Three-Dimensional Nonlinear Amplification of the Kathmandu Basin Sediments During the 2015 Mw 7.8 Gorkha Earthquake

We study the role of three-dimensional (3D) nonlinear site effects in Kathmandu Valley, Nepal, by combining a kinematic source model with large scale wave propagation simulations. We validate our model by comparing ground surface simulated records to strong motion data recorded during the 2015 $M_w$ 7.8 Gorkha mainshock. For the wave propagation analysis, we use a 3D generalization of the 1D linear-equivalent method implemented in the spectral element (SE) code SPEED, whereby a scalar measure of shear strain is computed at each time step and is used to select strain-compatible material properties across the domain for the next time step. This methodology, implemented in SPEED, has been shown to provide computationally efficient estimations of nonlinear site effects for ground motions inducing small to medium levels of strain. Results show that despite the exceptionally long period ground shaking that characterized the strong motion records of the Gorkha mainshock, soil nonlinearity induced by body and surface waves played an important role in modifying the amplitude and frequency content of ground shaking.