Abstract: Using seismic observation data of Shandong seismic network, we relocated 2 927 earthquakes(M_L≥0.2) recorded from Feb. 2017 to Apr. 2019 with double-difference algorithm in Changdao area. The fault plane parameters are calculated with 1 631 relocated earthquakes in the northern and southern earthquake swarms based on the simulated annealing and Gauss-Newtonian nonlinear inversion algorithms. There are two different earthquake swarms in both sides of 38°N. In order to distinguish the different earthquake swarms, we divide them into the northern earthquake swarm locating in the north of 38°N and starting from Feb. 2017, and the southern earthquake swarm locating in the south of 38°N and starting from Aug. 2017.
The stress field of Changdao area is inverted with 7 266 P wave polarities of 2 518 earthquakes in the swarms using the composite focal mechanism method. This method takes full advantage of all P wave polarities, thus avoiding the errors brought about by inverting focal mechanism with P wave polarities. The study region is divided into grids of 0.25°×0.25° before the stress field inversion for the northern and southern earthquake swarms. The rake on the fault plane of the northern and southern earthquake swarms is calculated using the stress field and fault plane parameters.
1 432 and 219 earthquakes are used to calculate the fault plane parameters for the northern and southern earthquake swarms, respectively. The result shows that the fault plane parameters are different between the northern and southern swarm. The strike, dip and rake of fault plane are 287.18°, 84.09° and -18.3° in the northern earthquake swarm, which is nearly the same with the previous results of shallow-depth acoustic reflection profiling. The fault plane parameters for the southern earthquake swarm are 269.67°, 67.46° and -3.6°. This result is similar to that of marine geophysical survey and the seismo-geological studies. The type of both fault planes is sinistral strike-slip according to the rake on the fault plane.
The stress field is inverted with a 50km radius smoothing in this paper. In general, the stress field calculated by this paper is basically identical with the previous results obtained by focal mechanism inversion and hydraulic fracturing in-situ stress measurement in Changdao area and is consistent with the stress field of the North China area. The stress field is controlled by pushing and subduction of the Pacific Plate from east to west. But there is a slight difference in the stress field between the northern and southern earthquake swarms. The compressive axis of stress field is rotated between the northern and the southern earthquake swarms. The stress field is in strike-slip regime in the northern earthquake swarm. The direction of P-axis is NEE-SWW, with a nearly horizontal plunge, and the direction of T-axis is NNW-SSE with a low plunge. In the southern earthquake swarm, the stress field is in a regime of normal faulting with a small amount of strike-slip component. The P axis is in NE-SW direction with plunges varying from 30° to 50°, and the T axis is the same as the northern swarm.
Based on the fault plane fitting, the seismogenic fault for the northern earthquake swarm is maybe the buried NW extension of the Dazhu Island-Weihaibei Fault, and the southern earthquake swarm occurred on a secondary EW-trending fault. According to the rakes of seismogenic faults, both of them are of strike-slip movement, and the stress field is in strike-slip regime in the northern earthquake swarm and normal with a small amount of strike-slip in the southern swarm. Both northern and southern earthquake swarms are controlled by the sinistral strike-slip Penglai-Weihaibei Fault, but the southern swarm is also under the influence of SN extension. We believe that the reason for the different fault plane parameters and stress fields is the different structure of the northern and southern earthquake swarms.

Key words: Changdao area, fault plane fitting, P wave polarity, composite focal mechanism, stress field

摘要： 文中基于小地震重定位数据计算了2017年长岛2个震群的断层面参数, 并使用综合震源机制法求解了长岛地区应力场。 通过应力场及断层面参数求得断层面上的滑动角, 确定了南部与北部震群发震断层的左旋走滑性质, 分析了南部与北部2个震群的应力场特征及发震构造, 并得到以下初步结论: 南部与北部震群的断层参数、 应力场存在一定差异; 北部震群断层面的走向为287.18°、 倾角为84.09°, 推断发震断层是大竹岛-威海北断裂NWW向延伸的隐伏断层; 北部震群受左旋走滑性质的蓬莱-威海断裂带控制, 呈NEE-SWW向挤压及NNW-SSE向拉张的走滑应力机制; 南部震群断层面的走向为269.67°、 倾角为67.46°, 其应力场呈NE-SW向挤压, 为近SN向拉张的正断兼走滑的应力机制, 其可能在蓬莱-威海断裂带左旋走滑的控制下还受到SN向的拉张作用, 北部震群的持续活动诱发EW走向的次级断裂错动产生南部震群。

关键词: 长岛地区, 断层面拟合, P波极性, 综合震源机制解, 应力场