注水诱发地震序列的震源机制变化特征:以四川长宁序列为例

Focal Mechanism Changing Character of Earthquake Sequence Induced by Water Injection:A Case Study of Changning Sequence,Sichuan Province

2006年4月以来,四川长宁、兴文、珙县交界区域相继出现注水诱发地震活动,至2013年4月30日共发生ML4.0以上地震16次,最大为2013年4月25日ML5.2地震.通过采用体波谱振幅相关系数方法研究长宁注水诱发地震序列ML3.0以上地震震源机制变化过程,结果显示序列震源机制散乱;利用CAP方法计算了序列中ML4.0以上12次地震的震源机制解,结果同样表明其机制紊乱.分析认为:钻孔加压注水引起附近岩体的孔隙压力增大及先存断层面的摩擦系数减小,这2种变化均导致库仑应力变化Δσf增大,从而促进地震的发生.因此,注水诱发地震活动期间,研究区构造应力场的应力强度无明显增强,对破裂方向也无明显的约束作用,故地震序列震源机制发散.

Earthquakes induced by water injection appeared in the border area of Changning,Xingwen and Gongxian counties, Sichuan Province since April,2006.Up to April 30,2013,1 6 ML ≥4.0 earthquakes occurred and the maximum is ML =5.2 on April 25,2013.In this paper,the earthquake focal mechanism changing process of Changning ML ≥3.0 earthquake sequence is studied by calculating correlation coefficient of body wave spectral amplitudes,and the results show that focal mechanisms of sequence events are scattering,furthermore,12 ML ≥4.0 earthquakes？？focal mechanisms are calculated by CAP method and they are in disorder also.The following analyses believe that water injection causes the pore pressure of rock body increasing as well as the friction coefficients of pre-existing fault planes decreasing,and those 2 factors lead to Coulomb stress change Δσf in-creasing,thus induce earthquakesoccurrence.Therefore,the earthquakes induced by water injection occur without obvious en-hancement of regional tectonic stress intensity,and there are no obvious restraining effects on the rupture directions of the earthquakes.As a result,the focal mechanisms of those earthquakes appear scattering.