【摘要】 利用衰减结构层析成像方法尝试对紫坪铺水库区域地壳Qs进行成像,获得该区浅层地壳静态及蓄水前、后的Qs变化特征,结合实验研究成果、岩体岩性、断裂构造、水文地质条件探讨库水的渗透作用及其对地壳介质的影响,对研究区内地震活动类型及发震成因的介质物性变化进行了讨论.研究结果表明:紫坪铺水库区域地壳Qs横向不均匀变化显著,库区周边近似存在低Qs值环形区域,主要包括水库东北、东南、西南、西北及库区中段五个区域.其中,水库东北、库区中段、西南低Qs区域与库水渗透关系密切,初步认为库水可能沿着通济场断裂中段和两端的岩石破碎带及节理、裂隙发育地区向地下渗透,使岩石孔隙中充满流体,内摩擦增大,地震波大大衰减,从而导致Qs值大幅下降.同时,在西南区诱发了水库小震群,在东北区和东南区诱发了部分水库小震群.另外,紫坪铺水库区域东北、东南、西南三个小震群基本位于高Qs值(低衰减)和低Qs值(高衰减)过渡区域,可能由于低Qs区地震波衰减大,不易积累能量,而高、低Qs值的过渡区域,介质介于"软"、"硬"之间,有可能积累应变能,孕育地震.当高、低Qs过渡区域岩石裂纹饱含水或部分含水后,水对裂纹边界起潮湿和润滑作用,降低了发震断层的抗剪强度,使滑动容易产生,从而诱发地震.更多还原

【Abstract】 The crustal Qs values are imaged in Zipingpu Reservoir Area to obtain shallow crustal characteristics of Qs variation before and after the impoundment of water through two-dimensional seismic attenuation tomography.The infiltration of reservoir water and its impact on the crust are investigated with experimental findings,rocks lithology,faults,hydrological and geological conditions.Simultaneously,we discussed the type of seismic activity and physical variety of medium about the cause of earthquake occurrence in the area.The results show that there is a significant lateral heterogeneity in Qs distribution in Zipingpu Reservoir Area.Approximately,there are low Qs value annular regions surrounding the reservoir,namely,the Qs values are low on the northeast,southeast,southwest,northwest sides and in the middle part of the reservoir.Particularly,the Qs values variation are most closely related to fluid infiltration in the middle part and on the northeast,southwest sides of the reservoir.We consider that the reservoir water infiltrates through from the middle part and two sides of the rock fracture zones,joints and fractured regions of the Tongjichang faults,filling with fluid in the rock pores,increasing the internal friction,causing significant attenuation of seismic waves,resulting in a substantial decline in the value of Qs,inducing small earthquake swarms on the southwest and partly on northeast and southeast sides.The small earthquake swarms in northeast,southeast and southwest regions are basically converged to transitional edge regions of high and low Qs values.This is perhaps a result of accumulation of strain energy in weak and soft zones.After full or partial filling of water in rock cracks,there are wet and lubricating effect on the crack boundary in the high and low Qs transition region,reducing the shear strength of seismogenic faults,inducing earthquakes.更多还原