Abstract:

Focal mechanism solutions, distribution of aftershocks and field investigation suggest that the 3 August 2014 Ludian, Yunnan M_S6.5 earthquake was spawned by the NW-trending steep left-slip fault that stretches along Baogunao-Xiaohe. The landsides, dominated by avalanches, induced by the shock are distributed in a rectangular area(15km×12km)with a NW-orientated long axis. The bedrock avalanches at many sites indicate that the primary direction of ground motion changes from SE in the north to SN in the south. The distribution of landslides triggered by the shock can be explained by the following two models of seismogenic structure: 1)Left-lateral strike-slip faulting occurred on a generally NW-trending arc-like fault, which caused seismic ground motion with the direction changing gradually from SE in the north to near-SN in the south; and 2)in addition to the left-lateral slip of the NW-trending fault, the NE-trending fault has also contributed to the hazard, which thrust from NW toward SE. So the Ludian M_S6.5 event was the result of the joint action of both the NW and NE directed faults, of which the NW left-lateral slip fault is the dominant and the NE thrust fault is the secondary. Another evidence is that most aftershocks are distributed linearly in NW direction, meanwhile some are clustered in nearly NE around the epicenter, which also implies the possibility that both faults moved simultaneously during the event. Besides, focal mechanism solutions of a lot of earthquakes in the Yongshan-Zhaotong area, northeastern Yunnan Province, where the epicenter of the 2014 Ludian M_S6.5 event is located, indicate that the seismogenic faults are primarily dominated by thrust faulting, associated with strike-slip faulting.

Key words: Ludian earthquake, rock falls and landslides, focal mechanism, seismogenic structure

摘要：

2014年8月3日的云南鲁甸6.5级地震震源机制解、余震震中分布及震后的地震地质调查表明, 发震构造为NW向包谷垴-小河断裂, 断层发生左旋错动;震源机制与余震精定位数据表明发震断层倾角较陡。崩塌、滑坡分布在一个长轴为NW向的矩形区域内(15km×12km), 基岩崩塌指示地震动主方向自北向南由SE向变为SN向, 与震源机制解揭示的主压应力方向NW-SE总体一致。地震诱发的次生地质灾害崩塌、滑坡的平面分布特征可以用2种发震模式来解释: 1)总体走向为NW的弧形断层发生左旋走滑错动, 由北向南, 地震动方向由SE向逐渐转变为近SN向;2)除NW向断层的左旋错动之外, NE向断裂也可能被牵动, 发生由NW向SE的逆冲运动。地震是由NE、NW 2组断层共同作用的结果, 以NW向断层左旋错动为主、NE向断层逆冲为辅。余震震中主要呈NW向线性展布, 同时在震中附近存在NE向分布的地震条带, 隐含2组断层同时错动的可能性;而鲁甸6.5级地震震中所在的滇东北永善、昭通地区, 区域多个地震的震源机制表明, 地震断层多以逆冲运动为主, 走滑为辅。

关键词: 鲁甸地震, 崩塌滑坡, 地震震源机制, 发震断层