A Systematic Analysis of Basin Effects on Surface Ground Motion

"Basin effects" referas to trapped, reverberating earthquake waves in a bowl-shaped loose sedimentary deposit, which significantly alter frequency content, amplitude, and duration of seismic shaking. This has played an important role on surface ground motion during past earthquakes such as the $M_w$ 8.0 1985 Micho\'acan, Mexico, $M_w$ 6.9 1995 Kobe, Japan, and $M_w$ 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 especially Ground Motion Prediction Equations (GMPEs). In this study, we perform a finite element analysis to conduct an extensive parametric study on the characteristics of surface ground motion associated with basin effects. We use an elastic medium subjected to vertically propagating SV plane waves and utilize idealized basin shapes to examine the effects of geometric 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 quantifying the depth of the basin relative to the dominant incident wavelength, and dimensionless distance quantifying the separation of the basin edges relative to the dominant wavelength. Our results show that basin effects can be characterised using at least three separate parameters, each of which can significantly vary the resultant ground motion. To demonstrate the application of dimensional analysis applied here, we investigate the response of the Kathmandu Valley during 2015 M_w 7.8 Gorkha, Nepal earthquake using an idealized basin geometry and soil properties. Our results show that a simplified model is able to capture notable characteristics of the ground motion associated with basin effects. This suggests that such studies can provide useful insights, relevant to the parameterization of basin effects in ground motion prediction equations and design code provisions.