Mohamed N. ElGabry

Neo-deterministic seismic hazard assessment in North Africa

North Africa is one of the most earthquake-prone areas of the Mediterranean. Many devastating earthquakes, some of them tsunami-triggering, inflicted heavy loss of life and considerable economic damage to the region. In order to mitigate the destructive impact of the earthquakes, the regional seismic hazard in North Africa is assessed using the neo-deterministic, multi-scenario methodology (NDSHA) based on the computation of synthetic seismograms, using the modal summation technique, at a regular grid of 0.2 × 0.2°. This is the first study aimed at producing NDSHA maps of North Africa including five countries: Morocco, Algeria, Tunisia, Libya, and Egypt. The key input data for the NDSHA algorithm are earthquake sources, seismotectonic zonation, and structural models. In the preparation of the input data, it has been really important to go beyond the national borders and to adopt a coherent strategy all over the area. Thanks to the collaborative efforts of the teams involved, it has been possible to properly merge the earthquake catalogues available for each country to define with homogeneous criteria the seismogenic zones, the characteristic focal mechanism associated with each of them, and the structural models used to model wave propagation from the sources to the sites. As a result, reliable seismic hazard maps are produced in terms of maximum displacement (Dmax), maximum velocity (Vmax), and design ground acceleration.

Infrasound detection of meteors

Abstract Meteorites that penetrate the atmosphere generate infrasound waves of very low frequency content. These waves can be detected even at large distances. In this study, we analyzed the infrasound waves produced by three meteors. The October 7, 2008 TC3 meteor fell over the north Sudan Nubian Desert, the February 15, 2013 Russian fireball, and the February 6, 2016 Atlantic meteor near to the Brazil coast. The signals of these three meteors were detected by the infrasound sensors of the International Monitoring System (IMS) of the Comprehensive Test Ban Treaty Organization (CTBTO). The progressive Multi Channel Technique is applied to the signals in order to locate these infrasound sources. Correlation of the recorded signals in the collocated elements of each array enables to calculate the delays at the different array element relative to a reference one as a way to estimate the azimuth and velocity of the coming infrasound signals. The meteorite infrasound signals show a sudden change in pressure with azimuth due to its track variation at different heights in the atmosphere. Due to movement of the source, a change in azimuth with time occurs. Our deduced locations correlate well with those obtained from the catalogues of the IDC of the CTBTO.

Assessment of the accuracy and stability of ENSN sensors responses

Abstract The Egyptian National Seismic Network (ENSN) is an advanced scientific tool used to investigate earth structure and seismic activity in Egypt. One of the main tasks of the engineering team of ENSN is to keep the accuracy and stability of the high performance seismic instruments as close as possible to the international standards used in international seismic network. To achieve this task, the seismometers are routinely calibrated. One of the final outcomes of the calibration process is a set of the actual poles and zeros of the seismometers. Due to the strategic importance of the High Dam, we present in this paper the results of the calibrating broad band (BB) seismometers type Trillium-40 (40 second). From these sets we computed both amplitude and phase responses as well as their deviations from the nominal responses of this particular seismometer type. The computed deviation of this sub-network is then statistically analyzed to obtain an overall estimate of the accuracy of measurements recorded by it. Such analysis might also discover some stations which are far from the international standards. This test will be carried out regularly at periods of several months to find out how stable the seismometer response is. As a result, the values of the magnitude and phase errors are confined between 0% and 2% for about 90% of the calibrated seismometers. The average magnitude error was found to be 5% from the nominal and 4% for average phase error. In order to eliminate any possible error in the measured data, the measured (true) poles and zeroes are used in the response files to replace the nominal values.