The 2014 Zigui Earthquake Sequence near the Three Gorges Dam in China

Seismicity in the Three Gorges Reservoir (TGR) region increased noticeably as the water level of the reservoir rises, since the Three Gorges Dam (TGD) was built in 2003. Here, we determined moment tensors of six earthquakes in the 2014 Zigui earthquake sequence (ZGS) in the TGR region using local and regional broadband seismic waveform data. We also determined the focal depth of the mainshock using its teleseismic waveform data and then relocated the epicenter using its travel times at local stations. High-precision locations of 64 events in the sequence were obtained by combining the double-difference relative location result and the mainshock's absolute location. The event locations and moment tensor solutions indicate that the ZGS occurred between depth 5 and 9 km on a previously unknown fault striking southwest and dipping similar to 80 degrees to the northwest. The event depth distribution and coulomb stress change estimation suggest that the ZGS were not induced directly by the reservoir water but triggered by the stress change from the annual reservoir water level variation. We estimated that the newly found fault has the potential for a magnitude 5.7 earthquake for which ground motion has a 16% probability to exceed the designed maximum intensity level at the TGD, though it would take more than 100 yr to accumulate the needed amount of slip.