V16N1C.indb EARTH SCIENCES RESEARCH JOURNAL Earth Sci Res SJ Vol 16, No 1 (June, 2012): 35 - 40 Introduction The 1,100 km long strike-slip Gulf of Aqaba-Dead Sea transform fault is a major active tectonic feature linking the southern Turkey-western Iran Taurus-Zagros area with the Red Sea rift It has been suggested that its left-lateral strike-slip motion is a result of relative oblique left-hand move- ment between Arabia and Africa which opened up the Red Sea (Quennell 1959; Freund et al , 1970) Lyberis (1988) indicated that movement in the southern Dead Sea transform fault, representing the Gulf of Aqaba, started in late Miocene and was associated with a strike-slip stress pattern (40° extension associ- ated with 130° compression) This movement produced the left lateral mo- tion between the Arabian plate and Sinai Peninsula and faulting has been the result of an E-W extension since the end of the Miocene, indicating a rotation of the regional stress pattern in the vicinity of the transform fault Al-Arifi (1996) found the main maximum compressive stress (l) direction to be 137° and the main minimum stress direction (3) to be 222° using the focal mechanism solution for 53 recent earthquakes in the Gulf of Aqaba, The Gulf ’s structure is dominated by en-echelon normal faults de- limiting three elongated basins (Ben-Avraham et al , 1979, Reiss and Hottinger 1984) The northern basin has a simple bathymetry and struc- ture, dominated by the flat-bottomed Eilat deep (<900 m) The central basin consists of two deeps: Aragonese deep (<1,850m), and Arnona deep (<1,550m); the southern basin includes the Dakar (<1,400m) and Tiran (3 5) for two sequences Where A is the 1993 sequence and B is the 1995 sequence Small and large arrows indicate the direction of the minimum and maximum compressive stresses respectively, according to small scale structures study (Reches 1987), and focal mechanism study (Al-Arifi 1996) figure 4. (a) Shows the local migration of strong aftershocks of the 1993 sequence in the Gulf of Aqaba (b) Shows profiles for jumping of strong aftershocks of the 1993 Gulf of Aqaba sequence The arrows indicate the direction of the local mi- gration and jumping est aftershock) (Figure 3); the region experienced a widely-felt earthquake on November 22nd 1995, located at 28 81°N, 34 75°E, having 12 km focal depth The mainshock (Md=6 2) was followed by 733 aftershocks (Md≥2 8) over a 40-day period (Md 5 3 for the largest aftershock) (Figure 3) The 1983 sequence was concentrated in the northern part of the Gulf of Aqaba (28 8°- 29 4°N and 34 3- 35 l°E) Most of this sequence was off- shore and coincided with the Eilat deep Although determining depth was difficult (due to a lack of stations before SSC stations were established), El-Isa et al , (1984) observed that surface waves had been clearly reported in Jordan University seismic station records, suggesting a very shallow depth for the 1983 sequence Their observation was supported by recent SSC recorded earthquakes located in the same area that suffered from the 1983 sequence; these earthquakes were located at a shallow depth not ex- ceeding 10 km (A1-Shaabi 1998) The 1993 sequence was concentrated in the Dakar and Tiran deeps in the southern part of the Gulf having focal depths not exceeding 26 km The 1995 sequence was distributed into two clusters, the more northerly of which was again concentrated in the Eilat deep, whereas the southern cluster was in the Aragonese and Arnona deeps in the central Gulf The 1995 sequence depth was less than the 1993 sequence; this may have been due to the aftershock area location where the 1993 aftershock zone was concentrated in the southern part (a) (b) (a) (b) Nassir S Al-Arifi, Aref A Lashin and Saad Al-Humidan38 of the Gulf whereas the 1995 aftershock area spread along the Gulf and aftershock density was greater in the northern than the southern part This may have indicated that earthquakes occurring in the northern part were shallower than those in the southern part of the Gulf and were related to the brittle-ductile transition zone (Al- Shaabi, 1998) This observation was supported by the 1995 aftershocks where the events in the northern cluster were shallower than those in the southern cluster The surface waves ap- peared clearly when the earthquakes were located in the north of the Gulf of Aqaba (A1-Shaabi 1998); however, they disappeared from the records when they were located in the south method HYPO71 software (Lee and Valdes, 1985) was used to relocate all events in this study (±1 0 km horizontal component error and ±2 0 depth error) of aftershocks’ local migration, both vertically with depth and hori- zontally in a NE-SW direction corresponding to the general state of exten- sional tectonic stress in the Gulf of Aqaba and the Dead Sea Transform, ac- cording to small-scale structure study (Richard 1987) and focal mechanism study (Al-Arifi 1996) The term ‘local migration’ has been used to denote a strong aftershock’s horizontal movement during a sequence; the term ‘jumping’ has been used to denote a strong aftershock’s vertical movement The term strong aftershock in this paper means the aftershocks which were felt in at least three towns in the study area Because all strong aftershocks occurred at a shallow depth not exceeding 20 km, all aftershocks having Md 4 9 and over were felt in at least three towns The 1993 mainshock oc- curred at the aftershock area’s extreme southern end: 28 45°N and 34 87°E (Figure 3) The aftershocks moved northward Figure 4 shows strong aftershocks’ local migration regarding the 1993 sequence On the same day in which the mainshock occurred, activity was concentrated in the north between 28 70-28 85° N and 34 75-34 9° E where two strong aftershocks (Md 5 6 and 4 9) occurred 80 days after the mainshock the activity moved northwards again where another strong aftershock occurred at 29 0°N and 34 83°E Activity leaped to the south- west (still north of the mainshock) on the 3rd of November 1993 with another strong shock that occurred at 28 62°N and 34 6°E; activity then moved northwards where the last strong aftershock occurred on the 4th of December 1993 at 28 86°N and 34 41°E Figure 4 shows that the 1993 focal depth started with more than 10 km for foreshocks, no foreshock having less than 10 km depth The mainshock had the depth of 15 km and then the depth reduced with time from 11-12 km for the first two strong aftershocks until the last strong aftershock was at a depth of 2 km The 1995 sequence was similar, aftershock focal depth becoming reduced as time elapsed Such ‘jumping’ could be explained by earthquakes migrating northwards into an area where all seismicity was shal- lower than in the south of the Gulf As regards 1995 sequence local migra- tion it was difficult to determine a specific direction for a strong aftershock (even though the aftershocks generally moved northwards) results The focal mechanism solution for the mainshocks and the largest aftershocks for both sequences (1993 and 1995, Figure 5) included the flowing: 1) The 1993 mainshock indicated normal and strike-slip fault dip- ping 34° to the northwest and striking N 21°E; figure 5. Shows the focal mechanism solution for the mainshocks and the largest aftershocks for both sequence of the 1993 and 1995 Migration of local earthquakes in the Gulf of Aqaba, North Red Sea 39 figure 6. Generalized model for the migration of large earthquakes in the Gulf of Aqaba 2) The largest 1993 aftershock occurring 3 hours and 50 minutes after the main shock indicated left-lateral strike-slip fault dipping 64° to the north and striking E 04°S; and 3) The 1995 mainshock indicated left-lateral strike-slip fault dipping 43° to the north and striking N 09°E; and 4) The largest 1995aftershock occurring 35 hours after the main- shock indicated normal and strike-slip fault dipping 31ρ to the northwest and striking 198° Observations regarding these focal mechanism solutions, mainshock locations and the largest aftershocks for both Gulf of Aqaba sequences (i e 1993 and 1995) indicated the following: 1) The seismic source for the main shocks for both sequences and their largest aftershocks were completely different (Figure 5); 2) The seismic source of the largest 1993 aftershock and the 1995 mainshock were located in the same fault zone (Figure 5); and 3) There was systematic south to north migration of the earthquakes’ seismic sources (Figures 4 and 5) Such systematic northward migration was influenced by the Gulf of Aqaba’s main maximum tectonic stress di- rection (137° according to Al-Arifi, 1996) Discussion It has been observed in many cases that mainshock epicentres are very often located at one end of an aftershock zone (Matsuzawa, 1979) This happened in the Gulf of Aqaba with the 1993 and 1995 sequences where the 1993 mainshock was located in the south end of the aftershock zone and below the bottom depth of the aftershock zone This also applied to the 1995 sequence where the mainshock was located near the southern end of the aftershock zone at a depth representing the bottom of the after- shock zone Most aftershocks were concentrated between the mainshock and the largest aftershock for both sequences (Figure 3) Aftershock characteristics give clues regarding the nature of the rela- tively long-term processes redistributing stress following instantaneous stress changes associated with a mainshock (Wesson, 1987) It seems likely that this was responsible for the concentration of stress in the hypocentral region of the largest 1993 aftershocks prior to and resulting in the nucleation of the 1995 mainshock Such systematic northward migration led to suggesting an initial model for earthquake migration in the Gulf of Aqaba (Figure 6), although real migration would likely have been more complicated However, the largest aftershock’s epicentre in this model represented the stresses nucle- ation area and location of the next large mainshock (Figure 6) Conclusions An earthquake mechanism for the Gulf of Aqaba fault system has proposed that seismic energy becomes gradually accumulated until it reaches failure point; a large drop in stress during a mainshock (the 1993 mainshock) then caused stress redistribution to the location of the largest aftershock thereby triggering the area to become an area of stress nucle- ation producing another large mainshock (the 1995 earthquake) Again, the last mainshock’s largest aftershock area could have received the next large mainshock Because a future large earthquake could easily nucleate in the Gulf of Aqaba, more effort is needed to fully understand stress mi- gration and seismic behaviour in this critical set of the southern Dead Sea fault system However, it should be mentioned here that triggering may have played an important role in the 1995 sequence even though further evidence is needed Acknowledgments This project was financed by King Saud University’s College of Sci- ence Research Centre Deanship of Scientific Research references Al-Arifi, N S , (1996) Micro-seismicity and lineament study of the East- ern side of the Gulf of Aqaba NW Saudi Arabia (1986-1994), Ph D Thesis, University of Manchester, UK Al-Shaabi, S , (1998) The seismicity of the Gulf of Aqaba, Northern Red Sea (1985-1995), Master Thesis, University of Leeds, UK Ben-Avraham, Z , Garfunkel, Z , Almagor, G , and Hall, J K (1979) Continental breakup by a leaky transform; 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