A Study On Reservoir-Induced Earthquake Location And Focal Mechanism Characteristics Of The Yangtze River Three Gorges Reservoir Region

The Yangtze River Three Gorges Project is one of the world’s largest hydro-junction projects, as well as the key project harnessing and developing the Yangtze River. From the end of May 2003 the first phase of the Three Gorges Reservoir began to store water. By the end of November 2009 the third phase of storing water completed, when the water level raised gradually from 69 meters up to 171 meters. In the process of the three phases, seismic activity in the reservoir region increased significantly, with an increasing trend of frequency and seismic geological hazard. Relying on the "Eleventh Five-Year Plan" National Science and Technology Supporting Programme, for the key project "Exploration Technology of Reservoir-induced Seismicty Conditions", a dense seismic monitoring station network consisting of 26 stations using REFTAK130 broadband recorder and L-22 short-period seismometers, has been constructed by Institute of Geology, China Earthquake Administration in 2009. 2995 earthquakes of ML-0.8~2.9 had been recorded by the dense network in the Three Gorges reservoir region during March to December 2009, and the rich and high-precision data make it possible to study location and focal mechanisms analysis of the reservoir-induced shocks in this region.High-accuracy earthquake location and focal mechanism solution characteristic analysis are the foundation of reservoir-inducted seismicity research. This used the double-difference location method (hypoDD) to relocate the 2837 earthquakes in the Three Gorges Reservoir region. The result of relocation shows that it makes the diffuse distribution of microseismic more concentrated and reveals horizontal lineations of hypocenters. The majority of epicenters in the Three Gorges reservoir area is distributed in the northern of Xinv fault and Jiuwanxi fault near the Xiangxi River, also on the two sides of the Yantze River west of Xietan town. Also there are several lineations of epicenter distribution in the Badong area with the same strike of the strata or fractures, one of which is the lineation from the south of Lianghekou to the north of Guandukou, and the two sides of the Gaoqiao fault, and also the two lineations in the west of the Yandu river. The epicenter clusters are distributed mainly within 5km away from the shore of the reservoir, and the hypocenters are very shallow, with an average depth of 4km.This work used the vertical digital records of Pg and Sg waves based on the program of focal mechanism determination using amplitude ratios and the first motions of Pg wave. 119 events with ML1 ~ 2.7 were selected to determinate the focal mechanism solutions. As a result, the focal mechanism solutions mainly are strike slip and normal and also vertical dip-slip faulting. There are also some differences of the characteristics in various districts. The normal and vertical dip-slip focal mechanism solutions are the majority on the two sides of the reservoir near the shore. The strike slip ones are the majority in the north of the Xinv fault and the two sides of the Gaoqiao fault; and there are several thrust ones near the Gaoqiao fault. This work also has made analysis of the characteristics of the focal mechanism solutions by systematic clustering procedure. The result shows that the principal compression stress field is in south to north or northeast to southwest near horizontal, different from the regional tectonic stress field.In combination with the water level change data of the Three Gorges reservoir, this work found that the seismic frequency increases relatively with the reservoir loading process, and the seismic frequency decreases relatively with the reservoir unloading process in the Three Gorges reservoir region,indicating the positive correlation between the frequency of seismic activity and the water level of the Three Gorges reservoir.In summary, the majority of earthquakes occurred in the Three Gorges reservoir region are induced by the reservoir, with several mechanisms. The earthquakes relatively closer to the shore of the reservoir are characterized by gravity-based rock unloading reservoir-induced seismicity with normal and dip-slip faulting; Those in two sides of the Gaoqiao fault in the north of Xinv fault far away from the shore may be of tectonic reservoir-induced seismicity. The lineation distribution of earthquakes from Guandukou to Majia country may be related to the karst in the Triassic limestone, and analysis suggests that these events are of karst collapse reservoir-induced seismicity.