丽江7.0级地震前地震活动特征

THE FOCAL MECHANISM OF THE WUDING EARTHQUAKE SEQUENCE

本文论述了丽江７０级地震前地震活动条带、能量释放加速、复发周期时间扫描、剪切应力值异常等地震活动性的中长期背景异常和ｂ值、缺震、振幅比、剪切应力场动态分布、地震窗等短期异常。

The focal mechanism solutions of the mainshock and 21 aftershocks of seismic sequence for the M 6 5 Wuding earthquake on Octorber 24,1995 are abtained from the sign data of P wave first motion.The two notal plane strikes of the mainshock are NWW(282°) with dip of NNE and dip angle of 70° and NNE (7°) with dip of 77°, and the principle pressure stress orientaiton is 324°,the angle of elevation 24° for euqivalent release stress field.Based on geological data, isoseismic form and epicentral distribution of aftershocks, the notal plane with NNE (7°) is selected as triggering seismic fault of the mainshock and the fault plane takes left-lateral strik-slip as dominant motion,with slight tension by the action of near horizontal stress field with SSE. The orientation of the principle pressure stress of equivealent release stress field for mainshock basically is in accordance with the dominant orientation of principle pressure stress of regional stress field in this region (325°-355°). Most of focal mechanism solutions for the strong aftershocks were similare with that for the mainshock, with a stable result; the difference between the orintations of principle pressure stresses for the strong aftershocks and for the mainshock was lower than 20° and the dislocation features of fault plane for the aftershocks were in accordance with that for the mainshock until November 10. The result of focal mechanism for aftershock sequence manifests unified and stable characteristics, which shows the mainshock source stress field that basically was in accordance with the regional stress field and the seismogenic fault control seismicity of the whole sequence.