Three-Dimensional V P and V P /V S Structure in the Longtan Reservoir Area by Local Earthquake Tomography

Abstract

High-resolution three-dimensional VP and VP/VS images in the Longtan reservoir area were obtained from local earthquake data by using 3,178 events with total 24,153 P-wave and 23,987 S-wave arrivals collected from 23 seismic stations. The tomographic images show that significant VP heterogeneities can be seen at layers of different depth in the Longtan reservoir area. Low-VP anomalies both beneath and around the main rivers in the reservoir area may be related to the composition of rocks which are mainly deposit carbonate and arenaceous shale, which contributes to water saturation. We deduced that the high porosity rocks beneath the main rivers may be fully saturated with water. The phenomenon that VP is relatively high in the area which is 10–20 km away from the rivers indicates that horizontal saturation of water is limited within a small range of area that is about 10–20 km from the main rivers. The characteristic is significant that seismicity in the Longtan reservoir area is coincident with the distribution of the low-VP area. VP/VS tomographic images show that VP/VS ranges from 1.8 to 2.05 in shallow layers above 4 km depth beneath the Longtan reservoir, suggesting the properties of the rocks are limestone and shale. At the depth of 7 km, the distribution of VP/VS image varies quite remarkably, especially in the dam area. This demonstrates that the range of influence by the saturation of water in the media below the reservoir surface can reach 4–7 km depth in the dam area.