龙滩库区水库地震震源机制及应力场特征

STUDY ON THE CHARACTERISTICS OF FOCAL MECHANISMS OF RESERVOIR INDUCED EARTHQUAKES AND STRESS FIELD IN THE LONGTAN RESERVOIR AREA

文中使用龙滩水库地震监测台网记录的波形数据,采用P波初动、SH波和P渡位移振幅比数据计算震源机制解的FOCMEC方法,获取了龙滩库区2006年9月蓄水至2008年底发生的73次M_L2.0以上地震的震源机制解,并在此基础上反演了库区应力场。龙滩库区2006年10月蓄水以来发生的2级以上地震以逆断层型为主,由震源机制解获得的2个地震丛集区应力场的主压应力都近于水平,取向都为NWW-SEE。反映出蓄水后库区仍为以水平NWW-SEE向压应力为主的应力场结构,且最大主压应力倾角更为水平。而最大主张应力及中等应力轴的分布则不一致,显示出在近水平的主压应力背景下,龙滩库区局部应力场的非一致性。通过对龙滩水库地震机制解特征及应力场的认识和讨论,初步提出了龙滩水库诱发地震的发震机理,认为载荷作用所引起的剪应力增大不是龙滩水库蓄水诱发地震的主要因素,而蓄水所产生的孔隙压力作用和库水渗透的润滑弱化作用的耦合作用可能是主要的成因。

The FOCMEC method was adopted to resolve focal mechanisms for earthquakes occurring from Sep. 2006 to the end of 2008 in the range of Longtan reservoir by using the first P polarization and SH/P amplitude ratio from earthquake waveforms recorded by the Longtan reservoir seismic network. Then, the stress field of the reservoir area was inversed. According to the results, most of the ME ≥2. 0 earthquakes are of thrust faulting since the impoundment in Oct. 2006. The stress fields in both the cluster A and B are oriented in NWW-SEE direction and almost horizontal,indicating that the horizontal compressive stress is still the dominant stress after the impoundment, and the dip angle of the maximum principal compressive stress is more horizontal than that before impoundment. However, the middle and minimum stress axes are somewhat inconsistent in the two clusters, which indicates the local disturbance of stress field. The genesis of reservoir induced earthquake is supposed as the coupling of pore pressure action with the fluid lubricating and weakening action under the loading,whereas the increase of shear stress due to water loading is not the causation of reservoir induced earthquake.