Source location-dependency site response in the Taipei Basin of Taiwan by using HVSR analysis

Abstract Techniques using horizontal-to-vertical spectral ratios (HVSR) have frequently been adopted in many studies, particularly of seismic site amplification. However, the influence of source location on the HVSR is not previously well-studied. This paper studies the variation of the S-wave HVSR in the Taipei Basin based on a variety of hypocenter locations. The earthquake records are classified into three categories, include depth, azimuth, and distance. We report the S-wave HVSR to have strong focal-depth-dependence, for which the HVSR of the deep events are larger than shallow events. Besides, the S-wave HVSR in the northern and northeastern parts of the basin also displays source azimuthal-dependence. However, there is no apparent difference in source distance-dependence. In contrast to the S-wave HVSR, the coda-wave HVSR all showed no apparent difference for various earthquake hypocenter locations. The synthetic modeling results support our idea that the cause of the variation of HVSR results for different earthquake focal depths is the different S-wave angles of incidence into the basin. Based on the dependency of the site amplification on the focal depth, a focal-depth-dependent empirical transfer function (ETF) was established. The focal-depth-dependent ETF was verified to improve the accuracy of the ground motion prediction by using the stochastic method in this study. We demonstrate that the source location effect, especially the focal depth, can significantly influence the S-wave HVSR and therefore needs to be taken into account in site response investigation.