Seismotectonics in the Three Gorges reservoir area is investigated by using the P-wave tomography with earthquakes that occurred before the impoundment of the reservoir. The result indicates that most of these events occurred in or around the velocity-gradient belts between high-velocity and low-velocity anomalies. These belts have similar characteristics to buried-fauh zones. Stresses generated by movement of partially molten upper-mantle materials and thermal stress may have jointly contributed to the seismic activities along the faults and such buried faults, and possibly activated new earthquake ruptures.
The three-dimensional P wave velocity structures of upper and middle crust beneath the Three Gorges reservoir dam and adjacent regions were reconstructed by means of the seismic tomography, using the absolute P-arrivals from the events observed by the Three Gorges seismic network. Our preliminary study of the deep structure shows: (1) the lithology of the Huangling anticline, integral and highly so-lidified, exhibits a uniformity. Down to a depth of 20 km, the Huangling anticline still shows high- velocity anomalies. Note that the characteristics of the velocity images with the different depths are not the same; (2) the depth of the sedimentary rocks beneath the entire Zigui basin is not constant, which is less than 6 km; (3) the top boundary of the crystalline basement varies from 0 to 7 km, whereas the bottom one between 14 and 16 km; (4) the partially molten substances of the upper mantle are located in the Badong-Zigui and its adjacent regions as well as nearby Peishi area. The development and evo-lution of the current tectonics are associated with the cooling and solidifying process of the molten substances within the upper mantle; (5) the different velocity structures of the deep crust in both east and west side of the Jiuwanxi fault are to be investigated in order to understand the origin of gravita-tional gradient zone in the Three Gorge areas; and (6) the velocities within the upper crust would be affected by reservoir water permeability.
LI QiangZHAO XuCAI JinAnLIU RuiFengLONG GuiHuaAN YanRu
