川西地区小震重新定位及其活动构造意义

使用双差地震定位法对川西地区1992～2002年的13367个小震进行重新定位， 初步分析了地震活动性与地表活动构造的关系及其揭示的构造信息. 重新定位后，地震活动沿活动断裂成线（带）状分布现象非常突出，呈现出与地表活动构造的密切关系：结构简单的单一走滑断层具有上宽下陡的花状结构特征，拉分盆地与逆断裂具有线性而发散的分布式结构特征，逆断裂之下还存在缺震层. 此外，沿活动断裂带地震活动还具有空间分段性，揭示出局部地段存在着隐伏活动断裂和可圈定为地震危险区的地震空区. 震源深度分布显示，川西高原在15～20km的深度范围内普遍存在厚度约5km的缺震层，以高温高压实验结果为基础，通过计算川西地区地壳强度表明，大约14～19km的深度范围花岗岩处于塑性流变状态，说明缺震层的出现具有地壳物质塑性变形基础.

Relocation of small earthquakes in western Sichuan, China and its implications for active tectonics

We relocated 13367 small earthquakes that occurred in western Sichuan from 1992 to 2002 using the double-difference (DD) earthquake location algorithm to improve relative location accuracy. The relocated seismicity forms highly organized structures that correlate well with the surface faulting, showing the flower-shaped structure across the simple strike-slip fault, and the organized but dispersed structure across the pull-apart basin and thrust fault. Beneath the thrust fault, there exists an aseismic layer. The seismicity also shows segmentation characteristics along the active faults. Other new findings, which were previously obscured by routine location errors, are also revealed by the relocated seismicity. Some blind faults are revealed by lineages of seismicity that are suggestive of faulting structures. Large seismic gaps appear with dimensions of tens of kilometers on the Xianshuihe-Anninghe-Zemuhe fault zone, which have been aseismic over a time interval of 10-year, suggesting that these segments are either locked or retarded and are storing strain energy for release in future large earthquakes. An aseismic layer with the thickness of 5km exists in most places beneath western Sichuan plateau at depths of 15 to 20km. Basing on the results from high-temperature and high-pressure experiments, we calculated the crustal strength envelopes for the western Sichuan. The result shows that the granite in upper crust at depths of 14 to 19km appears to be ductile, which is in good agreement with the thickness and depth range of the aseismic layer, suggesting that the aseismic layer may be the result of the ductile deformation of the granite in the upper crust.