Parametri izvora za značajne potrese u Egiptu (1992–1998), određeni iz magnitudnog spektra P-valova širokopojasnih seizmograma dalekih potresa

Using the P-wave magnitude spectra of the vertical component of teleseismic broadband seismograms, average source parameters have been retrieved for five significant earthquakes of Mw ≥ 5.7 occurring in Egypt namely, 1992 Cairo earthquake, 1998 Alexandria earthquake and three events which occurred in the Gulf of Aqaba region between 1993 and 1995. The magnitude spectrum represents the velocity amplitude density spectrum at the earthquake source, scaled in magnitude units. The maximum of the magnitude spectrum along with the period at which the maximum occurs are used to estimate the source parameters. For precise determination of the source parameters, two different methods for deriving the corner periods are applied. The obtained source parameters were compared with those derived in previous studies. The results show that within the moment magnitude range 5.5 ~ 7.2, the corner periods are 1.29 ~ 11.6 s, length of the fault ruptures are 5.5 ~ 50 km and the stress drops are 0.5 ~ 4.8 MPa. The derived stress drop shows an increasing trend with the seismic moment for the three Gulf of Aqaba earthquakes. The 1995 Gulf of Aqaba earthquake of Mw = 7.2, the largest earthquake to have occurred in Egypt in the last century is characterized by a higher complexity compared to the other events, that are much simpler. The values of the corner periods for this earthquake are azimuth dependent due to complexity and strong directivity of its rupture. For a detailed description of the complexity of 1995 earthquake additional source parameters are also estimated in terms of an inhomogeneous source model. These parameters are the asperity radius, displacement across the asperity, localized stress drop and ambient faulting stress. The average stress drop of Cairo, 1992 and Alexandria, 1998 intraplate earthquakes shows larger values compared with the interplate 1993 Gulf of Aqaba earthquakes. Generally, the estimated seismic moment using the magnitude spectra reflect good agreement with the estimates made from the other techniques for simple source while the complex source yields smaller values.Koriste}i magnitudni spektar P-valova vertikalne komponente širokopojasnih seizmograma dalekih potresa, određeni su prosječni parametri izvora za pet značajnih potresa s magnitudama Mw ≥ 5.7 koji su se dogodili u Egiptu (potres kod Kaira iz 1992, Aleksandrije 1998. i tri potresa u zaljevu u Aqaba regiji između 1993. i 1995.). Magnitudni spektar predstavlja spektar gustoće amplitude brzine u izvoru potresa, iskazan u jedinicama magnitude. Za procjenu parametara izvora korišten je maksimum magnitudnog spektra te period na kojem je opažen maksimum. Kritični period određen je pomoću dvije metode. Dobiveni parametri uspoređeni su s onima koji su izvedeni u prethodnim studijama. Rezultati pokazuju da u području magnituda 5,5 ~ 7,2 kritični periodi iznose 1,29 ~ 11,6 s, duljine rasjednih pukotina 5,5 ~ 50 km i pad napetosti od 0,5 ~ 4,8 MPa. Za tri potresa u zaljevu Aqaba pad napetosti pokazuje trend povećanja sa seizmičkim momentom. Potres iz 1995. u zaljevu Aqaba magnitude Mw = 7,2, najjači potres koji se u prošlom stoljeću dogodio u Egiptu, složeniji je u odnosu na druge potrese. Kritični period ovog potresa ovisan je o azimutu zbog složenosti i izražene usmjerenosti pucanja rasjeda. Za potres iz 1995. procijenjeni su i dodatni parametri izvora u smislu modela nehomogenog izvora. Ti parametri su radijus zone početnog rasjedanja, pomak duž nje, lokalni pad napetosti te okolna napetost rasjedanja. Prosječni pad napetosti za potrese u Kairu 1992. i Alexandriji 1998. veći je nego za potres u zaljevu Aqaba 1993. Procijenjeni seizmički moment dobiven upotrebom magnitudnog spektra uz pretpostavku jednostavnog izvora dobro se podudara s procjenama pomoću drugih metoda, dok pretpostavljeni složeniji izvor daje manje vrijednosti

Seismic hazard studies in Egypt

Abstract The study of earthquake activity and seismic hazard assessment of Egypt is very important due to the great and rapid spreading of large investments in national projects, especially the nuclear power plant that will be held in the northern part of Egypt. Although Egypt is characterized by low seismicity, it has experienced occurring of damaging earthquake effect through its history. The seismotectonic sitting of Egypt suggests that large earthquakes are possible particularly along the Gulf of Aqaba–Dead Sea transform, the Subduction zone along the Hellenic and Cyprean Arcs, and the Northern Red Sea triple junction point. In addition some inland significant sources at Aswan, Dahshour, and Cairo-Suez District should be considered. The seismic hazard for Egypt is calculated utilizing a probabilistic approach (for a grid of 0.5° × 0.5°) within a logic-tree framework. Alternative seismogenic models and ground motion scaling relationships are selected to account for the epistemic uncertainty. Seismic hazard values on rock were calculated to create contour maps for four ground motion spectral periods and for different return periods. In addition, the uniform hazard spectra for rock sites for different 25 periods, and the probabilistic hazard curves for Cairo, and Alexandria cities are graphed. The peak ground acceleration (PGA) values were found close to the Gulf of Aqaba and it was about 220 gal for 475 year return period. While the lowest (PGA) values were detected in the western part of the western desert and it is less than 25 gal

Seismological aspects of the Cairo earthquake, 12th October 1992

On 12 October 1992 a significant earthquake (MB = 5.8) occurred southwest of Cairo in the vicinity of the Dahshour region. For the mainshock, an average moment release M 0 = 9.70E17 n-m; a fault length = 13.8 km; an average displacement = 0.22 m; a stress drop = 0.760 MPa and maximum spectral magnitude = 5.78 were obtained. Spectral magnitude calculations were used in this analysis. The distribution of the well-recorded aftershocks over 15 months using a temporary seismic network installed immediately after the mainshock shows a zone of concentrated activity. Three composite focal mechanism solutions are constructed, using P wave polarity data for 30 earthquakes. These solutions have a mechanism, involving normal and right lateral strike slip motion along E-W to ENE-WSW trending fault plane. The P-wave polarity data of the individual 30 earthquakes are inverted to determine the stress tensor. The stress field estimated is extensional with s3 in the direction of NNE. The stress pattern determined from the inversion is in good agreement with the one estimated from the three composite fault plane solutions, while the neotectonic situation in Northern Egypt supports ENE-WSW extensional movement. However, the region of study has suffered both extensional tectonic activity of the Northern Red Sea rift zone and the compressional tectonic activity along the Hellenic arc. The inconsistency of the principal tension directions may belong to the interaction between the extensional and compressioal tectonics

Earthquakes focal mechanism and stress field pattern in the northeastern part of Egypt

Egypt is characterized by moderate size seismicity where earthquakes are distributed within several active regions. In the present study, we investigated the source mechanism of earthquakes using the digital waveform data recorded by the Egyptian National Seismic Network (ENSN) during the period from 2004 to 2008. The focal mechanisms are constructed with high reliability based on the polarity of the first motion of P-wave. These solutions are used to examine the mode of tectonic deformation and the present-day stress field pattern affecting on different tectonic provinces in the northern part of Egypt. The results demonstrate mainly a normal faulting mechanism with minor strike slip component generally trending parallel to the northern Red Sea, the Suez rift, Aqaba rift with their connection with the great rift system of the Red Sea and the Gulf of Suez and Cairo-Alexandria trend. The inversion technique scheme is used also in the present study for determining the regional stress field parameters for earthquake focal mechanism solutions based on the grid search method of Gephart and Forsyth (1984). The Results of the stress tensor using focal mechanisms of recent earthquakes show a prevailed tension stress field in N52°E, N41°E and N52°E for the northern Red Sea, Gulf of Suez and Gulf of Aqaba zone respectively

Seismological aspects of the Cairo earthquake, 12th October 1992

On 12 October 1992 a significant earthquake (MB = 5.8) occurred southwest of Cairo in the vicinity of the Dahshour region. For the mainshock, an average moment release M 0 = 9.70E17 n-m; a fault length = 13.8 km; an average displacement = 0.22 m; a stress drop = 0.760 MPa and maximum spectral magnitude = 5.78 were obtained. Spectral magnitude calculations were used in this analysis. The distribution of the well-recorded aftershocks over 15 months using a temporary seismic network installed immediately after the mainshock shows a zone of concentrated activity. Three composite focal mechanism solutions are constructed, using P wave polarity data for 30 earthquakes. These solutions have a mechanism, involving normal and right lateral strike slip motion along E-W to ENE-WSW trending fault plane. The P-wave polarity data of the individual 30 earthquakes are inverted to determine the stress tensor. The stress field estimated is extensional with s3 in the direction of NNE. The stress pattern determined from the inversion is in good agreement with the one estimated from the three composite fault plane solutions, while the neotectonic situation in Northern Egypt supports ENE-WSW extensional movement. However, the region of study has suffered both extensional tectonic activity of the Northern Red Sea rift zone and the compressional tectonic activity along the Hellenic arc. The inconsistency of the principal tension directions may belong to the interaction between the extensional and compressioal tectonics

Investigating the time-scaling behavior of the 2004–2010 seismicity of Aswan area (Egypt) by means of the Allan factor statistics and the detrended fluctuation analysis

The time dynamics of seismicity of Aswan area (Egypt) from 2004 to 2010 was investigated by means of the (i) Allan Factor, which is a powerful tool allowing the capture of time-clusterized properties of temporal point processes; and the (ii) detrended fluctuation analysis, which is capable of detecting scaling in nonstationary time series. The analysis was performed varying the depth and the magnitude thresholds. The 2004–2010 Aswan seismicity is characterized by significant three-fold time-clustering behaviors with scaling exponents ~0.77 for timescales between 10<sup>4.16</sup> s and 10<sup>5.14</sup> s, ~0.34 for timescales between 10<sup>5.14</sup> s and 10<sup>6.53</sup> s, and ~1 for higher timescales. The seismic interevent times and distances are characterized by persistent temporal fluctuations for most of the magnitude and depth thresholds