Dynamical characterization of the 1982-2015 seismicity of Aswan region (Egypt)

In this study, the seismicity that occurred in Aswan region from 1982 to 2015 is investigated using robust statistical methodologies. The completeness magnitude, estimated by using two different methods (MAXC and GFT) is 2.5 for the whole catalog with b ~ 1.07. By using the expectation maximization algorithm, two depth classes of events were identified with a threshold at about 12 km. The events deeper and shallower than the threshold could be likely generated by the same mechanism: the loading/unloading operation of the Lake Nasser reservoir. We suggest that the shallow seismicity occurs on shallow small fractures in correspondence of the intersection of N-S faults with E-W faults, which may form a minor pull-apart basin. The deep events mainly occur along the right-lateral, strike-slip, E-W Kalabsha fault and the seismicity is characterized by mainshock-aftershocks sequences that mask the annual periodicity if not properly aftershock-depleted. Indeed, before applying the declustering on the seismic catalog, the analysis of the time-clustering properties of the shallow earthquakes reveals already the presence of annual modulation that is not evident in the time dynamics of the deep earthquakes. Furthermore, the shallow events are featured by the Allan Factor scaling exponent (measuring the strength of the time-clustering in an earthquake sequence) lower than that of the deep events, indicating a tendency of the time dynamics of the shallow earthquakes to behave more regularly than the deep ones. The detrended fluctuation analysis of the magnitude series suggests that the earthquake series are weakly persistent, characterized by the tendency of events of similar value of magnitude to follow each other.