Shear-wave velocity map for Pohang Basin, South Korea, based on the P-wave seismogram method

Abstract

The 2017 ML 5.4 Pohang, South Korea earthquake, which caused severe damage, raised attention to the importance of the characterization of the Pohang Basin structure and the identification of active faults. We utilize 366 seismograms recorded at 102 densely deployed seismic stations during 18 microearthquakes with ML ranging from −1.11 to 1.70. We perform the P-wave seismogram method, which has been validated in various regions using seismograms for moderate earthquakes, to estimate the time-averaged shear-wave velocity from the surface to a depth of 30 m ( VS30) in and around the basin. We perform multichannel analyses of surface waves at the 32 sites to obtain shear-wave velocity ( VS) profiles, and additionally acquire the 22 existing VS profiles within the study area. We observe that velocity seismograms predict VS30 more accurately than acceleration and displacement seismograms do. The estimated VS30 values are in good agreement with the measured VS30 values. Approximately 93.3% of the measured and estimated VS30 values are located between the ±100% difference lines. We propose a VS30 map for the vicinity of the Pohang Basin by geospatially interpolating both the measured and estimated VS30 values, which is consistent with the local topography and geology. The proposed map exhibits areas with strong VS30 contrast within the Quaternary and Tertiary sediments, which might be attributed to the existence of faults.