Aftershock mechanisms from the 2010 Mw 8.8 Maule, Chile earthquake: detailed analysis using full waveform inversion

Abstract:

Since the earthquake rupture process is extremely heterogeneous, it is vital to understand how structural variations in the overriding plate and downgoing slab may control slip style along the subduction megathrust. The large-scale 3-D geometry of subduction plate boundaries is rapidly becoming well understood; however, the nature of any finer-scale structure along the plate interface remains elusive. A detailed study of earthquake source mechanisms along a megathrust region can shed light on the nature of fine-scale structures along the megathrust.

The M_w 8.8 Maule earthquake that struck central Chile in 2010 is the sixth largest earthquake ever recorded. Following the earthquake, there was an international deployment of seismic stations in the rupture area, making this one of the best datasets of an aftershock sequence following a large earthquake. This dataset provides a unique opportunity to perform a detailed study of megathrust earthquake source mechanisms.

Based on a high-resolution 3-D velocity model and robust earthquake locations [Hicks et al., 2014], we calculate regional moment tensors using the ISOLA software package [Sokos & Zahradnik, 2008]. We incorporate accelerometer recordings, important for constraining solutions of large earthquakes in the overriding plate. We also validate the robustness of our solutions by assessing the consistency of mechanisms with P-wave polarities observed at both onshore and offshore seismic stations, and compare them to already published solutions. We find that accurate earthquake locations are vital for the fine-scale interpretation of focal mechanisms, particularly for offshore events.

Our results show that most moment tensor solutions with thrusting mechanisms have a nodal plane dipping parallel to the subducting plate interface. Interestingly, we also find earthquakes with normal faulting mechanisms lying along to the megathrust plate interface in the south of the rupture area. This finding suggests that megathrust slip style can be highly variable across the megathrust, with implications for fault processes such as postseismic fluid release. We also perform 3-D full waveform forward simulation using the spectral element code, SPECFEM3D, to understand the sensitivity of our moment tensor solutions with respect to 3-D velocity structure.