2013年12月16日巴东M5.1地震序列及发震构造分析

ANALYSIS ON THE 2013 BADONG M5.1 EARTHQUAKE SEQUENCE AND THE SEISMOGENIC STRUCTURE

2013年12月16日湖北省巴东县发生M5.1地震,该地震是三峡库首区自2003年蓄水以来震级最大的1次。双差地震精定位结果显示,巴东地震序列呈NEE向线性展布,主震震源较深,余震震源深度逐渐变浅。垂直余震优势展布的震源深度剖面均显示出向SE变深的特征;倾角约50°左右,向深部变缓,揭示可能的发震断层走向NEE,倾向SE,倾角约为50°。利用矩张量反演得到了主震的震源机制解,与地震序列定位分析比较吻合的是NEE向节面,综合判断该节面可能为发震断层,倾向SE,倾角50°左右,性质以走滑为主兼逆冲分量。利用P波初动方法求解了ML2.0以上余震的震源机制,并采用FMSI程序反演得到了震源区的构造应力场,结果显示其方向与区域现代构造应力场方向一致,说明该地震序列受区域应力场的控制。地震序列发生在175m高水位运行期的水位缓慢降落阶段,基于ETAS模型对流体触发作用进行了定量检测,显示库水对地震序列的触发作用占17%,余震自激发占83%。综合分析认为,巴东M5.1地震为库水触发的构造型地震,NEE向断裂构造为可能的发震构造。

On 16^（th） September 2013,an M5.1 earthquake occurred in Badong County,Hubei Province,which is the biggest one since the first water impounding in 2003 in the head region of the Three Gorges Reservoir area. The crustal velocity information is needed to determine the earthquake location and focal mechanism. By comparison,the 1-D velocity structure model from Zhao was adopted in this study. Double difference location method was applied to determine the precise locations of the M5.1 earthquake sequence. Relocation results show that the dominant distribution of this sequence is along NEE direction. In order to understand its seismogenic structure,focal depth profiles were made.Profile AA＇ was along the sequence distribution,and the earthquake sequence extended about 12 km.Focal depth of mainshock is deeper than that of aftershocks,and earthquake rupture propagated laterally southwestward. The seismic profile BB＇ and CC＇ were perpendicular to profile AA＇,which represent the dip direction. Both profiles show that the focal depth becomes deeper toward southeast,and dip angle is about 50°. It means that the possible seismogenic fault strikes NEE and dips southeast. Focal mechanism could provide more information for judging the seismogenic structures.Many methods could obtain the focal mechanism,such as P-wave first motion method,CAP method,and some other moment tensor methods. In this paper,moment tensor inversion program made by Yagi Y is adopted. 12 regional seismic stations ranging from 100 ~ 400 km are picked up,and before the inversion,we removed the mean and trend. The seismic waveforms were band pass filtered between 0. 05 and 0. 2Hz,and then integrated into displacement. Green＇s functions were calculated using the discrete wavenumber method developed by Kohketsu. The focal mechanism of the M5. 1mainshock manifests that the NEE-striking fault plane probably is the possible seismogenic fault,which is consistent with the analysis of focal depth profiles. The focal mechanisms of the ML≥2. 0aftershocks are retrieved by P-wave first motion method,and the nodal plane I is in accordance with the earthquake sequence distribution and the fault plane of the mainshock. FMSI program was adopted to inverse the stress field in the earthquake area,and the results show that the earthquake sequence is under the control of the regional stress field. The earthquake sequence occurred on the stage of slow water unloading,and ETAS model was introduced to testify the influences of water level fluctuations on earthquakes. The results denote that the reservoir played a triggering role in the earthquake,however,the NEE-striking seismogenic fault is the controlling factor.