A Systematic Analysis Of Basin Effects On Surface Ground Motion

"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 earth-quakes such as the M-w 8.0 1985 Michoa' can, 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 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 M-w 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.