Abstract: Seismicity of the head portion of the three Gorges Reservoir region has increased obviously since the first impounding at the end of May, 2003. Most of the quakes are reservoir-induced, and mainly occurred on the Gaoqiao Fault west to the Zigui Basin, and the north end of the Xiannvshan Fault which locates between the Zigui Basin and Huangling Massif. The focal depths are usually less than 10 km, with the average of 4 km. Around UTC time 19:48 on June 16th, 2017, a M_S4.3 earthquake occurred near Zigui county (31.06°N,110.48°E). This earthquake has damaged more than 160 houses, and caused some collapses of medium-risk rock. Analyzing source parameters of the M4.3 earthquake can help us learn its seismogenic mechanism and provide reference for reservoir induced earthquakes. In this study, we use a total of 40 three-component broadband seismic stations within an epicenter distance of less than 3°. Two velocity models, Crust 2.0, a modified 1-D equivalent model, and the frequency-wavenumber (FK) method have been adopted to calculate Green's Functions at different depths. We use the CAP algorithm to invert focal mechanism of the M_S4.3 earthquake for each model. We also use the fundamental-mode Rayleigh wave amplitude spectra to determine the centroid depth. The results show that the focal mechanism has the best solution of plane I: 68°/59°/163° and plane II: 166°/75°/32°. The moment magnitude is M_W4.3 and the centroid depth is around 5 km. By comparing the results with those for the 2013 Badong earthquake, the 2014 Zigui earthquake and the 2011 Yangxin—Ruichang earthquake, we infer that the M_S4.3 earthquake might be a reservoir induced earthquake, but further study is still needed to determine the detailed seismogenic mechanism.

Key words: Focal Mechanism, CAP inversion, Rayleigh Wave Amplitude Spectra, Reservoir induced earthquake