丽江地震区应力场研究

通过现场调查工作，研究了丽江地区主要活动断裂的特征及现代活动性，分析了该地区构造应力场演化历史及现今构造应力场特征，探讨了丽江地震产生的机制，然后用离散元方法对该地区现今应力场进行了二维模拟，并进而深化了对丽江地震区应力场的认识。丽江地区处于川滇菱形断块的西南部，巨甸-九子海断块二级断块中的玉龙雪山-九子海三级断块上，该断块介于近南北向的楚波-白汉场断裂、北东走向丽江-小金河断裂和北西向中甸-永胜断裂之间，平面上成一顶角向东的三角形。在近南北向现今应力场的作用下，北东向的丽江-小金河断裂左旋走滑，北西向的中甸一永胜断裂右旋走滑，使两者所夹持的巨甸-九子海断块的主体部分发生向西的位移，在两者的复合部位，即断块的尖端附近，形成了局部的近东西向的拉张应力环境，从而使处于该部位的南北向玉龙雪山东麓断裂于1996年2月3日因发生了正断错动而导致了7．0级地震的发生。该地震后第三天，在其震中的正南方又发生了一次6．0级余震，其地震破裂是由北向南的一种撕裂，进而说明，本次地震的发生是由于玉龙雪山东麓断裂与中甸一永胜断裂复合部位拉张应力的强烈集中造成的。

A Study on the Stress Field in the Region of the Lijiang Earthquake

This paper studies the activity and characteristics of main active faults in the Lijiang region, discusses the evolution of structural stress field and features of modern stress field, and probes into the mechanism of the Lijiang earthquake on February 3, 1996. By using the discrete element method, the stress field is simulated, which deepens the understanding of the stress field of the Lijiang earthquake. The Lijiang region is situated in a triangle tectonic block bordered by the NS-trending Chubo-Baihanchang fault, the NE-trending Lijiang- Xiaojinhe fault and the NW-trending Zhongdian-Yongsheng fault. Geological and seismological studies indicate that in the region the maximum principal stress is in the NNW-SSE direction. In such a stress field, the NE-trending Lijiang-Xiaojinhe fault is dislocated reversely with the left-lateral movement, and the NW-trending Zhongdian-Yongsheng fault is dislocated right-laterally. It is such a dislocation pattern of faults that causes the Judian-Jiuzihai block between the faults to move westward, thus producing local tensile stress of the east-west direction, leading to normal faulting of the fault at the east of Yulongxueshan Mountain, generating the M 7. 0 earthquake on February 3 of 1996. The mechanism of an after-shock on February 5 with a magnitude of 6. 0 south of the epicenter further indicates that the earthquake was induced by the strong concentration of tensile stress at the intersection of the fault at the east slope of Yulongxueshan Mountain with the Zhongdian-Yongsheng fault.