Source parameters and fault plane determinations of the 28 December 1999 northeastern Cairo earthquakes

Abstract

The velocity records of the 28 December 1999 northeastern Cairo, Egypt earthquake sequence have been analyzed for relocation, focal mechanism, source parameters, and rupture directivity. A cross-correlation analysis was performed to recognize the waveform similarity and to obtain relative location on the basis of small time-delays between first arrivals. The three events exhibit good correlation and similarity in focal mechanisms. Actually, the fault plane solutions display normal faulting with strike-slip component. Dynamic source parameters were estimated for the three events using P-wave spectrum for 21 selected seismograms at the seven nearest stations. The corner frequency ( f o), low-frequency amplitude ( Ω o), and the average attenuation path were simulated by nonlinear least square best fitting. The mean of the obtained values shows that the mainshock occurred on a fault length of 822 m with relative displacement of 51 cm, seismic moment of 5.59×10 15 N m, and stress drop of 5.9 MPa. The azimuthal variation of the obtained low frequency spectrum amplitude was used to estimate the rupture velocity and direction. The mean rupture velocity, estimated to be eight-tenths of the local shear wave velocity, as well as the azimuthal variation, provides an indication for the rupture directivities of the three events that are of 175±5°. Estimated rupture directions were incorporated with P-wave focal mechanisms to identify the actual fault planes of these events. Therefore, the preferred fault planes are those that have the strike of 137°, 148° and 132°, dip of 59°, 45° and 51° and rake of −39°, −33° and −45°, respectively. In this case, the slip component in strike direction indicates a left lateral motion along the NW fault. The obtained results imply the reactivation of the Oligocene deep-seated faults in the Gulf of Suez-Red Sea trend (NW–SE).