Abstract

The temporal and spatial characteristics of 1924 post‐impoundment earthquakes (ML −0.3 to 3.1) recorded by 26 temporary stations from 16 March 2009 to 14 July 2010 in the Three Gorges reservoir area, Hubei Province, China, are investigated in this paper. The epicenters are mainly concentrated in three clusters and located along the Yangtze river within the range of 10 km from the reservoir waterfront. The hypocentral depths range from 0 to 15 km and appear to increase with the epicenters approaching the dam. Source parameters for 97 selected reservoir‐induced earthquakes of ML≥1.5 were estimated after applying corrections for geometrical spreading, frequency‐dependent Q, and site effects. The results show that the seismic moments (M0) are between 3.96×1011 and 4.07×1013  N·m, whereas static stress drops (Δσ) are 0.01–0.26 MPa. We find the static stress drops in this area vary with seismic moment for our data range, approximately as forumla. Apparent stresses we obtained lie between 0.0019 and 0.049 MPa and also increase with increasing seismic moment, indicating that large earthquakes radiate more energy per seismic moment than smaller ones in the Three Gorges reservoir area and do so more efficiently. Our results show that reservoir‐induced earthquakes appear to have systematically lower stress drop with respect to natural tectonic earthquakes, by about one order of magnitude. This may be attributed to the high pore pressures of the underground medium, and the presence of water decreasing the coefficient of friction.

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