Dynamics and radiation of thrust earthquakes with coseismic off-fault damage

Major earthquake ruptures occur predominantly in thrust faults producing devastating events and tsunamis such as the 2011 Mw 9.0 Tohoku earthquake, the 2004 Mw 9.2 Sumatra earthquake and the 1999 Mw 7.7 Chi-Chi earthquake. Understanding the mechanics of earthquakes in thrust faults and the effect of the free surface is thus crucial to explain their large shallow slip, their asymmetric ground motion and their damage patterns surrounding the fault and the free surface. In this work, we carry out 2D dynamic rupture simulations on thrust faults to accurately characterize a possible unclamping effect, its responsible physical mechanism, and to produce dynamically activated off-fault fracture networks. To conduct the simulations, we use the software tool based on the Combined Finite-Discrete Element Method (FDEM), HOSSedu, developed by Los Alamos National Laboratory. Our dynamic rupture models in an elastic medium confirm that unclamping occurs in thrust faults and increases significantly as the rupture reaches the free surface and for the fault models with lower dip-angles. We show that this is a consequence of the torque mechanism induced in the hanging wall, and the release of this torque when the rupture reaches the free surface produces a “flapping” in the toe of the wedge where the most significant unclamping (possibly leading to fault opening) is taking place. Our results indicate that the free surface produces a considerable reduction of the compressive normal stress when the rupture is propagating up-dip that facilitates the extension and the amount of slip close to the trench as observed for large thrust earthquakes.This significant normal stress change is reflected in the orientation of the principal stresses before and after the rupture, where under certain conditions, the greatest principal stress changes from subhorizontal to almost vertical leading to a post-rupture tensional stress state in the hanging wall that has been confirmed by observations of recent in-situ, seismological and geodetic studies. Finally, we investigate whether this dramatic normal stress reduction stands when we allow for the activation of coseismic off-fault damage and explore its role in the rupture dynamics, the near-field deformation and radiation patterns.