Significance of Nonplanar Rupture of the Mainshock and Optimal Faulting in Forecasting Aftershocks of the 2015 Mw 7.8 Gorkha Earthquake

Abstract

Abstract We computed the stress-change tensor around the 2015 Mw 7.8 Gorkha earthquake with two different rupture models: a simple uniformly dipping model and a complex ramp-flat-ramp-flat fault-slip model. In general, the Coulomb failure stress changes (ΔCFS) computed on the optimally orientated faults based on a ramp-flat-ramp-flat fault-slip model showed the best spatial correlation with the aftershock seismicity. This close relationship was further verified by the focal mechanism solutions of 17 intermediate-size aftershocks. The ΔCFS calculated from the known focal mechanisms of most events were close to the values computed from the optimal fault planes and slip directions using the complex slip model with a nonplanar rupture along the Main Himalaya thrust. We further computed the stress accumulation in the seismic gap regions located around the Gorkha earthquake and between the 1505 and 1934 Mw 8+ historical earthquakes. We found a significant increase of the Coulomb failure stress by 0.2–0.5 MPa caused by the three earthquakes, especially at the shallow ramp of the seismic gap, which indicates an enhanced seismic risk around the Kathmandu area.