AN UPDATED SEISMIC HAZARD ASSESSMENT FOR EGYPT EIGHTH INTERNATIONAL CONFERENCE ASSIUT 2015 ON THE GEOLOGY OF AFRICA AN UPDATED SEISMIC HAZARD ASSESSMENT FOR EGYPT Rashad Sawires1,2*, José A. Peláez2, Raafat E. Fat-Helbary3, and Hamza A. Ibrahim1. 1 Geology Department, Assiut University, 71516 - Assiut, Egypt. 2 Department of Physics, University of Jaén, Jaén, Spain. 3 Aswan Regional Earthquake Research Centre, 152 - Aswan, Egypt. * rashad.sawires@aun.edu.eg

Motivation of the study Declustered shallow seismicity (h ≤ 35 km). Largest instrumental events (magnitudes equal or above MW 5.0) that have been taken place in Egypt in the last 50 years.

Motivation of the study ECP-201 (2011) is still depending on a seismic hazard assessment that do not consider neither the historical earthquake events nor an up-to-date earthquake catalogue and seismic source model. (ECP-201, 2011) Thus, there is an urgent need to estimate up-to-date seismic hazard values and to supply this information for application and use in improving seismic zoning maps and building design and construction. Seismic hazard zoning map for Egypt (modified after ECP-201: 2011 and Riad et al., 2000).

Objectives/ METHODOLOGY Seismic hazard is defined as the probability of occurrence of an earthquake or earthquake effects of certain severity, within a specific period of time, in a given area. An earthquake recurrence model that describes the recurrence of events in time within the seismic source zones. The overall purpose of this work was to carry out a probabilistic seismic hazard assessment for Egypt, in terms of mean horizontal peak ground acceleration (PGA), and spectral acceleration (SA) values. The output results could be used to modify and update the seismic action in the upcoming building codes. A seismic source model that defines the spatial distribution of earthquakes within the region of concern. A ground-motion attenuation model that describes mathematically the manner in which earthquake ground-motions decrease with distance from an earthquake source for various magnitude levels. A probability model for calculating the expected maximum amplitude of ground-motion within a given period of time corresponding to a chosen probability level. Schematic description of the classical PSHA methodology.

Data and methodology 1. A Seismic Source Model

2. An Earthquake Recurrence Model Data and methodology 2. An Earthquake Recurrence Model b-values, annual rates of earthquakes, approximate completeness periods, and maximum observed and expected magnitudes for the delineated seismic sources. (*) The most recent event, (**) Macroseismic surface-wave magnitude (after Ambraseys et al., 1994). A Calculated using Robson-Whitlock-Cooke procedure. B Equal to maximum recorded magnitude. C Taken from Deif et al. (2009, 2011).

3. Ground-Motion Attenuation Equations Data and methodology 3. Ground-Motion Attenuation Equations GMPE Region Magnitude Type and Range Distance Definition (km) Spectral Period Site Conditions Faulting Mechanism Horizontal Component Definition Ambraseys et al. (1996) Europe + Middle East MS (4.0 - 7.5) rjb (0 – 200) PGA, 0.1 – 2.0 s Rock (VS ≥ 750 m/s), Stiff Soil (750 ≤ VS < 360 m/s), Soft Soil (360 ≤ VS < 180 m/s), Very Soft Soil (VS ≤ 180 m/s). Not Considered Larger Abrahamson and Silva (1997) Worldwide MW (4.4 – 7.4) rrup (0 – 220) 0.01 – 5.0 s Rock/ Shallow Soil (VS > 600 m/s) Deep Soil (VS < 150 m/s) Reverse/ Oblique-slip/ Others Average Zhao et al. (2006) Japan (5.0 - 8.3) (0 – 300) 0.05 – 5.0 s Rock (VS ≥ 600 m/s), Hard Soil (600 ≤ VS < 300 m/s), Medium Soil (300 ≤ VS < 200 m/s), Soft Soil (VS ≤ 200 m/s). Normal/ Strike-slip/ Unspecified Geometric Mean Boore and Atkinson (2008) California + Taiwan (4.27 – 7.9) (0 – 280) 0.01 - 10.0 s NEHRP B (VS ≥ 760 m/s), NEHRP C (760 ≤ VS < 360 m/s), NEHRP D (360 ≤ VS < 180 m/s), NEHRP E (VS ≤ 180 m/s). Cont. Function. Tavakoli and Pezeshk (2005) Eastern North America (5.0 – 8.2) (0 – 1000) 0.05 – 4.0 s Hard-rock Sites Youngs et al. (1997) (8.5 – 550.9) 0.075 – 3.0 s Rock (VS > 750 m/s), Soil (VS ≤ 750 m/s). Interface/ Inslab Characteristics of the considered GMPEs in the present study.

Data and methodology 4. Sensitivity Analysis PGA (for a return period of 475 years) corresponding to different values of Mmax for the selected cities. PGA values (for a return period of 475 years) corresponding to different values of beta for the selected cities. PGA values (for a return period of 475 years) corresponding to different values of activity rates for the selected cities. PGA (for a return period of 475 years) corresponding to applying different GMPEs for the selected cities. Administrative map of Egypt showing the geographical distribution of the selected cities considered in the hazard computations.

4. Logic-tree Formulation Data and methodology 4. Logic-tree Formulation Logic trees were originally introduced as a tool for treating uncertainties when performing seismic hazard assessment by Kulkarni et al. (1984). Nowadays, logic trees are widely used for seismic hazard analysis and have become a standard feature of PSHA (Coppersmith and Youngs, 1986; Reiter, 1990; Bommer et al., 2005). Cheng et al. (2007) Aldama-Bustos et al. (2009) Petersen et al. (2008) Sutiwanich et al. (2012) Kolathayar and Sitharam (2012) Logic-tree framework applied in the present study.

SEISMIC HAZARD COMPUTATIONS The computations was performed using the CRISIS 2014 software code (Ordaz et al., 2014) for grid points covering the Egyptian territory at a spacing of 0.1° x 0.1° (about 10 km x 10 km). The final seismic hazard for Egypt were computed for both rock and stiff-soil site conditions for mean PGA and for 5% damping SA at 0.1, 0.2, 0.3, 0.5, 1.0, 1.5 and 2.0 s spectral periods, with a 39.3%, 10% and 5% probability of exceedance in 50 years, which correspond to return periods of 100, 475 and 975 years, respectively.

For a return period of 475 years SEISMIC HAZARD RESULTS Region   For a return period of 475 years Significant Events Rock site Stiff-soil site PGA SA (0.1 s) (0.2 s) Gulf of Aqaba 0.36 g 0.89 g 0.76 g 0.41 g 0.93 g 0.97 g MW 7.2, November 22, 1995 mb 5.8, August 3, 1993 Aswan region 0.30 g 0.71 g 0.64 g 0.34 g 0.75 g 0.81 g MS 5.6, November 14, 1981 ML 5.6, April 23, 1982 Gulf of Suez/ Southern Sinai 0.19 g 0.50 g 0.40 g 0.23 g 0.52 g MS 6.9, March 31, 1969 mb 5.7, June 28, 1972 Red Sea (Abu Dabbab) 0.48 g 0.39 g 0.22 g 0.49 g MS 5.3, November 12, 1955 mb 5.1, July 2, 1984 Surroundings of Cairo 0.14 g 0.35 g 0.31 g 0.17 g 0.38 g 0.37 g MS 5.9, October 12, 1992 of Alexandria 0.09 g 0.20 g 0.11 g 0.24 g 0.25 g mb 6.5, September 12, 1955 MW 5.2, June 27, 2015

SEISMIC HAZARD RESULTS UHS, damped at 5%, for different locations and for rock and stiff-soil site conditions, for the different computed return periods.

SEISMIC HAZARD RESULTS City Soil 100 years 475 years 975 years PGA (g) SAmax Tmax (s) 0.2 s SA (g) 1.0 s Nuweiba R 0.156 0.382 0.1 0.331 0.063 0.294 0.742 0.642 0.146 0.383 0.978 0.848 0.204 S 0.178 0.406 0.404 0.076 0.332 0.806 0.2 0.175 0.433 1.000 0.1/0.2 0.245 Aswan 0.093 0.222 0.202 0.043 0.189 0.456 0.417 0.105 0.252 0.617 0.563 0.149 0.106 0.246 0.052 0.213 0.514 0.124 0.284 0.703 0.176 El-Tur 0.085 0.211 0.184 0.036 0.174 0.434 0.365 0.077 0.235 0.593 0.493 0.097 0.227 0.223 0.044 0.198 0.459 0.457 0.091 0.267 0.625 0.126 Sharm El-Sheikh 0.082 0.212 0.173 0.029 0.145 0.392 0.304 0.055 0.182 0.508 0.386 0.072 0.095 0.231 0.035 0.165 0.413 0.065 0.206 0.530 0.496 0.084 NEW Capital 0.154 0.137 0.030 0.142 0.348 0.299 0.197 0.487 0.411 0.166 0.037 0.159 0.367 0.078 0.219 0.108 Hurghada 0.070 0.177 0.150 0.027 0.134 0.349 0.286 0.054 0.463 0.373 0.080 0.190 0.032 0.371 0.360 0.064 0.199 0.474 Fayoum 0.060 0.130 0.283 0.058 0.186 0.461 0.390 0.160 0.033 0.153 0.354 0.351 0.071 0.488 0.100 CAIRO 0.324 0.280 0.062 0.441 0.379 0.086 0.167 0.148 0.075 0.475 0.104 Suez 0.151 0.131 0.285 0.443 0.388 0.090 0.164 0.353 0.201 0.484 0.107 6th of October 0.061 0.133 0.028 0.310 0.268 0.059 0.163 0.162 0.141 0.334 0.450 0.099 Zagazig 0.125 0.122 0.305 0.261 0.067 0.155 0.139 0.325 0.442 Seismic hazard values obtained at the selected cities for the three computed return periods and for both types of soil site conditions.

Mean PGA values (g) for selected cities. PRESENT STUDY ECP-201 (2004, 2008, 2011) Study ECP-201 (2004, 2008 and 2011) The current work Matruh 0.13 0.05 Alexandria 0.09 El-Arish 0.03 Cairo 0.15 Suez Nuweiba 0.20 0.29 Sharm El-Sheikh Hurghada 0.25 Assiut 0.10 0.07 Aswan 0.19 Possible Causes for differences: The unified earthquake catalogue. The seismic source model. Ground-Motion attenuation models. Application of logic-tree approach. Mean PGA values (g) for selected cities.

Summary and Conclusions Eighty-eight seismic sources covering the seismic activity (shallow and intermediate-depth) in different tectonic regions have been proposed. Seismicity parameters (b-values and activity rates) have been estimated for the defined seismic sources. In addition, the maximum expected magnitude for each seismic source was computed. A sensitivity analysis is also performed in order to evaluate the effect of different input parameters on the computed seismic hazard level. Six well-known Ground-motion attenuation models were selected in the current study to account for the epistemic uncertainty associated with not knowing the true attenuation characteristics of the region. Uncertainties in Gutenberg-Richter b-values, Mmax values and the GMPEs have been incorporated in the seismic hazard assessment using a logic-tree framework. The seismic hazard has been computed using the well-known total probability theorem, for both rock and stiff-soil site conditions, and for mean PGA and SA at 0.1, 0.2, 0.3, 0.5, 1.0, 1.5 and 2.0 s, for return periods of 100, 475 and 975 years.

Summary and Conclusions The seismic hazard maps and the UHS delineate the Gulf of Aqaba region with relatively higher seismic hazard from the rest of the country, which is characterized by its relatively moderate hazard levels. This region was experienced the biggest recorded earthquake since 1900 (MW 7.2, November 22, 1995 Aqaba event). Other seismic hazard nuclei appear in the surroundings of Nasser’s Lake, in the southern part of the Gulf of Suez, at the western coast of the Red Sea near the city of Marsa Alam, in the Greater Cairo region, and in the surroundings of Alexandria. A comparison of the computed hazard values with those considered ones in the most recent Egyptian building code has been performed. Differences were noticed especially for some very important cities (e.g., Nuweiba and Aswan). It is concluded that a change in the seismic hazard representation of the current Egyptian building code is required in terms of the definition of the seismic hazard zoning maps, seismic hazard values, and new definition of the elastic response spectra for each city.

AN UPDATED SEISMIC HAZARD ASSESSMENT FOR EGYPT