Quadratic Stress Drop Model of the 2013 M-w 6.6 Lushan Earthquake and Aftershocks Triggered by Blind Thrust Events

To reveal the distribution pattern of aftershocks of the M-w 6.6 Lushan earthquake on April 20, 2013, we analyzed finite-source slip models from seismic waveform inversions and calculated the stress changes on and off the main rupture. In the spatial domain, the fitted coseismic slip-stress relation on subfaults is much closer to the quadratic stress drop model than to the uniform stress model. In the wavenumber domain, the slip and stress change spectrum decay asymptotically as k(-3) and k(-2), respectively, where k is the wavenumber. And in this domain, we also find that the prediction of a quadratic stress drop model matches data better than a uniform stress drop model. In addition, we studied the effective friction coefficient on the fault. Aftershocks were clustered around a relatively narrow zone that counters the main rupture plane. The narrow zone has a main rupture width with a standard deviation of 2.7 km for M >= 3 events. For 12 M >= 4.8 aftershocks, approximately 33% of nodal planes were calculated to be located in the zone of positive shear stress changes, while 83% were in the zone of positive normal stress changes (unclamp), suggesting a high effective friction coefficient mu' >= 0.8 on the main fault. Combined with the investigation in aftershocks triggered by blind thrust events at Whittier Narrows (USA), Zemmouri (Algeria), and Gorkha (Nepal), we suggest that the correlation between aftershocks and positive Coulomb stress changes increases with the effective friction coefficient mu' and the effective friction coefficient and normal stress changes play an important role in aftershock triggering of blind thrust events.