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.

On March 3, 2017, an earthquake swarm of M_L 4.5 occurred near Dazhushan Island in Miaodao archipelago, Shandong Province, as of December 31, 2017, 2 453 aftershocks were recorded, including 46 earthquakes of M_L3 and above and 4 earthquakes of M_L4 and above. It is the most active and frequent earthquake swarm activity in Miaodao Islands area in the regional network records. On September 2, 2017, another earthquake swarm of M_L3.0 occurred near Beichangshan Island, about 15km away from the south of Dazhushan Island. More than 300 earthquakes were recorded, including one earthquake of M_L3 or above. According to the seismic data, two earthquake swarms were also recorded near Daheishan Island and Tuoji Island in Miaodao archipelago from February to March 1976. It is believed that these two swarms may be the “precursory earthquake swarms” of Tangshan strong earthquake with M=7.8 in 1976. The differences in spatial location, energy release and focal depth between the two swarm events are very similar to those in 2017. Therefore, in this paper, the three-dimensional velocity structure of P-wave in Miaodao archipelago area and the results of seismic precise relocation are obtained by using the double-difference tomography method, and the deep structural environment factors of the preparation of the earthquake swarms and the differences in the characteristics of the earthquake swarms are analyzed in combination with the fault activity and medium characteristics.
The velocity structure provides important information related to earthquake location and focal medium, and provides important basis for understanding the background of earthquake preparation and the mechanism of earthquake occurrence. Based on the observation report data of Shandong and Liaoning seismic networks, this paper selects 4 766 seismic events recorded clearly from January 2008 to December 2017 in Miaodao archipelago and nearby areas, and excludes the data with the difference of P-wave and S-wave travel time and time distance curve larger than 5s. After the difference grouping of earthquake events, 4 555 events recorded by 65 stations are finally selected for double-difference tomography inversion, and there are 26 430 P-wave absolute arrival data, 513 299 difference arrival data, 26 356 S-wave absolute arrival data and 508 482 difference arrival data. Limited by geographical conditions, the ray density is dense in the south and sparse in the north. After repeated test and selection of inversion parameters and model recovery test, high-resolution P-wave three-dimensional velocity structure image and high-precision earthquake positioning results are obtained in Miaodao archipelago, Shandong Province, and the following conclusions are obtained:
(1)The results of seismic precise relocation show that the convergence of seismic distribution near Miaodao islands is good, the NW direction zonal distribution of earthquake swarm activity is obvious, and the focal depth is mainly concentrated in the middle and upper crust. The characteristics of swarm activity show group occurrence in a short period of time, and there are obvious differences in the form of expression: the swarm near Tuoji Island has deep focal point, high frequency, large release energy, and wide distribution of focal area; the swarm activity characteristics near Daheishan Island and Beichangshan Island are just the opposite.
(2)The horizontal velocity structure shows that the lateral heterogeneity of velocity structure exists in every depth layer, which reflects the unbalanced uplift of crystalline basement and the zone filling of igneous rock. The velocity structure of the shallow crust is in good agreement with the known geological structure; the middle and shallow layers clearly reveal the basement uplifting area and the subsidence zone on both sides of Miaodao Islands; the velocity structure of the middle and lower layers is obviously affected by the deep faults and magmatic activities, and near the Dazhushan Island-Weihai North Fault, it shows obvious low-velocity characteristics, while the northern sea area of Jiaodong Peninsula is characterized by independent high-velocty abnormal blocks.
(3)The velocity structure profile shows that there is a certain correlation between the activities of faults and earthquake swarm and the velocity structure of P wave. There is an obvious low velocity region in the middle and upper crust in the profile passing through the Dazhushan Island earthquake swarm, and the earthquake swarm is nearly vertical and layered scattered in the relatively high velocity medium between the upper and lower low velocity bodies or near the velocity conversion zone, which is consistent with the characteristics of the Dazhushan Island-Weihai North Fault. The profile across the Beichangshan Island earthquake swarm reveals that there are small high-speed bodies in the middle and upper layers of the crust, and the earthquake swarm occurs at the edge of the high-velocity body.
(4)Earthquake swarm often occurs in places with dense fracture distribution, relatively weak medium and low strength. Based on the analysis of the characteristics of earthquake swarm and the three-dimensional velocity structure of P-wave in Miaodao archipelago area, it is considered that the two significant earthquake swarm activities in 1976 and 2017 are the energy release caused by the inhomogeneity of local medium and the low stress friction of regional fault during the process of regional background stress enhancement(adjustment).