A seismologic study of the Three-Gorges Reservoir (TGR) region, China
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For many years there have been a number of geologic investigations have been conduct in the Three Gorges Reservoir (TGR) region of the Yangtze River that provided useful information to hydro-power generation, engineering construction, flooding control and geohazard mitigation. However, there has not yet been a complete knowledge on the deep lithospheric structure, especially its seismic properties of the TGR region. The water impoundment of the 660-kilometer-long TGR makes the safety and geohazard mitigation a top concern in the TGR region and the entire central China. To address such concern, a new study of the lithospheric structure and seismotectonics of the TGR region has been carry out. Previous seismic studies mainly used the data acquired by the local permanent networks and focused on the event location and 1-D seismic velocity model building. However, the coverage of the local network cannot offer the sufficient coverage for the microearthquake monitoring and is not dense enough for reflection array analysis. In addition, due to the complex tectonic history, seismic velocity of this region is heterogeneous on surface and has high gradient at near surface. The 1-D velocity model doesn’t provide sufficient accuracy for event location and local structural studies. Here I established a hybrid event location method based on the combination of traditional location method and the modified master station method to overcome the velocity-depth ambiguity, and used a newly inverted 3-D velocity model to re-locate the earthquakes in the TGR region. The relocated hypocenters match with the subsurface features. To address the poor coverage of the local permanent network I deployed temporary dense networks of geophones in TGR region in the summer from 2008 to 2011. During the period of observation a number of microearthquakes less than ML0.1, which were missed by the local permanent network, were recorded. To overcome the limitation of frequency band we established the power spectral density ratio (PSDR) method to estimate the recoverable frequencies. By applying the PSDR method, different types of earthquakes, including an Mb 5.9 event with a 93-degree offset, are recognized from the geophone data. Application of the reverse time imaging method to the data reveal multiple-earthquake phenomenon at two nearby faults. Application of the ray-tracing modeling and virtual source imaging method to the acquired data revealed the layer structure of the crust and upper mantle. The imaged shallow basins matches well with the surface geological feature and have the bottoms at 5 km depth. An upper mantle reflector at around 50 km depth may have the relation with the complex tectonic history of the TGR region.