玉树地震震前甘孜—玉树断裂局部应力场时空偏转特征的离散元模拟研究

区域地质构造活动会改变地壳的应力状态, 构造地震是区域地质构造活动的极端方式, 了解震前发震断裂局部应力场的变化规律对断裂活动性的表征具有重要意义。为此, 利用颗粒离散元软件PFC2D, 以玉树地震台站YRY-4钻孔应变仪的监测数据为依据, 模拟2010年4月14日玉树MS 7.1级地震发震断裂, 甘孜—玉树走滑断裂, 震前局部应力场最大主应力方向的时空偏转特征。研究表明, 震前玉树台站位置的模拟最大水平主应力SH方向出现逆时针偏转, 在积累了一定应变能后偏转逐步恢复, 模拟局部应力方向的变化趋势与现场实际观测结果基本一致。断裂处不同位置SH变化规律不同, 距离断裂构造越近, 局部应力受到的扰动越明显; 位于拉应力集中区的SHT和压应力集中区的SHP同时向两者所在区域的分界方向旋转, 且偏转角随时步的增加逐步增大; 不同区域的应力积累程度不同, 且随着加载时间的推移而增强, 并在不同时刻发生破裂, 具有明显的时空异步效应。

Discrete Element Modeling of the Spatiotemporal Deflection Characteristics of the Local Stress Field of the Ganzi–Yushu Fault before the Yushu Earthquake

Regional tectonic activities alter the stress state of the crust, and tectonic earthquakes are extreme regional tectonic activities. Therefore, understanding the change in the local stress field of seismogenic faults before earthquakes occur has great significance in the characterization of fault activities. This study used the particle discrete element package PFC2D and monitoring data of the YRY-4 borehole strain gauge at the Yushu Seismic Station to simulate the spatiotemporal deflection characteristics of the maximum principal stress direction in the local stress field before the Yushu MS7.1 earthquake on April 14, 2010, and the Ganzi–Yushu strike-slip fault. The results showed that the SH direction of the maximum horizontal principal stress at the Yushu station before the earthquake displayed counterclockwise deflection. The deflection gradually recovered after a certain amount of strain energy had accumulated. The change pattern of the simulated local stress direction was generally consistent with the field observation results. The variation pattern of SH differed at different positions of the fault, and the disturbance of local stress was more obvious with nearer proximity to the fault structure. The SHT located in the tensile stress concentration area and the SHP located in the compressive stress concentration area rotated simultaneously toward the boundary direction of the two areas. The deflection angle increased gradually with increasing time step. The degree of stress accumulation varied among different regions and increased with loading time. Fractures occurred at different times, with obvious spatiotemporal asynchronous effects.