基于井下摆天然地震数据测量首都圈近地表波速结构

Near surface wave velocity structure in Chinese capital region based on borehole seismic records

收集了首都圈数字化地震遥测台网58个井下摆2003—2008年记录的102次M≥3.0地震的数据.通过鉴别直达波及对应地表反射波的波形,发现两者到时差与震中矩和方位角无关.基于斯涅尔定律,分析获得了这些台站上方厚约300m浅地表土层P波和S波平均速度结构以及波速比.结果表明,首都圈区域近地表P波和S波平均速度分别约为1.6—2.0km/s和0.34—0.48km/s,波速比约为4.0—5.3.进一步对比井下深度同为247m的11个台站的数据,显示该区域P波速度和波速比均无显著横向变化.清晰的震相图也表明,土壤层没有明显的速度断层.该结果对模拟强地面运动和计算精细的地壳速度结构,以及地震工程的一些领域均有一定意义.

Firstly we collected P and S waveform data from 102 earthquakes with M≥3.0 for the period 2003 to 2008 observed at 58 borehole stations obtained from the Capital Circle Digital Seismic Telemetry Network catalog.After identifying direct wave and surface reflection wave,we found their delay time varied little despite distinct epicentral distances and azimuths.Based on Snell＇s law,we developed travel time lags to compute average wave velocity in about 300 meters of soil layer above each station.Our results show that near-surface P-and S-wave average velocity in Chinese capital region ranges from 1.6 to 2.0 km/s and 0.34 to 0.48 km/s respectively,vP/vS is about 4.0—5.3.Furthermore,no obvious lateral variation of P-wave velocity structure or vP/vS is observed by comparing results of 11 borehole stations with the same depth of 247 m.The clear seismic phases for both P-and S-wave suggest no significant velocity discontinuity in the soil layer.The results are useful in both the simulation of strong ground motion and the calculation of detailed velocity structure in the crust or upper mantle,as well as some areas of earthquake engineering.