云南迪庆5.9级地震构造背景、深部流变结构与发震机制

2013年8月31日8时4分, 云南省迪庆藏族自治州香格里拉县(28.12°N, 99.4°E)发生5.9级地震, 震源深度10 km.区域范围内历史地震频繁, 为滇西北地震多发区.据震源机制解结果, 此次地震为正断兼左旋走滑型地震, NW向截面产状与德钦-香格里拉-中甸断裂基本吻合.利用EIGEN-6C2模型对震中附近进行布格重力异常探讨, 震源位置位于莫霍面起伏部位, 下部地壳厚度不稳定之处; 而从P波速度与地壳结构剖面可知研究区上地壳底部存在低速层, 认为韧性低速层与地震能量的积聚和存储关系密切.而韧性低速(高导)层与德钦-中甸断裂交接部位, 是流变界面能量释放的位置, 即本次地震的震源位置.这为板内地震3层次构造模式提供了一个新的案例. 

Seism-Tectonic Background,Deep Rheological Structure and Triggering Mechanism of Diqing Earthquake (M s 5.9),Yunnan

At 08∶04 of August 31st, 2013, an earthquake with a magnitude of M_s 5.9 occurred in Shangrila county of Tibetan Autonomous Prefecture, Yunnan Province (28.1°N, 99.4°E). The depth of the hypocenter is 10 km. The study area is seismically active in history and prone to earthquake in northwestern Yunnan. Then the focal mechanism solutions show that the Diqing M_s 5.9 earthquake is of normal faulting with dextral strike-slip and the attitude of the NW nodal plane is consistent to the Deqin-Zhongdian fault. In this paper, we discuss the bouguer anomaly near the epicenter using EIGEN-6C2 model, and the epicenter is located at the Moho depth undulating position, and place where the crustal thickness is unstable. From the profile of P wave velocity and crustal structure, we know that low-velocity layer exists at the bottom of the upper crust, and the layer is likely to be closely related to seismic energy accumulating and storing. The joint location of the edge of the low-velocity (high conductivity) layer and the Deqin-Zhongdian fractures is the location for the focus to release the accumulating energy. This provides a new case of the three-level tectonic model for intra-plate earthquake.