Evidence for earthquake interaction in central Chile: The July 1997September 1998 sequence (original) (raw)
Related papers
Tectonophysics, 2002
The 1943 Illapel seismic gap, central Chile (30–32°S), was partially reactivated in 1997–1998 by two distinct seismic clusters. On July 1997, a swarm of offshore earthquakes occurred on the northern part of the gap, along the coupled zone between Nazca and South American plates. Most of the focal mechanisms computed for these earthquakes show thrust faulting solutions. The July 1997 swarm was followed on October 15, 1997 by the Punitaqui main event (Mw=7.1), which destroyed the majority of adobe constructions in Punitaqui village and its environs. The main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault. This event is located inland at 68-km depth and it is assumed to be within the oceanic subducted plate, as are most of the more destructive Chilean seismic events. Aftershocks occurred mainly to the north of the Punitaqui mainshock location, in the central-eastern part of the Illapel seismic gap, but at shallower depths, with the two largest showing thrust focal mechanisms. The seismicity since 1964 has been relocated with a master event technique and a Joint Hypocenter Determination (JHD) algorithm, using teleseismic and regional data, along with aftershock data recorded by a temporary local seismic network and strong motion stations. These data show that the 1997 seismic clusters occurred at zones within the Illapel gap where low seismicity was observed during the considered time period. The analysis of P and T axis directions along the subduction zone, using the Harvard Centroid Moment Tensor solutions since 1977, shows that the oceanic slab is in a downdip extensional regime. In contrast, the Punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate, which becomes subhorizontal at depths of about 100 km. Analog strong motion data of the Punitaqui main event show that the greatest accelerations are on the horizontal components. The highest amplitude spectra of the acceleration is in the frequency band 2.5–10 Hz, in agreement with the energy band responsible for the collapsed adobe constructions. The isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock, the poor quality of adobe houses and probably local site amplification effects.
The Seismic Sequence of the 16 September 2015Mw 8.3 Illapel, Chile, Earthquake
Seismological Research Letters, 2016
On 16 September 2015, the M w 8.3 Illapel, Chile, earthquake broke a large area of the Coquimbo region of north-central Chile. This area was well surveyed by more than 15 high-rate Global Positioning System (GPS) instruments, installed starting in 2004, and by the new national seismological network deployed in Chile. Previous studies had shown that the Coquimbo region near Illapel was coupled to about 60%. After the M w 8.8 Maule megathrust earthquake of 27 February 2010, we observed a large-scale postseismic deformation, which resulted in a strain rate increase of about 15% in the region of Illapel. This observation agrees with our modeling of viscous relaxation after the Maule earthquake. The area where upper-plate GPS velocity increased coincides very well with the slip distribution of the Illapel earthquake inverted from GPS measurements of coseismic displacement. The mainshock started with a small-amplitude nucleation phase that lasted 20 s. Backprojection of seismograms recorded in North America confirms the extent of the rupture, determined from local observations, and indicates a strong directivity from deeper to shallower rupture areas. The coseismic displacement shows an elliptical slip distribution of about 200 km × 100 km with a localized zone where the rupture is deeper near 31.3°S. This distribution is consistent with the uplift observed in some GPS sites and inferred from field observations of bleached coralline algae in the Illapel coastal area. Most of aftershocks relocated in this study were interplate events, although some of the events deeper than 50 km occurred inside the Nazca plate and had tension (slab-pull) mechanisms. The majority of the aftershocks were located outside the 5 m contour line of the inferred slip distribution of the mainshock.
Complex distribution of large thrust and normal fault earthquakes in the Chilean subduction zone
Geophysical Journal International, 1983
We present the results of a systematic study of events with M , > 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,M,-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 Pwave 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.
Slab-pull and slab-push earthquakes in the Mexican, Chilean and Peruvian subduction zones
Physics of the Earth and Planetary …, 2002
We studied intermediate depth earthquakes in the Chile, Peru and Mexican subduction zones, paying special attention to slab-push (down-dip compression) and slab-pull (down-dip extension) mechanisms. Although, slab-push events are relatively rare in comparison with slab-pull earthquakes, quite a few have occurred recently. In Peru, a couple slab-push events occurred in 1991 and one slab-pull together with several slab-push events occurred in 1970 near Chimbote. In Mexico, several slab-push and slab-pull events occurred near Zihuatanejo below the fault zone of the 1985 Michoacan event. In central Chile, a large M = 7.1 slab-push event occurred in October 1997 that followed a series of four shallow M w > 6 thrust earthquakes on the plate interface. We used teleseismic body waveform inversion of a number of M w > 5.9 slab-push and slab-pull earthquakes in order to obtain accurate mechanisms, depths and source time functions. We used a master event method in order to get relative locations. We discussed the occurrence of the relatively rare slab-push events in the three subduction zones. Were they due to the geometry of the subduction that produces flexure inside the downgoing slab, or were they produced by stress transfer during the earthquake cycle? Stress transfer can not explain the occurence of several compressional and extensional intraplate intermediate depth earthquakes in central Chile, central Mexico and central Peru. It seemed that the heterogeneity of the stress field produced by complex slab geometry has an important influence on intraplate intermediate depth earthquakes.
TheMw8.1 2014 Iquique, Chile, seismic sequence: a tale of foreshocks and aftershocks
Geophysical Journal International
, M w 8.1 Iquique (Chile) earthquake struck in the Northern Chile seismic gap. With a rupture length of less than 200 km, it left unbroken large segments of the former gap. Early studies were able to model the main rupture features but results are ambiguous with respect to the role of aseismic slip and left open questions on the remaining hazard at the Northern Chile gap. A striking observation of the 2014 earthquake has been its extensive preparation phase, with more than 1300 events with magnitude above M L 3, occurring during the 15 months preceding the main shock. Increasing seismicity rates and observed peak magnitudes accompanied the last three weeks before the main shock. Thanks to the large data sets of regional recordings, we assess the precursor activity, compare foreshocks and aftershocks and model rupture preparation and rupture effects. To tackle inversion challenges for moderate events with an asymmetric network geometry, we use full waveforms techniques to locate events, map the seismicity rate and derive source parameters, obtaining moment tensors for more than 300 events (magnitudes M w 4.0-8.1) in the period 2013 January 1-2014 April 30. This unique data set of fore-and aftershocks is investigated to distinguish rupture process models and models of strain and stress rotation during an earthquake. Results indicate that the spatial distributions of foreshocks delineated the shallower part of the rupture areas of the main shock and its largest aftershock, well matching the spatial extension of the aftershocks cloud. Most moment tensors correspond to almost pure double couple thrust mechanisms, consistent with the slab orientation. Whereas no significant differences are observed among thrust mechanisms in different areas, nor among thrust foreshocks and aftershocks, the early aftershock sequence is characterized by the presence of normal fault mechanisms, striking parallel to the trench but dipping westward. These events likely occurred in the shallow wedge structure close to the slab interface and are consequence of the increased extensional stress in this region after the largest events. The overall stress inversion result suggests a minor stress rotation after the main shock, but a significant release of the deviatoric stress. The temporal change in the distribution of focal mechanisms can also be explained in terms of the spatial heterogeneity of the stress field: under such interpretation, the potential of a large megathrust earthquake breaking a larger segment offshore Northern Chile remains high.
Source characteristics of historic earthquakes along the central Chile subduction zone
Journal of South American Earth Sciences, 1998
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 (Ms=8.3), April 6, 1943 (Ms=7.9), December 1, 1928 (Ms=8.0) and January 25, 1939 (Ms=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 s. 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.Se analizan cuatro grandes terremotos, ocurridos a lo largo de la zona de subduccio ́n en Chile central. Estos eventos sismicos, de norte a sur, son: Nov. 22, 1922, en Copiapo ́-Vallenar, Ms=8.3; Abril 6, 1943 en Illapel-Combarbala ́, Ms=7.9; Dic. 1, 1928, en Talca, Ms=8.0 y Enc. 25, 1939, en Chilla ́n, Ms=7.8.Las formas de onda y polaridad del primer arribo de las ondas P a diferentes estaciones indican que el mecanismo focal de tres de estos terremotos es consistente con el fallamiento inverso de bajo a ́ngulo producido por el desplazamiento de la placa de Nazca bajo la placa Sudamericana. Sin embargo, el terremoto de 1939 no presenta estas caracteristicas sino que es producto de un fallamiento normal que ocurre en el interior de la placa que subducta.El terremoto de 1922 es, sin duda alguna, el evento ma ́s grande de esta serie. La funcio ́n de tiempo en la fucntc es compleja, de 75 s de duracio ́n, y se compone de tres pulsos que representan diferentes etapas de liberacio ́n de momento. La deformacio ́n en el fondo ocea ́nico asociada a este evento produjo como consecuencia un tsunami que alcanzo ́ una altura de 7m en la costa de la regio ́n epicentral (Caldera-Coquimbo), en tanto que en Japo ́n alcanzo ́ una altura de 30--70cm. El evento de 1943 presenta una funcio ́n de tiempo en la fuente relativamente simple que se compone de so ́lo un pulso de liberacio ́n de momento de 24s de duracio ́n. El tsunami generado alcanzo ́ localmente una altura de 4m y 10--30cn en Japo ́n. Por otra parte, el terremoto de 1928 tambie ́n presenta una funcio ́n de tiempo simple, de 28s de duracio ́n. Para este ultimo terremoto, tanto las re ́plicas como las mayores intensidades se localizan hacia el sur del epicentro indicando la existencia de un frente de ruptura que se propaga hacia el sur. La mayor liberacio ́n de momento se localiza a unos 50--80km hacia el sur del inicio de la ruptura, pero au ́n fuera de la regio ́n involucrada en el terremoto de 1939. El tsunami producido fue del orden de 1m frente a las costas de la regio ́n epicentral. Por otra parte, el terremoto de 1939 no ocurre sobre una falla inversa de bajo a ́ngulo como los anteriores, sino que sobre una falla normal. Nuestra mejor solucio ́n corresponde a un fallamiento producto de un campo tensional, a unos 80--100km de profundidad, con dos pulsos de liberacio ́n de momento y de una duracio ́n total cercana a los 60s. Las altas intensidades, ausencia de tsunami y la localizacio ́n epicentral hacia el interior del continente son consistenters con un evento intraplaca al interior de la placa que subducta. El evento de 1939 ha sido claramenta ma ́s destructivo que otros de taman ́o similar o mayores. En parte, esto puede ser debido a las caracteristicas inherentes de los terremotos intraplaca como tambie ́n a la mayor amplificacio ́n del movimiento del suelo en los valles del centro-sur de Chile.
Geophysical Journal International, 1995
The stress distribution along the subducting Nazca plate in northern Chile is analysed using focal mechanism solutions obtained from the inversion of long-period P, SV, and SH waveforms of 15 earthquakes (mb 2 5.5), and from 212 events with reported focal mechanisms, which occurred between 1962 and 1993. A joint hypocentral determination was carried out to control the depth of 261 events (mh 2 5.0) recorded at teleseismic distances. A change from tensional to compressional stress field along the upper part of the subducting slab is associated with the maximum depth extent of the coupled zone. This change occurs in northern Chile at -200-250 km from the trench, at depths of -60 f 10 km. This depth is larger than the maximum depth observed for the thrusting interplate events (40 + 10 km), probably meaning that, at depths of between 40 and 60 km, large low-dip angle thrust events do not nucleate. Seismic slip, however, probably extends down to 40 km in depth. The shallow dip angle (up to 60 km in depth) of the Wadati-Benioff zone does not show variations along the strike of the trench. However, a gradual southward flattening of the slab is observed at distances greater than 200-250 km from the trench. This change, observed from about 21"S, could be associated with a younger and probably more buoyant lithosphere than that observed to the north of this latitude. There are two gaps located between the three main clusters of seismicity; these gaps are clearly not related to detachments in the descending litosphere. The first cluster is located in and beneath the seismogenic interplate contact, and is characterized by reverse and thrust faulting events over a scarce tensional activity. In the second cluster, the compressional seismicity is scarce for teleseismic events and is located beneath the normal faulting events. The third cluster corresponds to tensional events. Therefore, these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab.
The 1 April 2014 Iquique, Chile, M 8.1 earthquake rupture sequence
Geophysical Research Letters, 2014
On 1 April 2014, a great (M w 8.1) interplate thrust earthquake ruptured in the northern portion of the 1877 earthquake seismic gap in northern Chile. The sequence commenced on 16 March 2014 with a magnitude 6.7 thrust event, followed by thrust-faulting aftershocks that migrated northward~40 km over 2 weeks to near the main shock hypocenter. Guided by short-period teleseismic P wave backprojections and inversion of deepwater tsunami wave recordings, a finite-fault inversion of teleseismic P and SH waves using a geometry consistent with long-period seismic waves resolves a spatially compact large-slip (~2-6.7 m) zone located~30 km downdip and~30 km along-strike south of the hypocenter, downdip of the foreshock sequence. The main shock seismic moment is 1.7 × 10 21 N m with a fault dip of 18°, radiated seismic energy of 4.5-8.4 × 10 16 J, and static stress drop of~2.5 MPa. Most of the 1877 gap remains unbroken and hazardous.