天然地震频率范围内首都圈地区近地表S波速度结构c

近地表沉积层的S速度结构是强地面震动模拟和地震灾害估计的重要参数，尤其是浅部的S波速度结构在工程上具有重要的应用意义．目前大部分资料来源于工程钻孔或工程地震探测，很少有地震波频率范围内的S波速度结构，或者深度达数百米的S波速度结构．通过对天然地震的井下摆波形记录的分析，提供了一种测量地震波频率范围深达数百米的S波速度的有效方法．收集了首都圈地区44个井下摆的近震记录，利用广义射线方法确认了直达S波及其在地表的反射波震相，并通过测量不同台站上两个震相的到时差，获得了首都圈地区浅层100——500m 深度范围的S波速度结构．研究发现，浅部100m 的平均S波速度低于300m/s．当深度增加到500m 时S波速增加到800m/s，平均速度梯度为0.8 （m/s）/m．研究结果表明，井下摆地震记录波形是研究沉积盆地浅层S波结构的重要资料，将为沉积盆地的强地面震动模拟提供重要基础参数． 

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.