基于近震转换波的沉积层地区震源深度测定方法

摘要

基于合成地震图,并与观测数据对比,对沉积层地区近震波形的频率成分、偏振和走时等特征进行了分析,确认了沉积-基底界面的Sp转换波.研究表明:在给定震中距时,Sp转换波与直达P波的到时差随震源深度的增加近似呈线性增加,可以用来较好地约束震源深度.以2015年4月19日河北文安M3.0地震和2006年7月4日河北文安M5.1地震为例,验证了使用近震Sp转换波测定沉积层地区震源深度的可行性.利用Sp转换波对2015年4月19日河北文安M3.0地震重新测定震源深度的结果为18 km左右,而不是地震目录中给出的29 km,说明该地震发生在中上地壳,而不是下地壳.本文给出的方法可应用于测定沉积层地区的震源深度.%A clear seismic phase is often observed between the direct P and S waves of local earthquakes in sedimentary regions.Based on time-frequency features,polarization of three-component seismograms and travel-time measurements from synthetic waveforms,we propose that this phase is the Sp converted wave that is generated by the interface between sedimentary layers and basement.It is found that the arrival-time difference between Sp and direct P waves increases almost linearly with the focal depth when the epicentral distance is short.This method is verified with case study of the 4 July 2006 Wen'an M5.1 earthquake with Sp phase.Then,this method is used to determine the focal depth of the 19 April 2015 Wen'an M3.0 earthquake,which is 18 km,instead of 29 km in routine catalogs,indicating that the earthquake occurred in the upper crust rather than the lower crust.This method will be helpful for determining focal depth in sedimentary regions.