Abstract: Many small earthquakes occurred intensively and continuously and formed an earthquake sequence after the M_L3.8 earthquake happened at Rushan County, Shandong Province on October 1, 2013. Up to March, 2017, more than 13 000 events have been recorded, with 3 429 locatable shocks, of which 31 events with M_L ≥ 3.0. This sequence is rarely seen in East China for its extraordinary long duration and the extremely high frequency of aftershocks. To track the developing tendency of the earthquake sequence accurately, 20 temporary seismometers were arranged to monitor the sequence activities around the epicenter of the sequence since May 6, 2014. Firstly, this paper adopts double difference method to relocate the 1 418 earthquakes of M_L ≥ 1.0 recorded by temporary seismometers in the Rushan earthquake sequence (May 7, 2014 to December 31, 2016), the result shows that the Rushan earthquake sequence mainly extends along NWW-SEE and forms a rectangular activity belt of about 4km long and 3km wide. In addition, the seismogenic fault of Rushan earthquake sequence stretches along NWW-SEE with nearly vertical strike-slip movement and a small amount of thrust component. Then we apply the P-wave initial motion and CAP to invert the focal mechanism of earthquakes with M_L ≥ 1.5 in the study area. The earthquakes can be divided into several categories, including 3 normal fault earthquakes (0.9%), 3 normal-slip earthquakes (0.9%), 229 strike-slip earthquakes (65.8%), 18 thrust fault earthquakes (5.2%), 37 thrust-slip earthquakes (10.6%)and 58 undefined (16.6%). Most earthquakes had a strike-slip mechanism in Rushan (65.8%), which is one of the intrinsic characteristics of the stress field. According to the focal mechanism solutions, we further utilized the LSIB method (Linear stress inversion bootstrap)to invert the stress tensor of Rushan area. The result shows that the azimuth and plunge of three principal stress (σ₁, σ₂, σ₃) axes are 25°, 10°; 286°, 45°; 125°, 43°, respectively. Based on the stress field inversion results, we calculated the focal mechanism solutions consistency parameter (θ)and the angle (θ₁)between σ₁ and P axis. The trend lines of θ and θ₁ were relatively stable with small fluctuation near the average line over time. Furthermore, the earthquake sequence can be divided into three stages based on θ and θ₁ values. The first stage is before September 16, 2014, and the variation of the θ and θ₁ values is relatively smooth with short period. All focal mechanism solutions of the three M_L ≥ 3.0 earthquakes exhibited consistence. The second stage started from September 16, 2014 to July 1, 2015, the fluctuation range of θ and θ₁ values is larger than that of the first stage with a relative longer period. The last stage is after July 1, 2015, values of θ and θ₁ gradually changed to a periodic change, three out of the four M_L ≥ 3.0 earthquakes (strike-slip type)displayed a good consistency. Spatially, earthquakes occurred mainly in green, yellow-red regions, and the focal mechanism parameters consistency θ was dominant near the green region (around the average value), which presents a steady state, and the spatial locations are concordant with the distribution of θ value. Moreover, all of M_L ≥ 3.0 earthquakes are located in the transitional region from the mean value to lower value area or region below the mean value area, which also indicates the centralized stress field of the region.

Key words: Rushan earthquake sequence, double difference location, focal mechanism solution, consistency parameter

摘要： 采用双差定位法对乳山台阵记录到的M_L≥1.0的1 418个地震事件（2014年5月7日—2016年12月31日）进行重新定位，利用P波初动和CAP方法对研究区内M_L≥1.5地震进行震源机制解反演，根据震源机制解资料进行应力张量反演，从而分析乳山地区应力状态以及震源机制一致性参数时空特征。结果显示：1）乳山地震序列沿NWW-SEE向展布，形成长约4km、宽约3km的矩形活动带；2）推测乳山地震序列发震断层走向为NWW-SEE、近直立、以走滑运动为主兼具一定逆倾滑分量；3）采用自助线性应力反演（LSIB）方法得到乳山地区主压应力轴方向为25°，倾角10°；中等应力方向为286°，倾角45°；主张应力方向为125°，倾角43°；4）乳山地震序列的震源机制一致性参数随时间变化整体呈现平稳状态，地震发生的空间位置与震源参数一致性空间分布关系较为吻合。

关键词: 乳山地震序列, 双差定位, 震源机制解, 一致性参数