A future scenario earthquake and ground motion hazards for Kathmandu, Nepal

Abstract

Abstract We constructed a new distribution of the rupture zones of great historical earthquakes along the Main Himalayan Thrust (MHT) by integrating the distribution in a previous study and the results of recent trench surveys. Additionally, recent Global Navigation Satellite System (GNSS) observations revealed that the boundary between the Indian and Eurasian plates is strongly coupled from the southern boundary of the MHT to a depth of approximately 10 km and there is almost no lateral change in the coupling. This implies that all regions along the MHT have similar rates of strain increase. Therefore, it is most probable that the rupture zone of the oldest previous event will rupture as a future scenario earthquake. In the new distribution, the 1255 earthquake is the oldest. However, large earthquakes have already occurred in 1934 and 2015 within its rupture zone. Thus, we adopted the area obtained by removing the 1934 and 2015 rupture zones from the western part of the 1255 rupture zone. As this area is close to Kathmandu, we assumed that it would be the rupture zone of a possible scenario earthquake for seismic hazards in Kathmandu. The relationship between the rupture zone size and seismic moment of the 2015 earthquake falls between the scaling formulas for crustal earthquakes and plate boundary earthquakes, but closer to the former formula. Therefore, we constructed a characterized source model based on the former formula. We simulated broadband ground motions in Kathmandu using this source model, our 3-D velocity structure model, and a hybrid method combining the finite difference method and the stochastic Green’s function method. We obtained the peak ground accelerations (PGAs) of simulated ground motions, and calculated the seismic intensities in the modified Mercalli scale from the PGAs as indexes of hazards for Kathmandu. Intensities IX were identified in the center of the Kathmandu Valley, and we mostly found intensities VIII and VII in the area surrounded by the sedimentary boundary and the southernmost part of the valley.