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

摘要：

2012年6月24日宁蒗-盐源M_S5.7地震,位于丽江-小金河断裂西北30km。区域范围内历史上地震频繁,为滇西北地震多发区。震区断裂构造复杂,主要发育NW向、NE向2组断裂,呈棋盘格式展布。经野外实地考察,震中附近发育NW向永宁断裂和NE向日古鲁-岩瓦断裂2条晚更新世活动断裂。永宁断裂由温泉断层、永宁断层和阿拉凹断层组成。在卫星影像上线性特征清晰,断层地貌明显。断裂对永宁、泸沽湖第四纪盆地具有严格的控制作用,沿线多处发育温泉。前所河的多条支流顺断层发育,八七—海衣角一带、日古鲁东山厝附近,多处河流右旋位错。阿拉凹一带断错T₂阶地上更新统沉积,被错地层最新年龄(TL)为(21.19±1.80)ka,是一条以右旋走滑兼正断性质的晚更新世活动断层。日古鲁—岩瓦断裂对岩瓦、日古鲁、利家咀等古近纪、新近纪盆地和永宁第四纪盆地有着明显的控制作用,断错中更新世和上更新世地层。中挖都—利家咀一带,有多条小溪呈现出同步左旋位错特征。断裂在晚更新世有着明显的活动迹象,以左旋走滑运动为主。据震源机制解结果,此次地震为正断兼右旋走滑型地震,NW向节面产状与永宁断裂基本吻合,地震破裂型式与永宁断裂运动学特征一致。地震烈度长轴方向、Ⅷ度烈度异常点线性分布以及构造地裂缝方向均与永宁断裂走向一致。分析认为,永宁断裂为此次地震的发震构造。此外,1996年丽江7.0级地震、1976年中甸5.5级地震以及本次5.7级地震,均具有明显的正倾滑分量。这些地震多分布在哈巴雪山和玉龙雪山新构造隆起周缘。根据区域地形条件分析,该地区的正断层运动作用很可能与地形巨大反差引起的重力势能有关。

关键词: 地震, 永宁断裂, 晚更新世, 正断层, 震源机制解