新丰江水库中上地壳P波三维速度结构特征及库水的渗透影响

摘要

利用震源位置和速度结构的联合反演得到2007年6月-2014年7月新丰江水库地区地震序列的震源位置及中上地壳P波三维速度结构模型,并进一步研究库区序列分布及速度结构特征.结果显示:库区中上地壳不同深度P波速度存在显著横向不均匀性,浅部库区速度高于周缘,在5～10 km上地壳从库坝下游白田至库尾锡场NW方向存在高速异常体以及2个低速间断区域,低速间断区分别位于人字石断裂与南山—坳头断裂交汇处以及1962年6.1级地震震源区,库水可能沿低速间断区的人字石断裂、石角—新港—白田断裂下渗至13～14 km的地壳.在10～14 km地壳以NE走向的大坪—岩前断裂为界,NW侧为最高速度6.2km·s-1的高速区域,SE侧从库区中部回龙至库坝下游白田为显著低速异常区域,是可能的库水渗透影响区域,亦是库区中强地震集中区.库区地震多发生在高速体内部、高低速过渡带或低速的渗水通道两侧.%In this paper,we determined the Xingfengjiang (XFJ for short) Reservoir area earthquake sequence location from June 2007 to July 2014 and 3D P-wave velocity structure by a simultaneous inversion method.On that basis,we study the 3D P-wave velocity structure feature and the distribution characteristics of earthquake sources.The results show that the P-wave velocity presents obvious lateral and vertical inhomogeneity in the middle-upper crust.The P-wave velocity of XFJ reservoir is higher than that in the margin in shallow crust.In 5 ～ 10 km depth upper crust,there exists a NW-trending high velocity zone (HVZ) and 2 low velocity discontinuity zones (LVDZ) from Baitian at the downstream of dam to Xichang at the end of the reservoir.The 2 LVDZs are respectively located in the M6.1 hypocentral area and at the intersection of Renzishi fault and Nanshan-Aotou fault.At the 2 LVDZs,we speculated that the reservoir water infiltrates into 13～14 km depth crust along the Renzishi fault and Shijiao-Xingang-Baitian fault.In 10～14 km depth crust,with the NE-trending Daping-Yanqian fault as a boundary,it is a HVZ with maximum velocity 6.2 km· s-1 in the NW side,in the SE side it is a significant low velocity zone (LVZ) from Huilong at the center of the XFJ reservoir to Baitian at the downstream of dam,it is the area of reservoir water infiltration-diffusion also,and it is the most concentrated area of mid-strong earthquakes too.Most of earthquakes occurred at the HVZ,the gradient belt between different velocity blocks,or the side of infiltration channel.The M6.1 hypocentral area is located on the gradient belt above 2 HVZs whose volume is about 5 × 5 × 5 km3 and 7 × 8 5 km3 respectively.