Attenuation of coda waves in the Aswan Reservoir area, Egypt

Coda attenuation characteristics of Aswan Reservoir area of Egypt were analyzed using data recorded by a local earthquake network operated around the reservoir. 330 waveforms obtained from 28 earthquakes recorded by a network of 13 stations were used for this analysis. Magnitude of these earthquakes varied between 1.4 and 2.5. The maximum epicentral distance and depth of focus of these earthquakes were 45 km and 16 km respectively. Single back-scattering method was used for estimation of coda Q (Qc). The Q0 values (Qc at 1 Hz) vary between 54 and 100 and frequency dependence parameter “n” values vary between 1 and 1.2 for lapse time varying between 15 s and 60 s. It is observed that coda Q (Qc) and related parameters are similar at similar lapse times to those observed for those for Koyna, India, where reservoir induced seismicity is also observed. For both regions these parameters are also similar to those observed for tectonically active regions of the world, although Aswan is located in a moderately active region and Koyna is located in a tectonically stable region. However, Qc does not increase uniformly with increasing lapse time, as is observed for several parts of the world. Converting lapse time to depth/distance it is observed that Qc becomes lower or remains almost constant at around 70 to 90 km and 120 km depth/distance. This indicates presence of more attenuative material at those depth levels or distances compared to their immediate surroundings. It is proposed that this variation indicates presence of fluid filled fractures and/or partial melts at some depths/distance from the area of study. The Qc values are higher than those obtained for the Gulf of Suez and Al Dabbab region of Egypt at distances greater than 300 km from the study area by other workers. The turbidity decreases with depth in the study area.