Topographic Effect on Body-Wave Polarization and Near-Surface Wave Speed Estimation

Abstract

This study examines the topographic effect on the body-wave polarization and, subsequently, on near-surface wave speed estimation. We first derive 3D P- and S-wave polarization angles in the presence of ground tilt, where the angles are functions of the ground tilt orientation, the near-surface wave speeds, and the incident wave direction. We find that S-wave polarization angle varies considerably (e.g., more than 100 %) when the incident angle is close to the critical angle. The counter-intuitive phenomenon for flat surface, that is, P-wave polarization being only sensitive to S- but not P-wave speeds, breaks down in the presence of ground tilt, i.e., P-wave polarization becomes sensitive to both P- and S-wave speeds. Examining the differences in the inferred wave speeds with and without the flat-surface assumption reveals that bias in wave speed estimates is, in general, higher for smaller incident angles, e.g., about 50 % or higher for a 15○ ground tilt and near-vertical (<5○) incidence. The effect on P-wave speed estimates is also significant when the S-wave incident angle approaches the critical angle. In order to investigate the topographic effect on wave speed estimates inferred using teleseismic polarization data, we revisit the near-surface wave speeds estimates at Hi-net stations from Park and Ishii (2018). Based on the ground tilt and strike angles measured at 300-m scale for each Hi-net site, we constrain P- and S-wave speeds utilizing the P-wave polarization data. We find that P-wave polarization data alone can effectively constrain not only S- but also P-wave speeds, especially when the ground tilt is sufficiently large (e.g., >5○). Furthermore, our additional test suggests that including S-wave polarization data with the tilt consideration will improve the near-surface wave speeds estimates significantly compared to when the tilt effect is ignored.