Seismicity and 3D velocity structure in the Aswan Reservoir Lake area, Egypt

Aswan reservoir lake in Egypt is the world's second largest reservoir to trigger induced seismicity. It produced an induced earthquake Ms 5.3 in the reservoir area in 1981. Since then the area is experiencing reservoir triggered seismicity, and large number of earthquakes including several swarms are recorded. A 13-station three component digital network is in operation in the area, and we have analysed about 1000 events that are recorded during 2004-2007. These data are analysed using the Local Earthquake Tomography (LET) method to image the 3D velocity structure and to relocate the events for better understanding the seismic processes and active faults in the area. The results depicted heterogeneous velocity structure with low and high velocity blocks at different depth ranges and revealed deep rooted active faults. It is observed that the upper shallow (depth < 5 km) crustal earthquakes are mostly influenced by the fluid saturated heterogeneous rock matrix, which is reflected by low Vp and higher Vp/Vs or higher Poisson ratio. The earthquakes below this depth range are more of tectonic origin rather than the reservoir triggered. The high velocity zones at deeper (10-15 km) depth at the fault ends are mostly the source zones for stress concentration for generating earthquakes.