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The Effects of Surface Topography and Basin Layering on the Earthquake Ground Motion Intensities in Intermontane-Basin Settings

Description: 

Many densely populated intermontane sedimentary basins in collisional tectonic settings possess high seismic risk due to widely observed basin-specific ground motion amplifications. The documented observations show that ground motion predictions in these basins are often poorly constrained due to oversight of surrounding surface topography and the absence of sub-surface information about deeper basin layering, leading to inaccurate hazard assessment. In this study, we systematically evaluate the implications of these two factors on high-frequency ground motion characteristics, which is crucial for earthquake engineering practices.

Using the Kathmandu sedimentary basin as our case-study area, we extract a 2D east-west cross-section of length 80km containing high relief surface topography on either side of the 20km-wide basin. We simulate the eastward travelling Rayleigh waves in a high-resolution domain allowing us to resolve frequencies up to 5Hz. Our results indicate that the topography reduces peak ground acceleration (PGA) in the basin by 40% as compared to scenarios when the topography is neglected, particularly shielding frequencies above 2 Hz. We also perform 3D simulations of a shallow thrust-faulting moment tensor source in the west of the Kathmandu basin to confirm the high-frequency attenuation of waves entering the basin area due to the surface topography. In a simultaneous analysis for basin-specific properties, we find that the deeper Kathmandu basin layers control the spatial variability in the observed amplification that has an order of magnitude difference within the basin compared to the scenarios excluding the deeper basin layers.

We conclude that neglecting topography in ground motion predictions may lead to an overestimation of ground motion amplification in the basin, particularly for higher frequencies. A strong emphasis must also be given to maximise the understanding of deeper basin layers to capture the spatial variability in the ground motions. This is particularly relevant to microzonation studies where high spatial resolution is needed for risk-mitigation measures. 

Session: Physics-Based Ground Motion Modeling - II

Type: Oral

Date: 5/3/2024

Presentation Time: 10:45 AM (local time)

Presenting Author: Himanshu

Student Presenter: Yes

Invited Presentation: 

Authors