关键词: 地震, 永宁断裂, 晚更新世, 正断层, 震源机制解

Abstract:

The June 24th 2012 Ninglang-Yanyuan M_S 5.7 earthquake happened at 30km northwest of the Lijiang-Xiaojinhe Fault, a region seismically active in history and prone to earthquake in northwestern Yunnan. Tectonics in the earthquake region is complex,where two groups of faults are developed,trending NW and NE,respectively,and distributed in a chessboard pattern. Field survey results reveal that there are the NW-trending Yongning Fault and the NE-trending Rigulu-yanwa Fault developed near the epicenter,both are active in late Pleistocene.The Yongning Fault,composed of Wenquan Fault,Yongning Fault,and Alaao Fault,shows obvious fault landforms and clear linear features on satellite imagery.The fault has played an obvious control role in the development of the Yongning Quaternary Basin and Lugu Lake Basin,with several hot springs developed along the fault. Several tributaries of Qiansuo River run along the fault,and there are dextral displacements observed in many parts of the rivers along the fault,such as between Baqi and Haiyijiao,Shancuo village east of Rigulu. Near Alaao,the fault offset the late Pleistocene deposit on the T₂ terrace,and the latest TL age of the offset stratum is (21.19±1.80)ka,indicating it is a normal with dextral strike-slip,late Pleistocene active fault.The Rigulu-Wayan Fault has played a noticeable control role in the development of the Tertiary Basins such as Wayan,Rigulu,and Lijiazui and the Quaternary Basin of Yongning. It offset the mid Pleistocene and the upper Pleistocene strata. Between Zhongwadu and Lijiazui,several streams were synchronously displaced left-laterally. There are obvious signs showing the fault was active in the late Pleistocene,dominated by sinistral strike-slip. According to the focal mechanism solutions,the Ninglang-Yanyuan M_S 5.7 earthquake is of normal faulting with dextral strike-slip,the attitude of the NW nodal plane is basically consistent to the Yongning Fault,and the seismic rupture pattern is identical to the kinematical characteristic of Yongning Fault.The major axis of the isoseismals,the linear distribution of intensity Ⅷ anomaly sites and the direction of tectonic ground fissures are all consistent to the strike of Yongning Fault. Through analysis,it is believed that Yongning Fault is the seismogenic fault of Ninglang-Yanyuan M_S 5.7 earthquake. Furthermore,the 1996 Lijiang M7.0 earthquake,the 1976 Zhongdian M5.5 earthquake and this M5.7 earthquake all have apparent normal dip-slip component. These earthquakes are located in the periphery of the neo-tectonic uplift of Haba Snow Mountain and Yulong Snow Mountain. Based on analyses of regional topography,the normal faulting in this area is most likely related to the gravitational potential energy resulting from the big topography contrast.

Key words: earthquake, Yongning Fault, late-Pleistocene, normal fault, focal mechanism solution