Abstract: The Wulong M_S5.0 earthquake on 23 November 2017, located in the Wolong sap between Wenfu, Furong and Mawu faults, is the biggest instrumentally recorded earthquake in the southeastern Chongqing. It occurred unexpectedly in a weak earthquake background with no knowledge of dramatically active faults. The complete earthquake sequences offered a significant source information example for focal mechanism solution, seismotectonics and seismogenic mechanism, which is helpful for the estimation of potential seismic sources and level of the future seismic risk in the region. In this study, we firstly calculated the focal mechanism solutions of the main shock using CAP waveform inversion method and then relocated the main shock and aftershocks by the method of double-difference algorithm. Secondly, we determined the seismogenic fault responsible for the M_S5.0 Wulong earthquake based on these calculated results. Finally, we explored the seismogenic mechanism of the Wulong earthquake and future potential seismic risk level of the region.
The results show the parameters of the focal mechanism solution, which are:strike24°, dip 16°, and rake -108° for the nodal plane Ⅰ, and strike223°, dip 75°, and rake -85° for the nodal plane Ⅱ. The calculations are supported by the results of different agencies and other methods. Additionally, the relocated results show that the Wulong M_S5.0 earthquake sequence is within a rectangular strip with 4.7km in length and 2.4km in width, which is approximately consistent with the scales by empirical relationship of Wells and Coppersmith(1994). Most of the relocated aftershocks are distributed in the southwest of the mainshock. The NW-SE cross sections show that the predominant focal depth is 5~8km. The earthquake sequences suggest the occurrence features of the fault that dips northwest with dip angle of 63° by the least square method, which is largely consistent with nodal planeⅡof the focal mechanism solution. Coincidentally, the field outcrop survey results show that the Wenfu Fault is a normal fault striking southwest and dipping 60°~73° by previous studies. According to the above data, we infer that the Wenfu Fault is the seismogenic structure responsible for Wulong M_S5.0 earthquake.
We also propose two preliminary genetic mechanisms of "local stress adjustment" and "fluid activation effect". The "local stress adjustment" model is that several strong earthquakes in Sichuan, such as M8.0 Wenchuan earthquake, M7.0 Luzhou earthquake and M7.0 Jiuzhaigou earthquake, have changed the stress regime of the eastern margin of the Sichuan Basin by stress transference. Within the changed stress regime, a minor local stress adjustment has the possibility of making a notable earthquake event. In contract, the "fluid activation effect" model is mainly supported by the three evidences as follows:1)the maximum principle stress axial azimuth is against the regional stress field, which reflects NWW-SEE direction thrusting type; 2)the Wujiang River crosscuts the pre-existing Wenfu normal fault and offers the fluid source; and 3)fractures along the Wenfu Fault formed by karst dissolution offer the important fluid flow channels.

Key words: Wulong earthquake, focal mechanism, double-difference relocation, seismogenic fault

摘要： 武隆M_S5.0地震是发生在七曜山-金佛山及彭水2条基底断裂所夹持的武隆凹陷束中的一次中强震，震源区位于芙蓉江断层、文复断层和马武断层的交会部位。文中采用CAP波形反演方法，计算了武隆M_S5.0地震的震源机制解与震源矩心深度，同时利用双差定位方法对武隆地震及其余震序列进行重定位，分析了本次地震的发震断层。结果显示，武隆M_S5.0地震震源机制解节面Ⅰ的走向为24°、倾角为16°、滑动角为-_<sup>￣108°，节面Ⅱ的走向为223°、倾角为75°、滑动角为-85°；重定位后的余震序列主要分布在主震的SW向，深度集中于5~8km。分析表明，余震分布特征与震源机制解节面Ⅱ一致，且符合文复断层走向SW、倾向NW、倾角60°~73°的产状特征。由此推测此次武隆M_S5.0地震的发震构造为文复断层，并初步探讨了其可能的发震模式。

关键词: 武隆地震, 震源机制, 双差定位, 发震断层