Seismicity and seismotectonics of the Gulf of Aqaba region
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ORIGINAL PAPER
Seismicity and seismotectonics of the Gulf of Aqaba region Zuhair H. El-Isa
Received: 9 October 2011 / Accepted: 23 May 2012 / Published online: 8 June 2012 # Saudi Society for Geosciences 2012
Abstract The historical seismicity of the last ten centuries and the instrumental data that occurred in the Gulf of Aqaba region during the period 1982–2008 are evaluated. It is found that 12 historical earthquakes have occurred with average recurrence periods 70–90 and 333–500 years for M≥6.0 and 7.0, respectively. Those with M≤6.5 appear to be incomplete and require further investigation. More than 98 % of the instrumental data has occurred in the form of swarms and sequences. The first have released about 32 % of the total energy and are most likely related to subsurface volcanic activities. Their epicentral distribution indicates that all regional faults of the gulf area are active in the present, but with clear concentration within the area bound by latitudes 28.2°–29.8° and longitudes 34.4°–35.2°. Regional strike-slip faults of the northern two basins appear to be as twice active as the normal, or more. An appreciable level of seismic hazard is envisaged as the “a” value is 6.0– 6.2 while the “b” value shows a temporal variation, mostly in the range 0.8–1.05. More than 95 % of the seismic energy was released from earthquakes shallower than 22 km. This indicates a brittle upper crust and a ductile lower crust and upper mantle. Tectonic movements at depths>22 km appear to be aseismic. The epicentral distribution of the five swarms indicates that the lengths of the causative faults varied in the range 45–70 km. The maximum expected magnitude is Mw06.8–7.2. This implies a seismic slip rate of about 0.54–0.8 Cm/year and some 20–30 % of aseismic tectonic movements. This and the sequence nature of the seismicity of this region result in a noticeable hazard reduction. Combining the seismicity data of the Gulf of Aqaba Z. H. El-Isa (*) Department of Geology, University of Jordan, Amman 11942, Jordan e-mail: [email protected]
region with other geophysical, geological, tectonic, and environmental data, clearly indicate that the seismicity of this region is as old as the initiation of the gulf itself. No apparent southward or northward migration of activity is observed. Keywords Gulf of Aqaba . Seismicity . Seismotectonics . Seismic hazard
Introduction The seismicity of the Gulf of Aqaba has been mistakenly described by some researchers as only “recent” (e.g., Salamon et al. 1996; Klinger et al. 1999). Contrary to this lots of geological, geophysical, tectonic, volcanic, environmental, historical, and other justifications are available in support of the fact that the seismicity of this region is as old as the initiation of the gulf. Historical information as obtained from old Arab literature confirms that more than ten destructive earthquakes (M≥6.0–6.5) were felt in this region during the last ten centuries (Poirier and Taher 1980; Maamoon et al. 1984; Ambraseys et al. 1994). Some of these were felt quite strongl
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