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3D Modeling of the Kathmandu Basin Effects on Ground Motions Using Seissol

Session: Physics-Based Earthquake Rupture Modeling and Strong Motion Simulations I

Type: Oral

Date: 4/22/2021

Presentation Time: 02:30 PM Pacific

Description: 

The 2015 Mw7.8 Gorkha earthquake unzipped a locked portion of the Main Himalayan Thrust, which caused nearly 9000 fatalities across Nepal. However, damage was less severe in the Kathmandu Valley when compared to empirical hazard estimates. Previous 3D strong motion simulations suggest that the Kathmandu sedimentary basin structures amplify ground shaking at a dominant frequency of 0.2 Hz (Wei et al 2018). It is believed that this dominant frequency within the basin caused the collapse of tall structures, while ground shaking at high frequencies above 1 Hz was limited, thereby reducing the severity of damage. A more detailed 3D model, one that includes complex topography and accounts for the complex geometries and structure of the sedimentary basin, is needed to effectively simulate seismic wave propagation and ground motions at higher frequencies beyond 0.3 Hz. Incorporating topography and sedimentary basin effects in physics-based ground motion modeling is a challenge in computational seismology. Capturing wave interactions with realistic geological structure, specifically in the form of surface and interface waves, is increasingly important for hazard assessment. In this study, we use SeisSol, a simulation software that uses unstructured, tetrahedral meshes and high-performance computing to approximate complex 3D model geometries. We generate a detailed model of the Kathmandu Valley and simulate seismic wave propagation and ground motions of the Gorkha earthquake and its large aftershocks in a complex 3D sedimentary basin geometry shielded by strong local topography contrasts. We investigate the correlation between the 3D sedimentary basin geometry and the dominant resonance of surface waves within the basin. We explore frequency-dependent tradeoffs between topographic shielding and sedimentary amplifications, and the effect of sediment structure by introducing layers in the sedimentary basin in our model. Ideally, this study of 3D site effects and subsurface tradeoffs of 3D structures will lead to improved probabilistic seismic hazard assessments.

Presenting Author: Amrit Bal

Student Presenter: Yes

Authors