ANALYSIS ON THE 2013 BADONG M5.1 EARTHQUAKE SEQUENCE AND THE SEISMOGENIC STRUCTURE |
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| Authors: | ZHANG Li-fen LIAO Wu-lin LI Jing-gang WEI Gui-chun SHEN Xue-lin |
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| Affiliation: | 1.Institute of Geophysics, China Earthquake Administration, Beijing 100081, China;2.Key Laboratory of Earthquake Geodesy, Institute of Seismology, China Earthquake Administration, Wuhan 430071, China |
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| Abstract: | On 16th 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 12km. 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 100~400km 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 M5.1 mainshock 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≥2.0 aftershocks 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. |
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| Keywords: | double-difference location method focal depth profile focal mechanism tectonic stress field ETAS model |
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