A Long-Lasting Relaxation of Seismicity at Aswan Reservoir, Egypt, 1982-2001

Abstract

The Aswan reservoir seismicity is accepted as an example of reservoirinduced seismicity with the M 5.4 event of 1981, which occurred 15 years after the reservoir impoundment started. During the 1982-2001 period, the Aswan seismicity separates into shallow and deep seismic zones, between 0 and 15 and 15 and 30 km, respectively. These two seismic zones behave differently over time, as indicated by the seismicity rate, depth migration, b-value, and spatial clustering. For the deep events, the rate decreases with time, the depth remains at a constant or increasing depth, and the b-value is lower than for the shallow events, which migrate toward the surface with a close to constant seismicity rate. Of all the previous parameters, only the seismicity rate correlates with seasonal variations of the lake level. This is positive evidence for the Aswan reservoir seismicity to be reservoir triggered in the 1982-2001 period. Clustering over time and space expresses that numerous aftershock sequences are activated, that is, a minimum of 35% of the seismicity, similarly to tectonic seismicity. The observed decrease of the b-value with depth is also a property suggested for natural events. Our results suggest that the Aswan seismicity emerges both from the water-level loading and the interplay between induced earthquakes themselves through numerous standard aftershock sequences. This latter process induces stochastic fluctuations in the seismicity patterns that inhibit a recovery of a simple seismic response to water-level changes. Even though aftershock sequences dominate the temporal fluctuations of seismicity, this does not negate the importance of influence of the reservoir in sustaining this unusual long-lasting seismic episode. During the 1982-2001 period, the Aswan seismicity appears as a powerlaw relaxation response to the 1981 M 5.4 shock, this response being decorated by long-term (larger than 100 days) water-level changes.

Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt

“…Nonetheless the existence of fluids in the porous medium is required to simultaneously explain seismicity triggering and low resistivity. Mekkawi et al [18] showed that during a period of ten years after the occurrence of the M = 5.4 earthquake the seismicity of this region correlated with the lake level, implying the presence of fluids and delayed transfer of pore pressure. The delay is 120 days for deep seismicity and 60 days for the shallow activity.…”

“…Annual level variations in Lake Aswan have been well correlated with the local seismic activity [18], suggesting that a transfer of hydrostatic pressure occurs through the faulted rocks. The goal of the current research is to confirm the presence of crustal fluids and thus, to explain the triggering mechanism for earthquakes occurring in this seismically active area; this was achieved by looking for a typical conductive signature of transcurrent faults such as the Kalabsha Fault [19].…”

“…Since 1989, the ASN consists of 13 field stations. As mentioned in the Introduction, Mekkawi et al [18] analyzed the Aswan seismicity patterns observed during the period 1982-2001 in an attempt to explain how water level loading could drive the seismicity. The Aswan seismicity may be subdivided into shallow (0.1-15 km) and deep (N 15 km) seismic zones (Fig.…”

Methodology for geohazard assessment for hydropower projects

“…(Snaebjörnsson, et al 2006;Sigtryggsdóttir et al 2013b) TM Reservoirs may trigger RTEs. (Gupta 2002;Telesca et al 2012;ICOLD 2011) Reservoir (Res) (Gráková et al, 2007) ) IW (Muço, 2013 Pandey and Chadha (2003), Mekkawi (2004), Gupta (2002) d Def is abbreviation for deformation Nat Hazards (2003) …”

Analysis of the cross-correlation between seismicity and water level in the Aswan area (Egypt) from 1982 to 2010

“…In all the other periods, the crosscorrelation coefficient was very small, leading to a generally weak relationship between the water fluctuations and seismicity. Mekkawi et al (2004) studied the 1982-2001 shallow (between 0 and 15 km) and deep (between 15 and 30 km) Aswan seismicity, finding that among all the possible seismic parameters, only the seismicity rate correlates with the seasonal fluctuations of the lake level, indicating positive evidence for the Aswan seismicity to be reservoir-triggered in the 1982-2001 period. Furthermore, time and space clusterization indicated that numerous aftershocks are activated, suggesting that the Aswan seismicity emerges both from the water level loading and the interplay between induced earthquakes themselves through the aftershock activation.…”