Crustal Normal Faulting Triggered by the Mw=8.8 Maule Megathrust Subduction Earthquake in Central Chile

The Mw=8.8 Chilean megathrust earthquake of 27 February 2010 ruptured an interplate zone extending nearly 500 km along the coast of central Chile. Contrary to expectations, there has not been any major aftershock associated with the main rupture. Instead, the largest postseismic events thus far, a Mw=6.9 and a Mw=7.0 that occurred within 15 minutes of each other on 11 March 2010, are located in the upper plate, within the continental crust near the northern edge of the megathust rupture and close to the Pichilemu coastal village. Moreover, records of the Pichilemu earthquakes provide evidences for an intraplate rupture process dominated by shallow normal faulting mechanisms with several events of magnitude Mw > 5.0. In this work, we present an analysis of seismograms from this sequence of events recorded by an array of short period seismometers deployed near Pichilemu since 27 February 2010, along with a time series from a cGPS (LEMU) also deployed in this area. The aftershock hypocenters and tomography reveal a fault zone striking N40°W and dipping 50°-60°SW that extends from the surface to the interplate zone. This fault zone coincides at the surface with a pre-existing reverse fault that is the boundary between crystalline Paleozoic rocks to the east, and a slightly younger accretionary prism to the west. We infer that the Pichilemu rupture sequence is related to a reactivation of a pre-existent reverse fault but with an opposite sense of slip. The GPS time series shows 50 cm of co-seismic subsidence related to the 11 March events that is kinematically compatible with normal faulting. Seismic tomography imaging shows high Vp/Vs ratios along the Pichilemu rupture zone suggesting the presence of fluid along the fault. The kinematic inversion of the intraplate fault and the presence of fluids suggest a weak fault behavior. Hence the long-term deformation of this region is consistent with what one would expect from the main thrust event, while the subsequent subsidence corroborates the extension expected from the Pichilemu earthquakes. We conclude that the 11 March earthquake sequence represents intraplate normal faulting triggered by stress relaxation from the 27 February 2010 event during the early post-seismic phase close to the northern end of the megathrust rupture.