Understanding and Predicting Nonlinear Basin Effects in Kathmandu During the 2015 Mw 7.8 Gorkha Earthquake

We study the role of basin effects in the ground motions recorded during the M_w 7.8 2015 Gorkha earthquake in Kathmandu, which experienced very low frequency shaking during the April 25 mainshock. We seek to understand the source of frequency and amplitude characteristics of the recorded motions, especially as they pertain to the lack of high frequency components. For this purpose, we couple a 3D kinematic source model of the mainshock to a model of the Kathmandu basin that accounts for nonlinear soil response in the shallow, unconsolidated sediment layers. Despite the scarcity of documented soil material properties, three-dimensional mapping of the regional basin geometry, and strong ground motion data - on reference as well as soft soil site conditions - our results show that our synthesized geotechnical model of the shallow basin layers, coupled to a kinematic source model, can significantly improve ground motion simulations compared to rudimentary 1D and elastic 2D basin analyses. Our goal is next to use the geotechnical validated model of the basin to derive synthetic ground motion prediction equations for Nepal's capital using earthquake forecasting models suitable for the area.