Coulomb stress change pattern and aftershock distributions associated with a blind low-angle megathrust fault, Nepalese Himalaya

On April 25th, 2015, the magnitude Mw 7.8 Gorkha earthquake ruptured the Main Himalaya Thrust (MHT) in central Nepal as a result of a thrust type event. Comprehensive evaluations of this event and its aftershocks have been undertaken to understand the locations and mechanisms of the aftershocks. Here we model co-seismic displacement and strain caused by the Gorkha earthquake using different slip models, and then compare those results with the focal information. We have the following conclusions: 1) previous work suggests that there are two aftershock belts after the Gorkha earthquake, and we further suggest that the negative ΔCFS lobe(s) beneath the maximum slip led to the seismic gap between the two seismic belts; 2) aftershocks associated with the Gorkha earthquake occurred not only along the MHT fault plane, but also 10–20 km deeper than the depth of the maximum slip zone, triggered by the ΔCFS increase, this stress change pattern and distribution of aftershocks coincide with the recently proposed ‘top-down’ effect, that the stress pulses produced by earthquake in upper crust can drive aftershocks in the lower crust; 3) we suggest that the normal-fault aftershock 40 km north of the maximum slip zone was triggered by the Gorkha earthquake, the ΔCFS on the nodal plane (strike 178, dip 52, rake −78) driven from different slip models all increase more than 1 bar; 4) The aftershocks' mechanisms are strongly related to the deformation field caused by the mainshock, the displacement directions always nearly parallel to the maximum principal stress directions or the shear directions of the aftershocks' focal mechanism solutions. Our studies give suitable explanations to the distribution of the aftershocks accompanied with the Gorkha earthquake, which is also of great significance for the analysis of aftershocks under the similar seismogenic environments.