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
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-fault 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.
