四川锦屏水库地区地壳剪切波分裂特征及蓄水影响初探

利用雅砻江流域地震台网2011年8月1日至2014年12月31日期间及四川省地震台网1个地震台站2008年5月1日至2015年8月31日期间记录的地震观测波形资料,采用剪切波分裂分析得到了四川锦屏水库地区中上地壳各向异性参数,即快剪切波偏振方向和慢剪切波时间延迟.结果显示,研究区内台站的快波优势偏振方向存在明显的局部特征,左侧4个台站的快波优势偏振方向与区域主压应力方向比较一致,右侧台站优势偏振方向各异.研究发现,台站MLI的快波偏振方向变化与水库水位的变化具有很好的相关性,在2013年7月,水库水位急剧升高到约1800m后,台站的快波偏振方向也发生了90°变化,这是一种被称为90°翻转(90°-flip)的现象.蓄水导致的应力增加(以及可能的渗水)产生的高孔隙压影响了剪切波分裂特征.

Seismic shear-wave splitting characteristics in the crust in the area of Jinping reservoir of Sichuan and influences from water impoundment

Using seismic data recorded by Yalong River network from August 1, 2011 to December 31, 2014 and 1 station of Sichuan seismic network from May 1,2008 to August 31,2015, the upper earth crust anisotropy parameters, namely the polarization of fast shear waves and time delay of slow shear waves were obtained in Sichuan Jiping reservoir by shear-wave Splitting Analysis Method(SAM).The results show that the spatial distribution of polarization of fast shear waves has the obvious localized characteristic. For example, the dominant polarization directions of fast shear waves from four stations in the left are more consistent with the regional principal compressive stress, while the polarization directions of fast shear waves from stations in the right are not the same. Moreover, the variation of the polarization directions of fast shear waves from MLI has good correlation with the water level of the reservoir. The polarization directions of fast shear waves had a 90 degree change when the water level increased rapidly to about 1800 meters in July 2013,which is known as a flip 90 degrees (90 degrees-flip) phenomenon. Further analysis speculates that the variation of the splitting characteristic of shear waves was induced by high pore pressure resulting from an increase in stress owing to impounding and possible water leakage.