The Major Gulf of the Aqaba Earthquake, 22 November 1995 Maximum Intensity Distribution

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1 Natural Hazards 22: 17 27, Kluwer Academic Publishers. Printed in the Netherlands. 17 The Major Gulf of the Aqaba Earthquake, 22 November 1995 Maximum Intensity Distribution EID AL-TARAZI Department of Earth and Environmental Sciences, Hashemite University, PO Box , 13115, Zarqa, Jordan (Received: 14 July 1998; in final form: 10 September 1999) Abstract. The major earthquake of 22 November 1995, with a moment magnitude M W = 7.1anda local magnitude M L 6.2, was the beginning of a seismic swarm that occurred in the central part of the Gulf of Aqaba. During this swarm, thousands of earthquakes occurred with local magnitude ranging between 2 and 6.2 from 22 November 1996 to 31 December 1997, when 2089 earthquakes were detected and/or analyzed by the Jordan Seismological Observatory (JSO). The major earthquake strongly affected the near shoreline cities (Figure 1). The maximum observed intensity on these cities was VIII on the modified Mercalli intensity (MMI) scale. A questionnaire was distributed in the main cities of Jordan one week after the major earthquake. The results of this investigation, which demonstrated the observed intensity distribution for Aqaba city, shows a relationship between local conditions, such as geological foundations and topographical characters, and the extent of the destruction. This conclusion was supported by the maximum peak ground acceleration (PGA) measurements inside Aqaba and Eilat cities. From the results of this questionnaire compiled inside Jordan, and other reports and readings compiled from other nearby countries outside Jordan, a preliminary regional iso-intensity map for this major earthquake of 22 November 1995 is presented in this study. Key words: Gulf of Aqaba (Eilat), earthquake swarm, Jordan, local site effect, maximum observed intensity. 1. Introduction Early on the morning of Wednesday, 22 November 1995, an major earthquake with a magnitude of 6.2 on the local scale M L, and a moment magnitude of M W = 7.1, (PDE bulletin, 1996), struck the shorelines cities of the Gulf of Aqaba, such as Aqaba (Jordan), Eilat (Israel), Hagel (Saudi Arabia) and Nuweiba (Egypt). Figure 1 shows the locations of these cities and the epicenter of the main shock. This major event was followed by 2089 earthquakes ranging in magnitude from 2 to 5.5 on local magnitude (M L ) recorded and/or analyzed by the Jordan Seismological Observatory (JSO bulletin, 1998). This seismic swarm activity began on 22 November 1995 and continues at least until the end of December The seismic parameters of the main events that occurred during this swarm are listed in Table I (after JSO bulletin, 1998).

2 18 EID AL-TARAZI Figure 1. The seismic swarm that began on 22 November 1995 and continued until the end of December Note the location of the major earthquake of 22 November 1995 with a M L = 6.2. As a result of the main event, and according to the PDE bulletin (1996), the Dar Al-Handasah report (1995) and the Civil Defense reports in Jordan (personal communication), at least 11 people were killed and 47 injured, including five killed and 11 injured at Nuweiba. Damage occurred in many parts of northeastern Egypt as far as Cairo. One person was killed and two slightly injured at Haqel. One person died of a heart attack, several people were injured and substantial damage occurred with power breakdowns and liquefaction experienced at Eilat. Damage occurred in Aqaba city and high waves were reported along the coast at Aqaba. The major

3 THE MAJOR GULF OF AQABA EARTHQUAKE 19 Table I. No Date Origin time Lat. Long. Focal Magnitude Remarks yr month day h min sec N E Depth (M L ) on GMT (km) M 0 = (Nm), M W = 7.1 Io =IX event was felt from Sudan in the south to Lebanon and Syria in the north (PDE bulletin, 1996). The Gulf of Aqaba is characterized by the swarms of seismic activity such as the 1983, 1993 and the last one in All of the previous seismic swarms remained for several months, but the 1995 swarm had the longest and strongest duration according to the activity and the amount of energy released. This paper aims to demonstrate and discuss the results of the questionnaires in order to quantify and qualify the effects of this major earthquake on Aqaba city, and the distributions of the damages in this city and in Jordan on the modified Mercalli intensity (MMI) scale. A regional iso-intensity map for the major event is presented in this study.

4 20 EID AL-TARAZI 2. Tectonic and Geologic Features of the Gulf of Aqaba (Eilat) The Gulf of Aqaba (Eilat) is about 180 km long and is considered part of the Jordan Dead Sea transform fault system (JDS). The JDS branches from the Red Sea rift that formed during the Cenozoic period when the Arabian plate broke away from the African plate (Goldberg and Beyth, 1991). The separation of these plates opened the Red Sea between them, while in the north a small part of the plate movement was taken up by the JDS (Joffe and Garfunkel, 1987). The Gulf of Aqaba (Eilat) is occupied by three elongated en-echelon basins, which strike 20 N 25 E. The bathymetric map shown in Figure 2 demonstrates that there are six distinct deeps along the floor of the Gulf, namely, Elat, Aragonese, Arnona, Dakar and Tiran (Ben-Avraham et al., 1979; Ben-Avraham, 1985). There are no continental shelves and coastal plains are absent or very narrow (Ben- Avraham et al., 1979). The eastern boundary slopes of the Gulf reach 25 30, while those on the western boundary average 16. Therefore, the Gulf is asymmetric in cross-section. Different structural features have been determined in the Gulf of Aqaba. Folds of different sizes are developed in the central and southern basins of the Gulf, which can be explained as diapiric (Ben-Avraham et al., 1979). The faults are the dominant structural element in the Gulf. These faults are typically en-echelon, producing basins between the faults which are rhomb-shaped grabens. Active normal faults also run along the western coast of the Gulf of Aqaba (Eilat). The Gulf of the Aqaba region is characterized by different types of exposed rocks that cover a long span of geologic time ranging from pre-cambrian, which is represented by the Basement rocks that are distributed in the eastern part of Aqaba city, to the Quaternary sediments that mainly consists of alluvium, sand dunes, sandstone, clay, conglomerate and reef coastal deposit, and ancient dunes. Three superficial deposit formations are outcropping in Aqaba city and the surrounding area namely: (a) The alluvial gravel and alluvial sands: it consists of poorly cemented conglomerates, friable silty sand, medium to coarse-grained sandstone, siltstone and silty claystone, with various types of igneous rock mainly granite and granodiorite distributed as gravel, pebbles, cobbles and a few boulders in beds or at random. This formation is distributed in the northern and southern part of Aqaba city. (b) Coral formation: this formation is outcropped in some places in the northern, central, and southern part of Aqaba city with a calcreted sand and limy matrix. (c) Wadi sediments (Holocene) : most of the sediments in the wadis composed of sand with gravel, pebbles, cobbles and boulders of granite and granodiorite that can be found in the eastern and southern parts of the city.

5 THE MAJOR GULF OF AQABA EARTHQUAKE 21 Figure 2. Bathymetry of the Gulf of Aqaba (Eilat). The inset maps show (top) the regional setting of the Gulf of Aqaba (Eilat) and the approximate plate boundaries, and (center) traces of major faults in the Gulf of Aqaba (Eilat) which define the rhomb-shaped grabens (hachures) separating the Sinai and Arabian plates, modified (after Ben-Avraham et al., 1979).

6 22 EID AL-TARAZI Figure 3. The distribution of the maximum intensity values observed in Aqaba city. 1 = VIII; 2 = VII; and 3 = VI on the MMI scale. S1 and S2 indicate the locations of strong motion stations at the Aqaba Hotel and the Civil Defense building, respectively. 3. Questionnaire Data and Analysis One week after the main earthquake, 800 questionnaires were distributed in the main cities of Jordan, 500 of which were distributed in Aqaba since it was the Jordanian city most affected by the main shock (Figure 1). From the other side it has suffered from some damages as has been cleared through this paper. The MMI scale is used to describe and quantify the damages on Aqaba city and the other cities in the region. An analysis of the questionnaires from Aqaba city is shown in Figure 3. From this figure it is clear that the observed MMI ranged from VI to VIII. The maximum observed intensity values in Aqaba city (I max ) equals VIII on the MMI and observed in the buildings located along the beach, such as the hotels along the northern beach and the industrial zone along the southern part of the city near the shoreline. To define any relationship between the maximum observed intensity values and the local conditions, more analysis is required. For this purpose the surveyed buildings in Aqaba city are classified into four groups according their type of foundation, those with very hard rock, hard rock, soft rock and no rock (soil or sand). The results are shown in Figure 4(a). From this figure it is clear that the structures build directly on sand and gravel suffered from the highest intensity values. To determine the topographical effects on the amount of damage, the sites of the studied constructions classified into wadi, hilly, flat and beach, the results are demonstrated in Figure 4(b). From this figure, it is clear that the highest intensity values observed on the constructions lay on flat and beach areas of Aqaba city that

7 THE MAJOR GULF OF AQABA EARTHQUAKE 23 Figure 4a. The relationship between geological foundation description (i.e., very hard rock, hard rock, soft rock and no rock), the numbers of surveyed buildings and the observed intensity on the MMI scale in Aqaba city. Figure 4b. The relationship between topography (i.e., wadi, hilly, flat and beach), the numbers of surveyed buildings and the observed intensity on the MMI scale in Aqaba city. were build on sand and gravel. Nevertheless, the topography influence is very clear in the beach area where intensity VIII is observed (Figure 3). This observation is supported by the peak ground acceleration (PGA) values recorded in Aqaba city, where the maximum PGA recorded on the Aqaba Hotel station (on beach) is almost three times larger than the value recorded on the Civil Defense station located on a hilly area (see Figure 3 and Table II). 4. Regional Intensity-Distribution Map Depending mainly on the questionnaire described above, and in addition to Hassoup et al. (1996), PDE bulletin (1996) and JSO bulletin (1998), an iso-intensity map for the major earthquake is shown in Figure 5.

8 24 EID AL-TARAZI Table II. I max (MMI) PGA (cm/sec 2 ) City Observed Calculated Recorded Calculated Nuweiba IX VIII Unknown 142 Hagel IX VIII VII Unknown 78 Eilat VIII VI 110 a 57 Aqaba VIII VI 66 and 157 b 53 a Personal communication, JSO. b For an explanation, see text. The maximum observed intensity contoured values in the different near cities around the earthquake epicenter in Figure (5) are compared with calculated values using Al-Tarazi s attenuation equation (1992), that derived for the area under consideration. This equation is valid for epicentral distances less than 150 km: I(M L,R)= 1.8M L R ln(r + 25), (1) where I is the maximum intensity in km at distance R from the earthquake with local magnitude, M L. The maximum intensity values calculated for the main earthquake near cities are summarized in Table II. Table II also lists the maximum PGA recorded on strong motion stations installed in Aqaba (Figure 3). To compare the recorded PGA values to calculated values a local PGA equation that derived by Al-Tarazi and Qadan (1997) for the area under study is used: PGA(M L,R)= 0.645e 1.514M L (R + 25) 1.036, (2) where PGA represents the maximum peak ground acceleration on bedrock for earthquake of local magnitude (M L ) and hypocentral distance (R). 5. Discussion The main earthquake of 22 November 1995 was felt in a 600-km radius, where the maximum intensity in the epicenter area was IX on the MMI scale (Table I). In Nuweiba (Egypt), which is about 22 km from the epicenter, five people were killed (four of whom died in the collapse of a three-story hotel), and more than 38 were injured. About 50 dwellings were also damaged. Thirty-three schools were affected; seven of which experienced considerable structural damage. Five hotels (four in Nuweiba and one in Mt. Sinai) suffered structural damage; one totally collapsed. Twelve monuments belonging to the Islamic & Coptic eras were damaged (Figure 5). The port and facilities of Nuweiba were severely affected by the main shock (Dar Al-Handasah-Report, 1995). As shown in Table II, the MMI

9 THE MAJOR GULF OF AQABA EARTHQUAKE 25 Figure 5. Regional iso-intensity map on the MMI scale for the major earthquake of 22 November observed was IX while the calculated was VIII, this difference can be explained depending on the local site effects since this shoreline city lay on soft sands and alluvial (Hassoup et al., 1996). The maximum PGA calculated for this city was 142 cm/sec 2, but unfortunately it cannot be compare to recorded value because there was no strong-motions station installed in the city (Hassoup et al., 1996). It is clear from Figure 3 and Table II that the maximum observed intensity on Aqaba city was VIII on the MMI scale, while the calculated intensity was

10 26 EID AL-TARAZI VII this can be explained depending on the topography effects and the very thick accumulations of soft loose sediments, such as sand, gravel and brittle corals. The field survey conducted after the main shock indicated that some hotels on the northern beach of Aqaba city suffered serious structural damage, where many gaps in walls were observed and the inner walls collapsed. Some concreteconstructed water tanks in the southern beach collapsed and were severely damaged. Some cracks in the earth parallel and perpendicular to the beach were observed and recorded. From Figures 4(a) and 4(b), it is clear that there is a relationship between the sediments characters (soil), topographical conditions and the rate of intensity on the different constructions. Structures, such as the hotels in the northwestern part of the city which were built on loose sand and gravel, suffered from the heaviest damage (Figure 2), and others build in the southern part near the shorelines where sand and gravel at least 30 m thick were detected by bore-holes drilled in the area (Arab Center, 1987). The structures built on the hilly areas were characterized by more compacted sediments and gravel since they were nearer to the mountainous zone in the eastern and northern parts of the city. These buildings suffered from the maximum intensity VI on the MMI scale. The above results were supported by the maximum PGA values recorded in two different stations inside the city, where it was 157 cm/sec 2 on the northwestern part of the city that suffered the maximum intensity VIII, and 66 cm/sec 2 on the eastern hilly area, where the maximum intensity VI was observed (see Figure 3). This may be an indicator for the occurrence of resonance phenomena in the city. The local site effect was also clear in the Sinai Peninsula, where the irregularities in the iso-intensity lines of contours VI and VII (shown in Figure 5), can be interpret according to the highly variations in topography and geology as one travels from south to north inside the Sinai Peninsula (Hassoup et al., 1996). Similar destruction and damage was recorded in Eilat city for buildings and hotels near the beach. Liquefaction and cracks were recorded on the beach of Eilat (PDE bulletin, 1996). In the city of Eilat the maximum observed intensity (VIII) was higher than the calculated value (VII). This can be explained depending on the local site effects. This result was supported by the high recorded value of PGA in comparison to the calculated PGA (Table II). Hagel city in Saudi Arabia suffered heavy damage, especially on the structures near the shorelines (PDE bulletin, 1996). It is important to note that the data compiled from Hagel and Eilat cities were insufficient to give an exact description of the amount of destruction in these cities. 6. Conclusions and Recommendations Resulting from the above discussion and analysis the following points are concluded: The Io for the major shock of 22 November 1995 was equal to IX on the MMI scale.

11 THE MAJOR GULF OF AQABA EARTHQUAKE 27 The geological foundation features and topography of the surveyed sites in Aqaba city affected the extent of the destruction. Liquefaction phenomena occurred on the ports of Nuweiba and Eilat. The performance of most engineered structures in Aqaba and other shoreline cities along the Gulf of Aqaba were satisfactory in minimizing structural collapse and loss of life. The following points are recommended: Detailed study for the liquefaction phenomena in the studied area is important to determine the seismic risk. Natural frequency studies are required for the existing constructions and for those intending to build in the future in the studied area. References Al-Tarazi, E. and Qadan, H.: 1997, Seismic hazard potential expected for dams in Jordan, J. Dirasat. 24, Al-Tarazi, E.: 1992, Investigation and assessment of seismic hazard in Jordan and the region around, PhD Thesis, Ruhr University Bochum, Germany. Arab Center for Engineering Studies: 1987, Site investigation for observation tower-aqaba-jordan, local report (unpublished). Ben-Avraham, Z.: 1985, Structural framework of the Gulf of Eilat (Aqaba), J. Geophys. Res. 90, Ben-Avraham, Z., Garfunkel, Z, Almagor, G. and Hall, J.: 1979, Continental breakup by a leaky transform: the Gulf of Eilat (Aqaba), Science 206, Dar Al-Handasah-Report: 1995, The Aqaba Gulf Earthquake of 1995, consulting company, local report (unpublished), Cairo, Egypt. Goldberg, M. and Beyth, M.: 1991, Tiran Island: an internal block at the junction of the Red Sea rift and Dead Sea transform, Tectonophysics 198, Hassoup, A., Albert, R., Mohamed A. and El-Hadidy, S.: 1996, Attenuation of intensity in Sinai Peninsula by the 1995 Gulf of Aqaba earthquake, Proceedings of the Second Regional Workshop on the Seismicity of the Gulf of Aqaba Region, 3 5 June 1996, Amman, Jordan. Joffe, S. and Garfunkel, Z.: 1987, Plate kinematics of the circum Red Sea a reevaluation, Tectonophysics 141, Jordan Seismological Observatory (JSO) bulletin: 1998, Jordan seismological observatory, annual bulletin, Earthquakes in Jordan and around 1995, No. 27, Natural Resources Authority, Amman, Jordan. PDE bulletin: 1996, Preliminary Determination of Epicenter, U.S. Geol. Survey, National Earthquake Information Center, U.S.A.

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