Abstract:

The Shandong-Jiangsu-Anhui segment of Tancheng-Lujiang fault zone is a key seismic monitoring and defense area in China due to its complex structural deformation and intense seismic activity. With the accumulation of digital seismic data from the digital seismic networks of provinces and cities in the area and its adjacent regions, the waveform quality is steadily advanced, and the calculation methods for the focal mechanism solution and the inversion methods of stress field are constantly improved, which makes it possible to obtain more reliable focal mechanism solution and more accurate stress field.
Based on the seismic waveform data recorded by regional seismic network, we calculated and obtained focal mechanism solutions of 825 moderate and small earthquakes in Shandong-Jiangsu-Anhui segment of Tancheng-Lujiang fault zone and its adjacent areas from 2001 to 2016, by using the initial motion and amplitude information of P wave, SH wave and SV wave. In addition, we collected focal mechanism solutions of 323 earthquakes from 1970 to 2000. A total of 1 148 focal mechanism solutions were obtained. With the focal mechanism solutions as the input data, we adopted the damped regional-scale stress method to inverse and calculate the spatial variation characteristics of the stress field by 1.0°×1.0°grid region of the study area, and discussed the structural boundary, block difference, stress environment, seismicity and related dynamic problems. The results show that the maximum principal stress direction of the study area presents continuous change spatially, with an overall rotation trend in EW, NEE and NE direction from west to east, and there are differences locally. The dominant stress type is strike-slip, followed by normal strike-slip, indicating that the study area is generally under the action of horizontal stress field, and the difference of stress types mainly reflects the difference of local geological tectonic environment and fault activity mode to a certain extent.
Taking the Tancheng-Lujiang fault zone as the boundary, the stress fields of the Ludong-Yellow Sea block and the North China Plain on the both sides are different. The direction of maximum principal stress in the North China Plain block on the west is near-EW and NEE, while that on the east is NEE and NE. The analysis shows that the near EW-directed stress field in the North China Plain block generally inherits the stress field pattern resulting from the eastward extrusion of the Qinghai-Tibet block, but is more influenced by the near EW compression of the Qinghai-Tibet block. The stress field of the Ludong-Yellow Sea block is obviously affected by the westward subduction of the Philippine Sea plate. Although the whole North China block is controlled by the combined action of the northward push of the Indian plate and the westward subduction of the Philippine Sea plate, the effects of various driving forces on different secondary blocks in the block are different due to the existence of the Tancheng-Lujiang fault zone which extends obliquely to the top of the upper mantle. It reflects significantly that the Tancheng-Lujiang fault zone plays a significant role as a block boundary fault.
Along the 33°N latitude of Tancheng-Lujiang fault zone, there is a significant difference in the stress field between the north and the south. The direction of the maximum principal stress at the 33°N and its north area begins to deflect anticlockwise from west to east; while in 32°N and to the south, it is deflected clockwise from west to east. The direction of the maximum principal stress gradually transits from NE in North China to NW in South China, showing the characteristics of the stress field in South China to some extent. It indicates that 31°~32°N latitude is the transition zone of the two primary blocks, the North China block and the South China block. The direction of the maximum principal stress of the area between 31°~33°N and 120°~122°E is complex and characterized by radial distribution. This region locates in a very complex tectonic environment and may be influenced by the dextral strike-slip of Tancheng-Lujiang Fault caused by the near EW—NEE movement of the North China Plain block as well as the westward subduction of the Philippine Sea plate. The moderate-strong seismicity in the study area is obviously related to the tectonic stress environment. The area with complex tectonic stress field is usually the area with moderate-strong earthquake activity.

Key words: focal- mechanism, stress field, dynamics, Shandong-Jiangsu-Anhui segment of Tancheng-Lujiang fault zone

摘要：

文中利用区域台网数字地震波形, 计算了2001—2016年郯庐断裂带鲁苏皖段及邻区825次中小地震的震源机制解, 并收集了1970—2000年模拟记录时期323个震源机制解, 共计获得1 148个震源机制解。 以震源机制解作为输入数据, 采用阻尼应力张量方法反演获得了研究区1.0°×1.0°空间应力场变化特征。 结果显示, 郯庐断裂带鲁苏皖段最大主应力方向呈现空间连续性变化特征, 由西向东总体呈EW、 NEE、 NE向逆时针趋势性旋转, 局部存在差异; 以郯庐断裂带为界, 东、 西两侧的应力场存在差异, 以西的华北平原地块最大主应力方向主要表现为近EW向和NEE向, 而以东的鲁东-黄海地块则表现为NEE向和NE向, 反映了W向倾斜延伸至上地幔顶部的郯庐断裂带使得各动力源对不同块体的影响存在差异, 显示了郯庐断裂带的边界作用显著; 沿郯庐断裂带33°N为界, 南、 北地区的应力场存在显著差异, 33°N及以北地区最大主应力方向由西向东出现逆时针偏转, 而以南地区最大主应力方向由西向东开始出现顺时针偏转的迹象; (31°~33°N, 120°~122°E)区域的最大主应力方向较为复杂, 呈放射状分布特征, 该区域处于非常复杂的构造环境下, 可能受到华北平原地块近EW—NEE向运动导致的郯庐断裂带右旋走滑作用和菲律宾海板块W向俯冲的共同影响; 研究区的中强地震活动与构造应力环境相关明显, 构造应力场复杂的地区往往是中强地震活跃的地区。

关键词: 震源机制, 应力场, 动力学, 郯庐断裂带鲁苏皖段