a total duration of approximately 45–50 seconds. The largest moment release occurs at approximately 35–40 seconds and is located ∼90km southeast of the rupture initiation. Approximately 70% of the seismic moment was released in the third pulse.¶We find that the 1996 event reruptured part of the rupture area of the previous event in 1942. The location of the 1996 earthquake corresponds to a region along the Peru coast with the highest uplift rates of marine terraces. This suggests that the uplift may be due to repeated earthquakes such as the 1996 and 1942 events.
In order to obtain a uniform magnitude catalogue, surface-wave magnitudes Ms and broad-band body-wave magnitudes mB have been determined for large shallow earthquakes from 1904 to 1980. In making the catalogue homogeneous, the author consistently adheres to the original definitions of Ms and mB given by Gutenberg (1945) and Gutenberg and Richter (1956). The determinations of Ms and mB are all based on the amplitude and period data listed in Gutenberg and Richter's unpublished notes, bulletins from stations worldwide, and other basic information. mB is measured on broad-band instruments in periods of ∼8 s. Consistency of the magnitude determinations from these different sources is carefully checked in detail. More than 900 shallow shocks of magnitude 7 and over are catalogued. The meaning of the magnitude scales in various catalogues is examined in terms of Ms and mB. Most of the magnitudes listed by Gutenberg and Richter (1954) in their “Seismicity of the Earth” are basically Ms for large shocks shallower than 40 km, but are basically mB for large shocks at depths of 40–60 km. The surface-wave magnitudes given by “Earthquake Data Reports” are higher than Ms by 0.2 unit whether the combined horizontal amplitude or the vertical amplitude is used. mB and the currently used 1 s body-wave magnitude are measured at different periods and should not be directly compared.
Moment tensor source mechanisms for the largest twenty-nine shallow earthquakes from 1981 (M_s ≥ 6.5) have been determined using long-period Rayleigh-wave spectra and P-wave first-motions, recorded by the International Deployment of Accelerograms (IDA), Global Digital Seismic Network (GDSN), and World-Wide Standardised Seismic Network (WWSSN). These are compared with the centroid moment tensor mechanisms presented by Dziewonski and Woodhouse, which were obtained in a contrasting manner. In most cases, the source is well represented by a single double-couple mechanisms, with good agreement of scalar moment and orientation between the two studies. Apparent mismatches for other cases may be interpreted in terms of source complexity.
We emphasise the objectivity of comparing moment tensor elements; apparent discrepancies in the moment and orientation of double-couple fault plane solutions are usually due to differences in the Mxz and Myz moment tensor elements, which are poorly constrained by long-period data.
In order to arrive at a uniform basis for a study of the global seismicity, the surface-wave magnitude Ms of shallow earthquakes for 1898–1917 is newly determined by using maximum amplitudes of surface waves recorded by undamped Milne and Omori seismographs. The effective gain of these instruments needs to be calibrated with great care, because of the lack of a damping device. The calibration of Milne instruments is based on a comparison of amplitudes measured by Milne instruments with those by more modern, damped instruments. The effective gain varies from 7 to 20 in the range of Ms from 6 to 8.5. The reported amplitudes for the years 1910–1912 are twice as large as the standard, and need to be corrected. By applying these results to a wealth of the Milne data, Ms is determined for a large number of earthquakes between 1898 and 1912. To supplement the data for 1913–1917, records of undamped Omori seismographs are used. For the entire period from 1898 to 1917, 308 shallow earthquakes with are listed here. Both this list and the previous catalogue of Abe (1981) for 1918–1980 provide a catalogue of large shallow earthquakes much more uniform than any previously established. Between 1899 and 1910 the annual frequency of shocks with is 20, which is twice as large as the annual average of 10 during 1911 and 1980.
Centroid-moment tensor solutions are presented for 175 earthquakes that occurred during the fourth quarter of 1984. The solutions are obtained using corrections for aspherical Earth structure.
Making use of data of tsunamis which occurred in the region of South America, frequencies of tsunami generation and of tsunami which propagated to Japan are investigated. Refraction diagrams and the distributions of tsunami height for seven tsunamis which propagated across the Pacific Ocean are shown. Especially, these phenomena along the coast of Japan are shown in detail with the aid of mareograms. The spectral analysis of records obtained in Japan is made for five tsunamis. The generating area of the Chilean tsunami of 1960 is estimated by means of an inverse refraction diagram. This source includes the area of aftershock activity extending about 800 km in an elongated shape.|南米太平洋沿岸で発生した歴史津波の資料によると,波源付近の波高が10m程度の津波によつて日本,ハワイは数回相当な被害を受けている.1900年以後は我が国においても検潮儀によつて7個の津波が観測された.このうち1960年11月20日のペル沖で起きた地震(M=6.75)によつて,微小な津波を観測した例もある.マグニチュードが8.3以上の地震に伴なう津波は,例外なく全太平洋沿岸地域で観観されているが,8.0以下の地震による過去の津波は,日本沿岸で波高50cm以下の微弱なものである.
The 1977 Caucete (San Juan) earthquake considered in the present paper occurred near the Sierra Pie de Palo in the Sierras Pampeanas tectonic province of western Argentina. In the study reported, coseismic surface deformation is combined with seismic observations (main shock and aftershocks, both teleseismic and local data) to place constraints on the geometry and slip of the main fault responsible for the 1977 earthquake. The implications of the 1977 event for long-term crustal shortening and earthquake recurrence rates in this region are also discussed. It is concluded that the 1977 Caucete earthquake was accompanied by more than 1 m of vertical uplift.
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