暴雨触发局部地震活动的一个典型例子:2010年6月广西凌云—凤山交界3级震群活动

A typical example of locally triggered seismicity in the boundary area of Lingyun and Fengshan following the large rainfall event of June 2010

在前期严重干旱的背景下,2010年6月27～30日广西西北部出现大暴雨过程,大范围积水成涝.降雨量最大的凌云、凤山交界于6月28日17时开始出现密集的微震活动,形成显著的震群事件.自6月28日至7月15日共记录地震2739次,其中2～2.9级(M_L,下同)41次,3级以上地震3次,最大为7月1日10时27分3.2级地震.凌云—凤山震群微震活动与暴雨过程具有明显的时空相关特性,在时间上略有滞后.震中附近大范围内碳酸盐岩类岩溶等浅层构造非常发育,震中位于Nw和NE向断裂交汇区域,断层破碎带的存在,成为流体下渗的优先通道.震群空间分布集中、震源深度浅,63%地震的空间范围(半径)或震源深度小于1.6 km、88%的小于2.5 km,98%的小于3.4 km.较大地震震相特征显示,震群活动有流体参与并显示岩溶塌陷特征.由于流体沿断层面的快速下渗,微震似有沿断裂带展布的特征,但较大地震震源机制结果不支持震群活动缘于断层构造运动的猜想.位于震中区流动台记录的大多数地震的垂直向初动向下,且较小地震初动向下的比例更高,意味着大多数微震活动可能缘于岩溶塌陷或裂隙闭合.基于传染型余震序列模型(Epidemic Type Aftershock Sequence model,简称ETAS模型)的定量检测结果显示,流体对凌云—风山震群的触发作用非常强烈,同时震群地震自激发亦较强.基于孔压的一维扩散方程,模拟了流体渗透导致不同深度孔压随时间的变化,结果显示,流体渗透导致的孔压增加是凌云—风山震群发生的主要力学动因.在上述研究的基础上,初步提出凌云风山震群可能的发震机理.

An intensive rainfall occurred from June 27 to June 30 2010 in the northwest part of Guangxi province, large areas became waterlogged. Before that, a serious drought situation continued for several months in this region. Following the large rainfall, an earthquake swarm with thousands events occurred in the boundary area of Lingyun and Fengshan, where maximum precipitation appeared. There are 2739 earthquakes being recorded since June 28 to 15 July, among them, 41 events with magnitude from ML2. 0 to ML2. 9 and 3 events with ML^3. 0 （the maximum event is MI,3. 2） occurred on 1 July. There is an obvious spatio-temporal correlation between swarm and large rainfall, the highest seismicity is a little behind the maximum precipitation of the rainfall in time. Shallow karst structure consisting of carbonatite rock materials are prolific in a large area around the epicenter, the epicenter is located at the crossing area of NW faults and NE faults. The fault fracture zone is the preferential channels for fluid intrusion. Earthquakes concentrate in a cluster and the focal depths are shallow. The size of earthquake distribution （radius） and focal depths are smaller than 1.6 km for 63% earthquakes, 2.5 km for 88% earthquakes and 3.4 km for 98% earthquakes. The seismic phase analysis on relatively large earthquakes shows that the fluid has been involved in the swarm and it also shows some features of karst collapse. Due to the quick migration of fluid along the fault surface, it seems that the small earthquakes are distributed along the fault, but the focal mechanism solution of relatively large earthquakes does not support the guess that the swarm is resulted from the fault movement, in another words, there is no relationship between the swarm activity and the tectonic movement. The U-D first motions of most earthquakes, recorded by the field seismic sensor in the epicenter area, are downward and the ratio of down direction is larger for smaller earthquakes, this means that most of small earthquakes may be produced by karst collapse or crack closure. The results of quantitative detection based on ETAS model show that the fluid triggering action is very strong on the swarm activity. Meanwhile, the earthquake self-generation is also very strong. According to the 1-D diffusion equation, the temporal variation of pore pressure in different depths, caused by the fluid intrusion, has been simulated, it pointed out that the increment of pore pressure caused by the fluid intrusion is the major mechanical reason of the swarm. The seismogenic mechanism of the Lingyun-Fengshan swarm has been proposed in the final.