从震源机制中准确识别发震断层面对重构应力场的意义

Significance of accurate selection of the seismogenic faults from the earthquake focal mechanisms for stress field reconstruction

基于震源机制的应力场反演,需要从震源机制的两个节面中准确地识别出发震断层面,然后通过拟合发震断层面的滑动方向获得应力场.然而,目前的技术和方法不能保证能够从所有的震源机制中准确地识别出发震断层面.因此,用于应力场反演的断层数据中会参入小部分辅助断层面,这部分假断层的存在会造成重构的应力场与实际应力场产生多大程度的偏离?本文通过合成实验揭示出,从部分震源机制中错误地选择断层面可能会造成重构的应力场方向和主应力相对大小(R)与实际产生较大偏离,重构应力场的偏离程度主要与实际的应力R值、错误选择断层面的震源机制所占的比例以及震源机制中包含的噪声大小这三个因素有关.当实际的应力R值为中间值附近时,只有准确地选择断层面才能准确地重构R值,当实际的应力R值较小时,准确地选择断层面有助于准确地重构应力方向,故准确地从震源机制中选择断层面可以保证在不同的背景应力场下都能准确地重构应力场方向和R值,这对于根据重构的应力场合理解释构造运动具有重要意义.

Stress field inversion based on focal mechanisms requires accurate identification of seismogenic faults from the two nodal planes given by the focal mechanism,and then obtaining the stress field by fitting the slip direction of the seismogenic faults.However,current techniques and methods cannot guarantee that seismogenic faults can be accurately identified from all focal mechanisms.Therefore,a small fraction of the auxiliary fault planes are included in the fault data used for stress field inversion,and to what extent does the presence of this fraction of pseudo faults cause the reconstructed stress field to deviate from the actual stress field?In this paper,we have shown through synthetic experiments that the incorrect selection of faults for part of the focal mechanisms may cause the reconstructed stress direction and the relative magnitude of the principal stresses(R)to deviate significantly from the actual situation.The amount of deviation in the reconstructed stress field is mainly related to three factors,which are the actual stress R value,the fraction of focal mechanisms that incorrectly selected faults,and the noise level contained in the focal mechanisms.When the actual stress R value is around the median,the only way to accurately reconstruct the R value is to accurately select the fault planes.When the actual stress R value is relatively small,accurate selection of the fault plane helps to accurately reconstruct the stress orientation.Therefore,accurate selection of faults from the focal mechanisms can ensure that the orientation and R value of the stress field can be accurately reconstructed under different background stress fields,which is important for reasonable interpretation of tectonic movement based on the reconstructed stress field.