Crustal extensional faulting triggered by the 2010 Chilean earthquake: The Pichilemu Seismic Sequence

We report a sequence of crustal quakes that began after the M w = 8.8 thrust‐subduction Maule earthquake that affected the Central Chile margin on 27 February 2010. This activity lasted by several months, having the most important events on 11 March 2010 ( M w = 6.9 and M w = 7.0) with normal focal mechanisms. Seismicity shows a rupture oriented along a NW‐striking and SW‐dipping normal fault from the surface down to the interplate contact. Seismicity can be correlated with neotectonics extensional structures similarly oriented in the region, which have coexisted with NNE‐SSW reverse faults since the late Pliocene, even though both have older periods of activity since the Paleozoic. This crustal rupture would have been triggered by the high Coulomb stress change produced by the Maule earthquake, enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high Vp/Vs ratio. The occurrence of relevant neotectonic activity in coincidence with short‐term deformation suggests a relationship with long‐term tectonic features of this region, which would have been acting as a barrier during the interseismic period, increasing the strain accumulation and triggering contractional faulting in the crust, as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting. Crustal faulting in Pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the Maule earthquake.