A systematic analysis of basin effects on surface ground motion

Peyman Ayoubi,Kami Mohammadi,Domniki Asimaki

doi:10.1016/j.soildyn.2020.106490

Peyman Ayoubi, Kami Mohammadi + Show 1 more

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https://doi.org/10.1016/j.soildyn.2020.106490

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Abstract

“Basin effects” refers to trapped and reverberating earthquake waves in soft sedimentary deposits overlying convex depressions of the basement bedrock, which significantly alter frequency content, amplitude, and duration of seismic waves. This has played an important role on shaking duration and intensity in past earthquakes such as the Mw 8.0 1985 Michoa'can, Mexico, Mw 6.9 1995 Kobe, Japan, and Mw 7.8 2015 Gorkha, Nepal. Although the phenomenon has been understood and addressed in the literature, it has not been fully incorporated in seismic hazard analysis, and Ground Motion Prediction Equations (GMPE). In this study, we perform an extensive parametric study on the characteristics of surface ground motion associated with basin effects using finite element simulations. We use an elastic medium subjected to vertically propagating SV plane waves and utilize idealized basin shapes to examine the effects of basin geometry and material properties. We specifically study the effects of four dimensionless parameters, the width-to-depth (aspect) ratio, the rock-to-soil material contrast, a dimensionless frequency that quantifies the depth of the basin relative to the dominant incident wavelength, and a dimensionless distance quantifying distance of the basin edges relative to the dominant wavelength. Our results show that basin effects can be reasonably characterized using at least three independent parameters, each of which can significantly alter the resultant ground motion. To demonstrate the application of dimensional analysis applied here, we investigate the response of the Kathmandu Valley during the 2015 Mw 7.8 Gorkha Earthquake in Nepal using an idealized basin geometry and soil properties. Our results show that a simplified model can capture notable characteristics of the ground motion associated with basin effects which suggests that such studies can provide useful insights, relevant to the parameterization of basin effects in GMPEs and design code provisions.

Highlights

The effects of material, layering, and geometry of the shallow crust on surface ground motion is referred to as “site effects”
We demonstrate the effectiveness of our parameterization by comparing our long-period simplified simulations to the (c) recorded ground motions at the Kathmandu Valley in Nepal during the M7.8 2015 Gorkha earthquake
The sediment-rock material contrast plays an important role since it determines the amplitude of the seismic pulse that enters the basin, and regulates the amount of time it takes to reach the basin surface. (b) Vertically incident waves from the base, and laterally propagating surface waves generated at the basin edges travel toward the basin center. (c) Waves interact while waves propagate toward the basin center. (d) Maximum amplification occurs when they constructively interfere at the center

Summary

Introduction

The effects of material, layering, and geometry of the shallow crust on surface ground motion is referred to as “site effects”. One subcategory of site effects, “basin effects”, refers to the influence of bowl-shaped loose sedimentary deposits on the amplitude, frequency and duration of earthquake ground motions, that affects the structural performance ([12], [24]). These changes have played an important role in many past events including the 1985 Mw 8.0 Michoacan, Mexico, the 1994 Mw 6.7 Northridge, USA, the 1995 Mw 6.9 Kobe, Japan, and the 2015 Mw 7.8 Gorkha, Nepal ([28], [37], [27], [4]). Limitations of the model to simulate basin-modified seismic wavefields included the Rayleigh ansatz, the small periodic length, and the truncation of wavenumber components

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