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.

The Gaoyou-Baoying M_S4.9 earthquake on July 20, 2012 occurred in the Gaoyou Sag in the Subei Basin. This earthquake was a relatively rare medium-strength earthquake in the weak seismicity region of eastern China. Although studies on the seismogenic structure of this earthquake have been conducted previously, the seismogenic structure itself is still under debate and needs to be further studied. This paper uses the methods such as distribution of seismic intensity, precise positioning of earthquake sequence, focal mechanism, regional tectonic stress, seismic exploration, etc. to comprehensively study the seismogenic structure of this earthquake.
The characteristics of earthquake sequence show that the seismic structure is a high dip-angle fault spreading along the NNE direction, dipping ESE. The result of focal mechanism solutions shows that the strike of one of the two nodal planes is NNE, and the fault plane shows high dip angle. The earthquake is mainly characterized by strike-slip motion. Through the seismic exploration lines(GYL1, GYL2)laid at the epicenter area of the earthquake, a fault structure is identified, which strikes nearly NNE and dips near ESE. This fault is located between the Linze sag and the Liubao low uplift, coinciding with the distribution of the Liuling Fault, the boundary fault in the northwest of the Gaoyou Sag, so it can be judged that all the detected breakpoints belong to the Liuling Fault. The “Y-shaped” breakpoints detected by the two seismic exploration lines are characterized by high dip angle. There is a very obvious wave group disorder area at the distance of 6 500~9 000m on the GYL1 seismic exploration line. This area is about 2.5km in width displayed on the post-stack migration profile and shows an uplifting trend. The disordered uplifting of wave group is caused by intrusion of soft material into the structural breakage and weakness, squeezed by horizontal stress. The GYL2 post-stack migration profile shows obvious uplift appearing in the reflection wave group(T_g)on the top of the bedrock. This arc-shaped uplift also reflects the effect of strong compression of horizontal stress.
In order to further discuss the seismogenic structure of the Gaoyou-Baoying M_S4.9 earthquake, we used the focal mechanism data to invert the modern tectonic stress field in the Northern Jiangsu-South Yellow Sea Basin where the earthquake occurred. The maximum principal stress in this area is NE-SW, while the minimum principal stress is NW-SE; both of them are nearly horizontal, and the intermediate principal stress is nearly vertical. According to Zoback's rule for dividing the types of dislocation in the direction of the force axis, the distribution of principal stresses in the Northern Jiangsu-South Yellow Sea Basin is equivalent to a strike-slip dislocation.
To sum up, the stress characteristics reflected by the Liuling Fault are consistent with the horizontal forces on the P-axis and T-axis shown by the focal mechanism solution results, and also consistent with the horizontal state of the stress in the tectonic stress field in this region. The above characteristics indicate that the development of the Liuling Fault is affected and controlled by modern tectonic activities. At the same time, the characteristics of the strike and dip of the seismic fault reflected by the methods of seismic intensity investigation, precise earthquake positioning, focal mechanism solution and seismic exploration, etc. are consistent with each other. Therefore, the occurrence of this earthquake may be the result of continuous stress accumulation and sudden instability and rupture of the NNE-trending Liuling Fault under the long-term compression of the NE-direction principal stress.

Using the seismic waveform data recorded by regional seismic network of Yunnan and Sichuan and the method of CAP, we calculate and obtain the focal mechanism of 268 earthquakes with the magnitude of M_L≥4.0 occurring in Yunnan during Jan. 1999 to Aug. 2014; then, we analyze the types and the regional feature of the focal mechanism of earthquakes in Yunnan, on the basis of the focal mechanism of 109 earthquakes analyzed by Harvard University. Based on the data of the above focal mechanism solutions, we adopt the method of damped regional-scale stress inversion to calculate the best-fitting tectonic stress tensor of every grid in Yunnan; and adopt the method of maximum principal stress to calculate the direction of maximum horizontal principal stress in Yunnan. The result shows that: (1)the strike-slip type is the most principal type of the earthquake focus in the study area and the second is the normal faulting type; while, the reverse-fault type is relatively small. The spatial distribution of focal mechanism is obvious. This reflects that the dynamic source and acting force are different in different parts of the study area. (2)The direction of the stress field in Yunnan shows a certain spatial continuity. Maximum horizontal principal compressive stress is mainly clockwise from north to south and counterclockwise from the west to the east. The direction of stress field shows inhomogeneity in space. There exist two stress conversion zones respectively in EW and NS direction. The inversion result of stress field shows that the stress field in Yunnan is complex and the principal stress direction changes greatly; and there are obvious differences in different regions.