用应变地震观测求解震源矩张量的基本原理  

Fundamental principle to determine seismic source moment tensor using strain seismographs

作　　者：邱泽华[1] 唐磊[1] 赵树贤[1] 郭燕平[1] QIU ZeHua;TANG Lei;ZHAO ShuXian;GUO YanPing(Institute of Crustal Dynamics,China Earthquake Administration,Beijing 100085,China)

机构地区：[1]中国地震局地壳应力研究所

出　　处：《地球物理学报》2020年第2期551-561,共11页Chinese Journal of Geophysics

基　　金：国家自然科学基金(41974018);中国地震局地壳应力研究所中央级公益性科研院所基本科研业务专项资助项目(ZDJ2017-10,ZDJ2018-09)联合资助

摘　　要：震源机制解,即对地震矩张量的推断,对于地震研究具有至关重要的意义.应变地震观测是张量观测,与摆式地震仪的位移矢量观测不同,可以为地震研究提供新的数据源.本文讨论用应变地震观测求解震源矩张量的基本原理.在距离震源足够远的地方,地震波可以看成平面波,其性质决定于震源矩张量.假设平面地震波的应变张量可以由震源矩张量通过坐标变换计算得到,就可以通过观测应变地震波求解震源机制.这个假设至少对于双力偶震源机制是成立的.由此可以证明,在理想的无限介质中,只要有两个以上不同地点的应变地震波观测,就可以解出震源矩张量.这为解决震源机制问题提供了新的方法.目前的地震矩张量求解方法需要两方面的条件:或者需要很多观测点(例如体波反演),或者需要长周期地震波资料(例如面波反演).这些方法只适用于分析比较大的地震.对于小震,因为通常其震中周围不会有足够多的摆式地震仪观测点观测到其地震波,而地震波周期又短,难以利用传统方法给出可靠的震源机制解,所以只需少数观测点就能求解震源矩张量的新方法特别有意义.用应变地震观测求解震源机制,可以给出更为精确的结果.Focal mechanism solution,which is here referred to the inversion of seismic source moment tensor,plays a key role in studies of earthquakes.Strain seismograph is of tensor observation and provides new information for earthquake studies since it is physically different from the traditional pendulum seismograph that concerns vector displacements.In this paper,the fundamental principle of determining source moment tensor with strain seismic observation is discussed.Seismic waves can be regarded as plane waves when travelled sufficiently far from the source and their natures are determined by the focal mechanism.Assuming that the strain tensors of seismic waves can be calculated using the coordinate transformation approach from the source moment tensor,we can inverse the moment tensor from strain seismographs.This assumption is proved to be right for the case of double couple model.In an ideal elastic medium,merely two sites of strain seismological observation will make it possible to calculate the focal moment tensor of a seismic event.It offers a simple approach to solve the problem of focal mechanism.Presently,the prevailing approaches to solve this problem require numerous observation stations(for instance,body-wave inversion)or/and long-period wave data(for instance,surface-wave inversion).Such methods have worked well for moderate and large earthquakes.Particularly,for a small event,because usually there are not enough pendulum seismograph observation sites available closely around the epicenter,and the seismic wave periods are short,it is difficult to give a reliable focal mechanism solution by traditional methods.In such cases the approach using strain seismographs of few sites will be a good substitute.

关 键 词：震源机制 应变地震仪 地震矩张量 地震波应变张量 钻孔应变仪 

分 类 号：P315[天文地球—地震学]