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.

A notable swarm occurred in Rushan, Shandong Peninsula and its activities continue since Oct. 2013 till now. Up to Sept. 30, 2014, more than 7 000 events have been recorded, in which locatable shocks exceed 2000, and 18 events with M_L≥3.0. The swarm is rarely seen in East China for its extraordinary duration time and surprising high frequency of aftershocks. 18 temporary seismometers have been deployed around the swarm since May 6, 2014, and composed a seismic array for monitoring the swarm activities. Based on data from permanent networks and temporary array, we relocated the earthquake sequence by using hypoDD method. It has been shown that, there is obvious difference between permanent network results and temporary array results. The permanent network of Shandong has a relative large coverage gap(more than 200°)for this swarm. Its location results therefore should not be reliable. There are maybe other errors in the permanent network result due to some problems in the raw data, such as too few stations for most locatable events(3 stations), and relative lower proportion of located events in final result(74.3%, while 95.1% in temporary array result). It can be found by comparing location results from permanent network and temporary array that, using temporary array's data can improve the location accuracy significantly. The results of temporary array are: aftershocks distribution of Rushan swarm is in NWW direction, the dip-direction of fitted fault plane is SW, and the strike and dip angle agree with focal mechanism of the mainshock. Focal depths of aftershocks are at 4.5～8km; the swarm is restricted in a small area about 3km×3km×1km, and has some characteristics such as clustering, staged activities, and etc; the aftershock activities are in accord with crack growth behavior pattern, hence we deduced that there may be fluid intrusion in source area. Finally, we discussed the seismogenic structures and active mechanisms of this swarm combined with relative geologic knowledge. We draw some conclusions as follows: 1)Rushan swarm probably occurred at the boundary of rock bodies of Duogu Mountain and Haiyangsuo super-unit; 2)The seismogenic structure is a blind fault, which should be a part of adjacent Heishankuang-Jilincun Fault, or might be a new fault at rock body boundaries; 3)Rushan swarm might be an evidence for the existence of the disputed Shidao Fault.

An M_S4.6 earthquake occurred at noon on Nov. 23, 2013 at Laizhou, Shandong Province, China. This earthquake is the largest event since the Sept. 20, 1995 Cangshan M_S5.2 earthquake in Shandong area, and shook the whole Shandong Peninsula. The local area has low seismic activities, only one M_L3 earthquake sequence was recorded from 1970 to 2012. But since 2012, small shocks break out every now and then, up to the recent M_S4.6 sequence.We investigate the faulting process of the 2012—2014 Laizhou M4.6 earthquake sequence by combining relocated hypocenters and focal mechanisms. CAP method and additional bootstrap technique are employed to stably invert the moment tensor solution and to estimate its uncertainties. The average faulting parameters are: A. strike=239.6°, dip=75.0°, rake=174.4°; B. strike=331.1°, dip=84.6°, rake=15.0°, and error range of P, T axes is about 20°。We use HASH method to solve the focal mechanism solutions for 12 small events(M_L≥3.0)in the sequence, and adopt double difference method(HypoDD)to analyze precisely the aftershock distribution.Relocation images show that, except 3 small shocks away from the swarm, the concentrated area of Laizhou sequence presents a NE-oriented major axis, and the sources distribution indicates a NW dipping fault, with a dip angle about 70°, which is in accord with the solutions for small events retrieved by HASH method.Finally, a discussion on the structural features of seismic tectonic and faulting process is made by using of all the results and relative geological data, and several opinions are concluded as follows:(1) There was an ordered rupture process at the earlier stage. At the very beginning(Jan 1, 2012 M_L 3.2), rupture spread towards northeast. After the M_S4.6 mainshock, rupture of the aftershocks became disordered, and sources distribution became more stochastic.(2) Small events before the mainshock scattered around the main rupture area; the occurrence of M_S4.6 event filled up the gap.(3) Strike-slipping is the dominant faulting type in the earlier stage of the sequence. Two foreshocks right before the mainshock display some thrust component. This maybe implicates the strengthening of regional stress relative to the mainshock. The focal mechanism variation of small aftershocks indicates stress field's adjustment at deep source area after the mainshock.(4) Slipping vectors of the fault are in accord with accurate location results, which reveals the dynamics of faulting process.(5) The seismotectonic characters of Laizhou earthquake sequence revealed by this paper are consistent with other regional geology data. Focal mechanisms conform to the orientation of regional maximum horizontal principal compressive stress. This implies that Laizhou earthquake sequence occurred under the regional stress field, and has relationship with the relative motion between tectonic blocks.

In the paper,we relocate the small earthquakes occurring in Linfen region from 1981 to 2010 by using the double-difference earthquake location algorithm. Our result shows that the distribution of relocated earthquakes is characterized by cluster activities,and most of the them occurred at the depths of 5～11km and 15～27km,which suggests that the two main seismogenic layers in this region are located in the upper and mid-upper crust. Earthquake frequency is high along both sides of the Subu Fault zone in the northern Linfen Basin,and most of the earthquakes in the north of Subu Fault zone are deeper than that in the south of Subu Fault. To some extent,this result indicates that the regional tectonic setting plays a major control role in the distribution of earthquakes. Analysis on the distribution of earthquake focal depths reveals that there exist deep faults which cut through the crust into the mantle in the middle of the Linfen Basin and connect with the active faults around the basin in the upper crust. The dislocations along the active faults on both sides of the basin are a reflex on the surface of faulting of the deep faults beneath the central basin.