Long-term and annual influence of Aswan Reservoir (Egypt) on the local seismicity: a spatio-temporal statistical analysis

Abstract

Earthquakes continue to occur in the Kalabsha embayment region near the Aswan reservoir for more than three decades. These earthquakes, including the 1981 mainshock (M 5.3), have been considered to be reservoir triggered and pore pressure appears to be the main factor for their occurrence. However, influence of reservoir water levels on the earthquakes of the region, which is significant in the initial period, is reported to be weakening with time. We statistically analyse the influence of the reservoir water levels on spatio-temporal distribution of the earthquakes for the period 1982–2013. We divided the seismicity of various zones in different time windows, depending upon their clustering, and employed the power spectrum and singular spectrum analysis (SSA). The influence of long-term and annual variations in the water level on the seismicity is very much apparent in SSA. In the 45 years of reservoir filling history, episodes of water levels more than 170 m, which lead to filling of the embayment region, appear to be correlated with the increase in seismicity of the Aswan region. In the power spectrum analysis, a strong annual peak is found for water level time series. However, in the earthquake time series, on an average, a moderate and weak annual period cycle is present in the initial (1982–1988) and in the later (2001–2013) period, respectively. In the intermediate period (1989–2000) when the water level was the least in the embayment area, a very weak annual cycle is present. Non-consideration of spatio-temporal variation in the seismicity leads to erroneous and misleading results of no or weaker reservoir influence in the later period. This also highlights that the time and space dependent pore pressure diffusion process is likely responsible for earthquake triggering in the Aswan region.