Basin Effects in Strong Ground Motion: A Case Study from the 2015 Gorkha, Nepal Earthquake

TL;DR

This study investigates how basin effects influenced the strong ground motion during the 2015 Gorkha earthquake in Nepal, using simplified elastic and nonlinear models to understand low frequency reverberations and energy focusing in the Kathmandu basin.

Contribution

It demonstrates that a simplified 2D elastic model can effectively capture low frequency basin responses and that coupling with 1D nonlinear analysis improves simulation accuracy.

Findings

Low frequency reverberations are well modeled by elastic simulations.

Nonlinear analysis extends the frequency range of accurate ground motion prediction.

Basin geometry significantly influences seismic wave amplification and duration.

Abstract

The term "basin effects" refers to entrapment and reverberation of earthquake waves in soft sedimentary deposits underlain by concave basement rock structures. Basin effects can significantly affect the amplitude, frequency and duration of strong ground motion, while the cone-like geometry of the basin edges gives rise to large amplitude surface waves through seismic wave diffraction and energy focusing, a well-known characteristic of basin effects. In this research, we study the role of basin effects in the mainshock ground motion data recorded at the Kathmandu basin, Nepal during the 2015 Mw7.8 Gorkha earthquake sequence. We specifically try to understand the source of the unusual low frequency reverberating pulse that appeared systematically across the basin, and the unexpected depletion of the ground surface motions from high frequency components, especially away from the basin edges. In order to do that we study the response of a 2D cross section of Kathmandu basin subjected to vertically propagating plane SV waves. Despite the scarcity of geotechnical information and of strong ground motion recordings, we show that an idealized plane-strain elastic model with a simplified layered velocity structure can capture surprisingly well the low frequency components of the basin ground response. We finally couple the 2D elastic simulation with a 1D nonlinear analysis of the shallow basin sediments. The 1D nonlinear approximation shows improved performance over a larger frequency range relative to the first order approximation of a 2D elastic layered basin response.