Large extensional aftershocks in the continental forearc triggered by the 2010 Maule earthquake, Chile

Abstract

Artículo de publicación ISIThe Mw 8.8 Maule earthquake occurred off the coast of central Chile on 2010 February 27 and was the sixth largest earthquake to be recorded instrumentally. This subduction zone event was followed by thousands of aftershocks both near the plate interface and in the overriding continental crust. Here, we report on a pair of large shallow crustal earthquakes that occurred on 2010 March 11 within 15 min of each other near the town of Pichilemu, on the coast of the O’Higgins Region of Chile. Field and aerial reconnaissance following the events revealed no distinct surface rupture. We infer from geodetic data spanning both events that the ruptures occurred on synthetic SW-dipping normal faults. The first, larger rupture was followed by buried slip on a steeper fault in the hangingwall. The fault locations and geometry of the two events are additionally constrained by locations of aftershock seismicity based on the International Maule Aftershock Data Set. The maximum slip on the main fault is about 3 m and, consistent with field results, the onshore slip is close to zero near the surface. Satellite radar data also reveal that significant aseismic afterslip occurred following the two earthquakes. Coulomb stress modelling indicates that the faults were positively stressed by up to 40 bars as a result of slip on the subduction interface in the preceding megathrust event; in other words, the Pichilemu earthquakes should be considered aftershocks of the Maule earthquake. The occurrence of these extensional events suggests that regional interseismic compressive stresses are small. Several recent large shallow crustal earthquakes in the overriding plate following the 2011 Mw 9.0 Tohoku-Oki earthquake in Japan may be an analogue for the triggering process at Pichilemu.GEER Fugro William Lettis Associate

Comparison of postseismic afterslip models with aftershock seismicity for three subduction-zone earthquakes: Nias 2005, Maule 2010 and Tohoku 2011

“…A concentration of thrust-faulting aftershocks was also observed between the main patches of co-seismic slip (Agurto et al 2012). In the northern part of the rupture zone, intense seismicity also occurred in the overriding plate in the Pichilemu sequence, which started on 2010 March 11 with a M w = 6.9 and M w = 7.0 normal-faulting aftershock doublet (Farías et al 2011;Ryder et al 2012;Lieser et al 2014). Notably, the largest aftershock (M w = 7.4) occurred in the outer rise adjacent to the southernmost limit of the rupture approximately 90 min after the mainshock.…”

Cambios en el relieve generados como consecuencia del terremoto Mw = 8,8 del 27 de febrero de 2010 en el centro-sur de Chile

“…Las réplicas en el área de ruptura se concentraron entre marzo y mayo de 2010, decayendo progresivamente. El 11 de marzo de 2011, se produjo un nuevo evento sísmico Mw = 6,9 en el área de Pichilemu (34,4º S), que reactivó una falla normal cortical de orientación NW-SE descendiendo el bloque sur Ryder et al, 2012) provocando una subsidencia mayor al alzamiento cosísmico del 27 de febrero de 2010. Esta falla se reactivó como consecuencia del cambio de esfuerzos en la placa Sudamericana luego de la ruptura del 27 de febrero de 2010.…”

Improving seismotectonics and seismic hazard assessment along the San Ramón Fault at the eastern border of Santiago city, Chile

“…Almost all the cities in the Chilean country have experienced a megaor large-thrust earthquake in the last century. In addition to that, seismic hazard is also associated with large inland intermediate-depth earthquakes like the M s 8.3 Chillán earthquake in 1939 (Campos and Kausel 1990), less recorded shallow crustal events which have occurred mainly along the main Andes Cordillera (M w 6.3 Las Melosas earthquake in 1958, Alvarado et al 2009;Sepulveda et al 2008;Legrand et al 2007; M w 6.3 Aroma earthquake in 2001, Farías et al 2010; M w 6.5 Curicó earthquake in 2004) and large magnitude aftershocks like those following the 2010 Maule earthquake which occurred near Pichilemu in March 11th, 2010 (Farías et al 2011;Ryder et al 2012). Reliable assessment and mitigation of seismic hazards along active tectonic zones like the Chilean subduction margin are therefore challenging problems with clear economic and societal implications.…”