永登地震剪切波分裂及电性各向异性的变化特征  

The Temporal Evolution of Shear-wave Splitting and Geoelectrical Anisotropy During Yongdeng Earthquake

作　　者：阮爱国 李清河[2] 荣代潞[1] 赵和云[1] 

机构地区：[1]中国地震局兰州地震研究所,中国兰州730000 [2]江苏省地震局,中国南京210014

出　　处：《中国地震》2000年第4期316-326,共11页Earthquake Research in China

基　　金：甘肃省自然科学基金!资助课题 (ZS991 A2 5 0 13 Z) ;中国地震局"九五"重点项目! (95 0 7 436 ) ;地震科学联合基金!资助课题 (1

摘　　要：依据永登地震前后小震三分量数字记录和中法合作松山电磁台的观测资料 ,基于EDA裂隙模型 ,从各向异性的角度 ,将地震剪切波 (S波 )分裂方法和大地电磁测深 (MT)方法共同应用于地震孕育过程中前兆的各向异性特征的研究。结果表明 ,永登地震前 ,快S波偏振方向由北偏东逐渐转为北偏西 ,发震后又急转为北偏东 ,快、慢S波的时间延迟在震前也是急剧上升的。另外 ,MT电性主轴的计算表明 ,各频点的电性主轴方向基本一致 ,原与断层近于垂直的电性主轴 ( ρxy) ,从震前为北偏东 17°左右逐渐转为北偏西 15°并发震 ,震后恢复到北偏东。相应的视电阻率变化表明 160～ 2 2 6秒周期的变化最大 ,其对应的深度与该次地震的震源深度相当。而震前平行于断层的电性主轴其视电阻率 ρyx变化相对小些。这次地震的震源机制解表明其发震应力场的P轴是北偏西 15°。结论是 :( 1)从各向异性的方向特征看 ,不同方法得到的 3个方向在总体上是一致的 ,时间演化过程是协调的 ,其机理就是应力所致的EDA裂隙定向排列。( 2 )不同频率电性主轴变化的一致性表明 ,震前应力所致裂隙定向排列而导致的各向异性在地壳一定范围内是广泛存在的。 ( 3)视电阻率量值的变化也存在各向异性并有其自己的特点。The temporal evolution of shear\|wave splitting and electrical anisotropy of earth media during the preparation and occurrence of Yongdeng earthquake ( M S=5 8) is studied based on the digital seismic records and continuous MT records. The results show that fast S\|wave polarization gradually turned from the northeast to northwest until earthquake occurred, after the earthquake, it recovered to the northeast. Moreover, the time delay between fast S\|wave and slow S\|wave were accumulating to the largest until earthquake occurrence. On the other hand, the directions of geoelectrical principal axes were also changing before earthquake, and exhibited identical characteristics in different frequences. The direction of the axis ( ρ xy ), which originally perpendicular to fault near station, varied from N17°E to N15°W, and recovered to the northeast after the earthquake. Consequently, the most obvious change of resistivity occurred in the period of 160s～226s, but resistivity variation of the other axis ρ yx was not so obvious as ρ xy . The epicenter mechanism solution of the earthquake showed that the P axis was N15°E. Conclusions are:(1) The fast S\|wave polarization, one of the geoelectrical principal axes and P axis identically changed with earthquake preparation, which is the result of the alignment of EDA cracks; (2) The geoelectrical axes identically varied with frequency demonstrates that EDA is really extensively distributed in the crust of some range; (3) MT resistivity variation also showed anisotropy characteristic with its own special features. The static anisotropy of resistivity is controlled by geo\|structure near station, the dynamic anisotropy is caused and affected by stress field, the most obvious resistivity occurred in the axis which is parallel to P axis, while the difference with frequency shows that resistivity variations depend not only on direction but also on buried depth.

关 键 词：永登地震 剪切波分裂 电性各向异性 大地电磁测深法 

分 类 号：P315.72[天文地球—地震学] P315.31[天文地球—固体地球物理学]