四川西部理塘—巴塘地区的活动断裂与1989年巴塘6.7级震群发震构造研究

RESEARCH ON ACTIVE FAULTS IN LITANG-BATANG REGION, WESTERN SICHUAN PROVINCE, AND THE SEISMOGENIC STRUC- TURES OF THE 1989 BATANG M6.7 EARTHQUAKE SWARM

四川西部的理塘-巴塘地区以近SN向的断裂构造为主,并发育1组NNE和NW向的共轭剪切破裂带.文中在详细的TM卫片和航空照片判译的基础上,以活动构造地貌学为主线,重点解析了该地区断裂构造晚更新世-全新世以来活动的表现形式,确定近SN向的金沙江断裂带晚第四纪以来的近EW向的缩短速率为2 ～3mm/a,NNE向巴塘断裂的右旋水平滑动平均速率为1.3 ～2.7mm/a,NW向理塘断裂的左旋水平滑动平均速率为2.6 ～4.4mm/a.结合地球物理场、震源机制解、GPS测量等资料,分析了该地区现今地壳运动的总体态势及其所导致的块体运移规律,指出1989年巴塘6.7级震群的成因,是由于NNE向巴塘断裂和NW向理塘断裂共轭剪切所派生的近EW向正断层的张性破裂所致,揭示了一个挤压构造环境内正断层发震的典型震例.

The Litang-Batang region of western Sichuan Province is located at the eastern section of the Tibet Plateau. Since the Late-Cenozoic, the East Tibet Block has thrust over the Sichuan-Yunnan Block from west to east along the NS-trending Jinshajiang Fault zone, resulting in the nappe tectonic belt of 30 km width accompanied by the formation of the NNE-trending Batang Fault and the NW-trending Litang Fault, a pair of conjugate shear faults. In the vicinity of Daoxu, south of Derenduo, the generally NS-trending Jinshajiang Fault zone extends along NNW direction and displaces left-laterally a series of gullies for about 120～140m. At south of Yarigong, the fault extends along NNE direction and dislocates right-laterally a series of gullies for about 180～210m. In contrast, no evidence of horizontal displacement is observed on the NS-trending segment of the fault. It is suggested, therefore, that the Jinshajiang Fault zone should be characterized by shortening in nearly EW direction. Based on the results of GPS measurement, the shortening rate is determined to be about 2～3mm/yr. The NW-trending Litang Fault is dominated mainly by left-lateral shear movement. According to the dislocation value and the initiation time of dislocation obtained at southeast of Kangga, southeast of Litang and northwest of Heni etc, the average horizontal slip rate on the Litang Fault is estimated to be 3.2～4.4mm/yr on the Litang-Dewu segment, and 2.6～3.0mm/yr on the segment to the north of Litang. The NNE-trending Batang Fault cuts obliquely the main part of the Jinshajiang tectonic zone, displaying mainly right-lateral shear movement. At Mangling Village of Markam County, Tibet, the average horizontal slip rate on the fault is estimated as 2.0～2.7mm/yr, wiile in the vicinity of Batang it is estimated to be 1.3～1.9mm/yr. The connecting line of the eqicenters of the 1989 Batang M6.7 earthquake swarm and the long-axis of the aftershock distribution all extend nearly along EW direction, and the focal mechanism solution indicates an EW-trending normal faulting. Therefore, we tend to believe that the NNE-trending Batang Fault and the NW-trending Litang Fault to be a pair of conjugate shear ruptures. The southern block between these two faults slips southwards, resulting in EW-trending normal fault at the intersection of the two faults. It was the tensile rupturing on the normal fault that induced the Batang M6.7 earthquake swarm, as evidenced by the development of EW-trending normal faults on the southern edge of the Maoya basin and the nearly EW-trending surface rupture of about 2 km in length produced by the Batang M6.7 earthquake swarm.