关键词: 活动断裂, 滑动速率, 块体运动, 地震群, 四川西部

Abstract: 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 M 6.7 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 M 6.7 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 M 6.7 earthquake swarm.

Key words: active fault, slip rate, block movement, earthquake swarm, western Sichuan Province