Seismotectonics of the 26 November 2005 Jiujiang‐Ruichang, Jiangxi, Ms 5.7 Earthquake

Abstract:The 26 November 2005 Jiujiang‐Ruichang, Jiangxi, Ms 5.7 earthquake occurred in a seismotectonic setting of moderate earthquake. The northwest‐trending Xiangfan‐Guangji fault (XFG) does not enter into the epicenter vicinity, but the northeast‐trending Ruichang‐Wuning fault (RWF) as a regional fault extends to the epicenter nearby, appearing as the Ruichang basin and its marginal faults. Tilting of the Ruichang Basin (RCB) in the Quaternary was controlled by the RCB southeast‐marginal, buried fault (RSMBF). Shallow geophysical survey reveals that the RSMBF caused an offset of the reflection layers. Drill hole columnar section demonstrates that there are about 10–12 m displacement in the lower section of the middle‐Pleistocene Series along the RSMBF, but no disruption is found in the upper section of the middle‐Pleistocene Series. The RSMBF not only has activity in the Quaternary, but also coincides with the nodal plane I from the focal mechanism of the Jiujiang‐Ruichang Ms 5.7 earthquake. This evidence, including aftershock distribution and isoseismic lines, strongly suggests that the RSMBF might be the seismogenic tectonics. The RWF is discontinuous at the surface, and consists of three en echelon Quaternary basins, which are the Ruichang, Fanzhen and Wuning basins. Three moderate earthquakes, the Fanzhen ML 4.9 earthquake, the Yejiapu ML 4.1 earthquake and the Jiujiang‐Ruichang Ms 5.7 earthquake, have happened in the basins since 1995. The seismogenic tectonics of the Jiujiang‐Ruichang Ms 5.7 earthquake is not isolated, but may be controlled by the RWF at depth, the slip of which causes the accumulation of energy for earthquake occurrence.