SsPmp震相地壳探测方法

Probing the Moho interface using SsPmp waves

SsPmp波是远震S波经地表反射转换的P波在莫霍面发生反射后被地表台站接收得到的震相.震中距在30°～50°之间的远震S波震相经地表反射转换的P波射线参数较大,在莫霍面发生全反射,使得台站接收的SsPmp波具有较强的能量,能够从地震记录中清楚地识别出来,为探测台站附近的莫霍面形态提供新的途径.本文通过合成理论地震图分析了SsPmp震相与地壳厚度、射线参数和Pn波速度之间的关系.结果表明：对于水平界面,地壳厚度只影响SsPmp与Ss波之间的相对到时差;Pn波速度只影响SsPmp的相位;射线参数既对SsPmp波的相对到时有影响,也会引起SsPmp波的相位变化.对于复杂的界面,SsPmp反映的深度与速度梯度最大的深度接近,而反映的Pn波速度与实际的Pn波速度一致.

The Moho is one of the most important discontinuities in the earth.Its shape is associated with tectonic deformation and evolution of the crust.In orogenic belts,such as the Tibetan plateau,the crustal thickness is about 60～80km,however,in extensional regions,it is only 30～40km even less than the average global value.Detecting the depth of the Moho is helpful to understand tectonic environments.The virtual deep seismic sounding（VDSS）,a new method to measure the crustal thickness,can detect the Moho robustly.A systematic study of VDSS,however is absent now.In this paper,we use synthetic theoretical seismograms to analyze the seismic phase SsPmp in VDSS and its application in estimation of crustal thickness.Teleseismic S waves convert into P waves at the ground,and these down-going P waves will be reflected by the Moho,so SsPmp waves can be received after Ss phases.The most suitable epicentral distance for this observation is between 30°～50°,in which the waveforms of VDSS is protected from interfering with other seismic phases and the SsPmp becomes a prominent phase.As the ray parameters increase,the down-going P waves can be fully reflected and the energy ofSsPmp will become very strong.We analyze the relationship between the delay time and the phase shift of SsPmp and the ray parameter,uppermost mantle velocity and the thickness of the crust by synthetic seismograms.Our study suggests that the crustal thickness can be measured robustly by waveform fitting with a single model even if the Moho is a complex transition layer.The relation between SsPmp phases and the crustal thickness,ray parameter and uppermost mantle velocity（Pn velocity）was analyzed by synthetic waveforms.Differences of crustal thickness only cause the variation in the delay time of the SsPmp phases relative to Ss phases.Different Pn velocities only result in the phase shift of SsPmp variation.As the Pn velocity become faster,the phase shift becomes larger.With the ray parameter increasing,the delay time between SsPmp and Ss decreases,and the phase shift becomes larger.In general,SsPmp can be used to detect the thickness of the crust.This phase is powerful than the Ps used in receiver function.SsPmp,as a full-reflection phase,is strong enough to neglect these reflections and multiples from shallow crustal structure.