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Source characteristics and tectonic implications of moderate earthquake, northeastern Cairo Prefecture

Abdel-Fattah A.K., Badawy A.

Acta Geophysica Polonica

2004

Vol. 52, nr 1

29-43

Using local seismograms of the Egyptian National Seismological Network (ENSN), source characteristics of a moderate earthquake Mw = 4.5 (28 December 1999) are analyzed. In this analysis, the Empirical Green's Function (EGF) deconvolution technique is applied. The records of an appropriate aftershock are taken as the EGF and are used to deconvolve the mainshock seismograms, thus obtaining a Relative Source Time Function (RSTF) at each station. The deconvolution is performed using P waves in frequency domain. From the time-domain analysis of the RSTF, the resulting source time functions indicate a simple rupture process. The azimuthal dependencies of the RSTF pulse amplitudes and widths are used to estimate rupture velocity and rupture direction for the mainshock. The azimuth of rup-ture direction is obtained using a global optimization method. We found that the rupture direction of the main event propagated toward S175 E with an averaged rupture velocity around 0.75 Vs. The result obtained for rupture direction is in agreement with one of the nodal planes of focal mechanism. From the rupture directivity analysis, focal mechanism and geological evidence it follows that the investi-gated event reflects a reactivation of a NW-SE Oligocene deep-seated normal faulting with sinistral movement. Source parameters were estimated using RSTFs of the mainshock, including seismic moment of 2.85×1015 Nm, fault radius of 344 m, fault length 1460 m, and static stress drop of 3.071 MPa.

Discrimination of microearthquakes and quarry blasts: a case study in northern Egypt

Badawy A.

Acta Geophysica Polonica

2001

Vol. 49, nr 3

331-341

In northern Egypt, both microearthquakes and quarry blasts are observed in complicated tectonic region. Therefore, reliable event identification, preferably based on a simple method that can be easily used in a routine processing, is essential for accurate and detailed seismotectonic studies. Amplitude spectra of microearthquakes and quarry blasts recorded by stations of the Egyptian National Seismograph Network (ENSN) have been computed. Most quarry blasts in northern Egypt are ripple-fired with time delays ranging from 20 to 25 ms; consequently, we seek an event discrimination method based on the recognition of ripple-firing patterns in the signal. For this end, we compared the spectra of complete seismogram signals of 55 microearthquakes and 150 quarry blasts at the same stations to eliminate wave-path and distance effects. The obtained results clearly indicate that the microearthquakes spectra seem to contain high frequency energy more than thee spectra of quarry blasts. We found a prominent spectral modulation in quarry blasts spectra but not in the spectra computed from microearthquake seismograms. The observed spectral modulation is mainly due to ripple-firing explosion and it is independent of time in the coda. Finally, the quarry blasts can be discriminated from non-ripple-firing events (e.g., earthquakes) on the basis of low frequency (less than 15-20 Hz) content and the spectral modulations.