Resolving near surface S velocity structure in natural earthquake frequency band: A case study in Beijing region

Near surface S velocity of sedimentary layers is crucial for strong motion simulation and seismic hazard assessment. S velocity of the top hundreds' meters is particularly significant for earthquake engineering. While S velocity can be measured by well-logging or shallow seismic survey, it is deduced from the data with frequency band much higher than the band of seismic signals from earthquakes, or the depth profile is too shallow. We present an effective method for resolving near surface velocity structure by modeling borehole seismograms. By analyzing seismograms from hundreds of earthquakes in Beijing region, we have identified direct S and surface-reflected S waves by using generalized ray theory, and determined S velocity structure for the depth of 100-500 m by modeling the interval between the two S waves. The average S velocity is found to be lower than 300 m/s at top 100 m, but increases to about 800 m/s at the depth of 500 m, With an average gradient of 0.8 (m/s)/m. Therefore borehole seismograms are valuable data for studying shallow S-wave velocity structure of sedimentary basins and can provide basic parameters for strong motion simulation in the basins.