Source model and Coulomb stress change of the 2015 Mw 7.8 Gorkha earthquake determined from improved inversion of geodetic surface deformation observations

The 2015 Mw 7.8 Gorkha, Nepal, earthquake produced a approximate to 140km rupture on the Main Himalaya Thrust (MHT) fault at the convergence zone of the Indian and Eurasian plates. The coseismic surface deformation, fault slip model and induced stress change are comprehensively investigated with the geodetic observations in this study. The surface deformation is mapped by the combination of the GPS observations with the descending and ascending track ALOS-2/PALSAR-2 SAR images. An improved forward modeling of the surface displacement considering the effect of topographic variation on the geodetic data inversion is developed, and applied to determine the fault geometry and slip distribution associated with the event. The derived faulting model shows that the thrusting motion occurs on a fault plane with a low dip of approximate to 6.2 degrees, and the significant slip concentrates at depths of 5-13km. More than 90% of the seismic moment is released on the middle fault segment with a maximum slip of approximate to 6.7m. Two significant slip deficit zones located on the northwest and southeast of the fault are recognized from the source model. The shallow portion of the Main Frontal Thrust fault with little slip during the event is considered as the lower edge of the locked zone, which plays an important role in accommodating the convergence between the Eurasia and India plates. The change in Coulomb failure stress shows a significant increase for the slip deficits and the locked zone on the Himalayan front, which indicates that the accumulated strain on the MHT fault has been partially alleviated by the major quake and the aftershock sequence, but will continue to balance the slip deficit and high stress regimes, and drive the seismicity in the Himalayan front.