Abstract: At 3:05, September 4, 2017, an M_L4.4 earthquake occurred in Lincheng County, Xingtai City, Hebei Province, which was felt obviously by surrounding areas. Approximately 60km away from the hypocenter of Xingtai M_S7.2 earthquake in 1966, this event is the most noticeable earthquake in this area in recent years. On the one hand, people are still shocked by the 1966 Xingtai earthquake that caused huge disaster, on the other hand, Lincheng County is lack of strong earthquakes. Therefore, this quake has aroused widespread concerns by the government, society and seismologists. It is necessary to clarify whether the seismogenic structure of this event is consistent with the previous seismicity and whether it has any new implications for the seismic activity and seismic hazard in this region. Therefore, it is of great significance to study its seismogenic mechanism for understanding the earthquake activity in Xingtai region where a M_S7.2 earthquake had occurred in 1966.
In this study, the Lincheng earthquake and its aftershocks are relocated using the multi-step locating method, and the focal mechanism and focal depth are determined by the "generalized Cut and Paste"(gCAP)method. The reliability of the results is analyzed based on the data of Hebei regional seismic network. In order to better constrain the focal depth, the depth phase sPL fitting method is applied to the relocation of focal depth. The inversion and constraint results show that aftershocks are mainly distributed along NE direction and dip to SE direction as revealed by depth profiles. Focal depths of aftershocks are concentrated in the depths of 6.5~8.2km with an average of about 7km. The best double-couple solution of the mainshock is 276°, 69° and -40° for strike, dip and slip angle for nodal plane I and 23°, 53° and -153° for nodal plane Ⅱ, respectively, revealing that it is a strike-slip event with a small amount of normal-fault component. The initial rupture depth of mainshock is about 7.5km obtained by the relocation while the centroid depth is 6km derived from gCAP method which was also verified by the seismic depth phase sPL observed by several stations, indicating the earthquake is ruptured from deep to shallow. Combined with the research results on regional geological structure and the seismic sequence relocation results, it is concluded that the nodal plane Ⅱ is the seismogenic fault plane of this earthquake.
There are several active faults around the hypocenter of Lincheng earthquake sequence, however, none of the known faults on the current understanding is completely consistent with the seismogenic fault. To determine the seismogenic mechanism, the lucubrated research of the M_S7.2 Xingtai earthquake in 1966 could provide a powerful reference. The seismic tectonic characteristics of the 1966 Xingtai earthquake sequence could be summarized as follows:There are tensional fault in the shallow crust and steep dip hidden fault in the middle and lower crust, however, the two faults are not connected but separated by the shear slip surfaces which are widely distributed in the middle crust; the seismic source is located between the hidden fault in the lower crust and the extensional fault in the upper crust; the earthquake began to rupture in the deep dip fault in the mid-lower crust and then ruptured upward to the extensional fault in the shallow crust, and the two fault systems were broken successively. From the earthquake rupture revealed by the seismic sequence location, the Lincheng earthquake also has the semblable feature of rupturing from deep to shallow. However, due to the much smaller magnitude of this event than that of the 1966 earthquake, the accumulated stress was not high enough to tear the fracture of the detachment surface whose existence in Lincheng region was confirmed clearly by the results of Lincheng-Julu deep reflection seismology and reach to the shallower fault. Therefore, by the revelation of the seismogenic mechanism of the 1966 Xingtai earthquake, the seismogenic fault of Lincheng earthquake is presumed to be a concealed fault possessing a potential of both strike-slip and small normal faulting component and located below the detachment surface in Lincheng area. The tectonic significance indicated by this earthquake is that the event was a stress adjustment of the deep fault and did not lead to the rupture of the shallow fault. Therefore, this area still has potential seismic hazard to a certain extent.

Key words: M_L4.4 Lincheng earthquake, source parameter, seismogenic structure, tectonic implications

摘要： 2017年9月4日河北省邢台市临城县发生了M_L4.4地震，临城及周边地区震感明显。此次地震发生在地震较少的临城县，是邢台地区近年来的一次显著地震，研究其发震机理对认识曾于1966年发生M_S7.2地震的邢台地区的地震活动性具有重要意义。文中基于河北区域数字地震台网资料，采用多阶段定位方法对临城M_L4.4地震序列进行了精定位，然后利用gCAP方法反演了此次地震的震源机制解和震源深度，并对该结果进行了可靠性分析，最后采用sPL深度震相对震源深度进行精确测定。结果显示，临城地震序列主轴沿NE向分布，深度剖面揭示倾向SE，深度集中在6.5~8.2km，平均约7km。主震最佳双力偶解节面I的走向、倾角和滑动角分别为276°、69°和-40°，节面Ⅱ的走向、倾角和滑动角分别为23°、53°和-153°，由此可知该地震为具有少量正断分量的走滑型地震。起始破裂深度约7.5km，矩心深度6km，多个台站观测到的sPL深度震相结果也验证了这一矩心深度的可靠性，地震由深部向浅部破裂。结合区域地质构造资料和地震序列定位结果分析，认为节面Ⅱ为发震断层面。根据1966年邢台地震发震机理的启示，推测发震断层为临城地区滑脱构造面之下的一条走滑兼具较小正断分量的潜伏断层。本次地震是深部断裂的一次应力调整，并没有导致深浅部断裂的贯通破裂，因此该地区仍然具有一定的地震危险性。

关键词: 临城M_L4.4地震, 震源参数, 发震构造, 构造意义