1. Seismicity and seismotectonics of the Red Sea Region Zuhair Hasan El-Isa Geology Department, University of Jordan, Amman, Jordan. elisaz@go.com.jo

5. Crustal Structure Beneath the Red Sea # Magnetic data has long been interpreted to indicate an oceanic crust beneath the axial and main troughs of the southern Red Sea. Magnetic anomalies of the axial trough were observed to extend beneath the main trough but with lower amplitude and larger wavelength. These anomalies were interpreted by some to indicate an oceanic crust beneath the main trough as well. Deep seismic data were taken later to support the idea. Other workers however interpreted both sets of data to conclude continental type of crust beneath the main trough only. # Beneath the Northern Red Sea, magnetic, gravity and seismic data were interpreted to conclude a continental type of crust east of the axial trough and an oceanic crust beneath it and to the west, some few tens of km east of the Egyptian coasts. Other workers however interpret the same data to conclude extended and intruded continental crust beneath the whole northern Red Sea excluding the locations of a few scattered deeps beneath the axial trough.

6. Characteristics of the seismicity of the Red Sea region as deduced from previous studies 1. Moderate-low seismicity 2. Shallow, mostly less than 40km 3. Swarm and sequence type 4. Concentration along axial trough indicates present day continuous rifting 5. Further concentration where NE trending strike-slip faults cross the axial trough 6. Good correlation with regional geology and tectonic 7. Apparent lower activity in the Northern part

7. Compiled Historical Seismicity Criteria Earthquakes felt in the area between latitudes 13⁰ & 30⁰ and longitudes 32⁰ & 46⁰ Earthquakes that were felt with intensities I ≥ V – VI During the period 627 – 1955 Earthquakes with intensities IX were assigned magnitude 7 All earthquakes with intensities VIII and VII - VIII were assigned magnitudes in the ranges M = 6.3 – 6.6 and 6 – 6.2 respectively. All other events which had intensities ≤ VI were assigned magnitudes 5.7 – 6.0 depending on damage reports.

8. NumberYear MagnitudeLatitudeLongitudeLocation / Comments 16275.824.339.35Al-Madinah, Hejaz 26415.724.639.6Al-Madinah, Hejaz 3742615.445.4Destroyed Ma'reb Dam, Yemen 48276.11444.5Sana'a & Aden 5859615.444.3Sana'a 68736.42739Taima'a, Saudi Arabia 795562833Gulf of Suez 899762634Egypt, Red Sea 91068729.835Gulf of Aqaba 1010726.314.543.2Zabid, Yemen 1110915.628.534Gulf of Suez 1211216.823.537Northern Red Sea 1311226.324.339.35West of Al-Madinah 1411455.81444Ta'iz, Yemen 151154614.744.6300 dead in Sana'a 1611966.525?36.2?Northern Red Sea 1712035.713.541.9W. Bab Al-Mandeb 1812126.829.334.6Gulf of Aqaba 1912565.724.739.7East of Yanbu' 2012595.915.443.9Swarm in Sana'a 2112616.315.3344.14Sana'a 2212655.824.540Al-Madinah, S. Arabia 2312695.621.140.5Mecca region, S. Arabia 2412705.721.1540.2Al-Ta'ef, Saudi Arabia 251293624.439.75W. of Mainah, S. Arabia 2613126.428.534Southern Sinai 2713596.214.3543.8Strong swarm, 51 dead in Zabid 2813876.113.344.8Many days long swarm in Aden 291400613.441.8Dubbi volcano, Red Sea 3014085.721.539.8Mecca, Saudi Arabia 311413714.144.2Himyar, Sana'a, Yarim 32142562933Gulf of Suez 3314276.31444Sequence, 60 dead in Zabid & Ta'iz 3414326.31442.5Southern Red Sea 3514325.71543N. Zabid, Yemen 3614345.815.142.2Southern R. Sea 3714445.615.142.2Southern R.Sea 3814636.414.343.3Sequence, 10 dead in Zabid 391466614.343.3Zabid, Yemen 401481621.239.7Mecca, Saudi Arabia 4114845.714.243.3Zabid, Yemen 4214855.614.243.5Zabid, Yemen 4315046.315.4644Zabid, Zaila, Yemen 441509614.243.3ground cracks, mud in Zabid wells 4515106.113.343.6Bab Al Mandeb, Aden 4615115.814.243.3Zabid 4715115.713.643.5Bab Al Mandeb, Aden 4815886.328.534.6Sinai, Gulf of Aqaba 491613613.244East of Bab Al Mandeb 5016195.816.444N. sana'a, E. of Red Sea 5116375.715.743.8NW of Sana'a 5216445.715.443.9Large induced landslide in Sana'a 5316665.615.543.9Sana'a 5416676.315.444.2Felt all over Yemen 5516746.21544.1Damar region, Yemen 5616755.61544.1Damar region, Yemen 5716795.615.541.8Southern R. Sea 5817105.72933Gulf of Suez 5917335.615.739E. Sudan, W. Red Sea 6017546.629.632.2N. Gulf of Suez 6117755.813.943.4Hais Area 621778627.534.1Ras Mohammad, Gulf of Suez 63178861542A year long swarm 6417896.414.943.3North of Zabid 6518105.81742.8Najran 6618145.62933Gulf of Suez 6718325.915.541.8Southern R. Sea 681839628.534Gulf of Suez-S. Sinai 6918455.715.541.8Southern R. Sea 7018465.815.142.2Southern R. Sea 7118506.515.444.2300 houses destroyed in Sana'a 7218546.61339Southern Red Sea 7318735.614.243.3Zabid, Yemen 741878614.5544.33 months swarm in Damar, Yemen 751879(*)62933Gulf of Suez 7618815.916.943.8Yemen 771884(*)6.215.739.6Western Shores of southern Red Sea 7818876.425.534.5Egypt, Northern Red Sea Region 791895613.443.6West of Ta'iz 801900629.334.6Gulf of Aqaba 81 1909 61544Strong swarm, 300 dead in Wa'lan 8219105.729.933.8Gulf of Suez 8319135.817.238.8Western Shores of Southern Red Sea 8419156.81639Western Shores of southern Red Sea 8519215.615.540.5 Southern Red Sea 8619385.618.537.5Western Shores of southern Red Sea 8719415.61643Eastern shores of Southern R. Sea 881955625.334.5Western Shores of Northern Red Sea

9. R. S. Proper 13 earthqs W. R. Sea 12 “ E. R. S. 57 “ Yemen 39 “ N. R. Sea 31 “ S. R. Sea 57 “ M N.R.Sea S.R.Sea 7 1 1 6.8 2 1 6.6 1 1 6.5 1 1 6.4 3 2 6.3 2 6 Average Recurr. Periods M ≥ T(years) 5.6 16 6.0 28 6.5 166 7.0 644 Sequences & swarms Volcanic related

12. 52%48%

13. 43.6%56.4%

14. 62.7%37.3%

16. Characteristics of the Historical Seismicity Mmax = 7.0 T= 16y, 28y, 166y & 644y for M=5.6, 6. 6.5 & 7.0 Mostly occur in sequences & swarms Volcanic related Seismicity of both N. & S. Red Sea parts are comparable, but low in middle part Good correlation with regional geology and tectonics Relatively moderate hazard

17. M ≥ N 2.0 7391 2.0-3.9 6430 4.0 1310 5.0 96 6.0 12 6.5 3 7.0 1 7.1 (G. Aqaba) 6.9 (entrance G. Suez) 6.7 (SW Jeddah, S.R.S.) All others are M ≤ 6.4 Correlation with tectonics G. Aqaba: 99% correl. S. Lat. 27.8: 83% axial Concentration NE cross. Lats 16.5-17, 19.5, 27,… Onshore activity Lat. 25 And in Yemen 1960 - 2010

18. Sequence & Swarm Types of Activity The 1983, 1990, 1993 & 1995 swarms of the G. Aqaba account for 97.8% of its seismicity. At least some of these are related to subsurface volcanic activities. The 1967, 1969 & 1995 swarms account for 82.5% of the total seismic moment of the Red Sea region. Much of the seismicity is observed in the catalogues to occur in sequences and swarms It is concluded therefore that not less than 90% of the seismicity of the Red Sea region occurs in sequences and swarms, and much of it, particularly that of the axial trough (83% of the seismicity of the Red Sea proper) are volcanic related.

21. Tectonic RegionNo of Earthquakes (M≥2) Seismic Moment *10^27 dyne.cmPercentage Whole Red Sea Region () Lat. 13⁰ - 30⁰ 7391 1.437 100% Red Sea Region () Lat. 13⁰ - 27.8⁰ 1430 0.8128 56.6% Both Gulfs 5961 0.618 43% Northern Red Sea () Lat. 21⁰ - 30⁰ 6488 0.966 67.2% Northern Red Sea () Lat. 21⁰ - 30⁰ Excluding the Gulf of Aqaba 0.400927.9% Northern Red Sea () Lat. 21⁰ - 27.8⁰ 527 0.348 24.2% Southern Red Sea () Lat. 13⁰ - 20.99⁰ 903 0.417 32.8% Axial Trough () Lat. 13⁰ - 27.8⁰ 845 0.6747 47% = 83% of 56.6%

22. Fig. (12) The frequency-magnitude results of: (A) 7391 instrumental earthquakes M ≥ 2.0, and (B) 1310 instrumental earthquakes M ≥ 4.0.

23. Seismicity Data ab Recurrence Period (years) for M = 6.0 6.5 7.0 Historical (years 627 – 1955), M = 5.6 – 7.0, 88 events8.641.17 33 150 583 (28) (166) (644) Historical (88 events) + Instrumental (29 events), M = 5.6 – 7.18.51.13 31 114 423 Instrumental south of Latitude 27.8, 1430 events, M =2-6.9 4.88 0.62 Instrumental south of Latitude 27.8, 588 events, M =4-6.9 6.710.96 Instrumental Axial Trough south of Latitude 27.8, 845 events, M =2-6.9 4.570.59 Instrumental Axial Trough south of Latitude 27.8, 381 events, M =4-6.9 6.18 0.89 Instrumental (1960 – 2010), M = 2 – 7.1 (7391 events)5.960.81 Instrumental (1976 – 2010), M = 2 – 3.9 5.380.67 Instrumental ( 1976 – 2010), M = 4 – 7.1 7.251.03

25. Region No. of Earthqs & Magnitudes Total Seismic Moment X 10 27 Dyne.cm Percentage of Seismic Moment at the depths(km): 0-1010.1-2020.1-3030.1-40≥40.1 Whole Red Sea Region 7391 M = 2 - 7.1 1.437 52.4% 6.5% 11.1% 26.3% 3.7% Red Sea Region () Latitudes13⁰&27.8⁰ 1430 M = 2 - 6.9 0.819 57% of Total 17.9% 10.5% 18.7% 46.5% 6.4% Axial Trough () Latitudes 13⁰&27.8⁰ 845 M = 2 - 6.9 0.67472 47% of Total 16.1% 0.7% 21.6% 54.3% 7.3% Northern Axial Trough () latitudes 21⁰ & 27.8⁰ 398 M = 2 – 6.9 0.28995 20.2% of Total 1.8%0.24%0.13%97.8%0.03% Southern Axial Trough () latitudes 13⁰ & 20.99⁰ 447 M = 2 – 6.7 0.38477 26.8% 26.9%1%37.7%21.4%13% Both Gulfs 5961 M= 2-7.1 0.618 43% of Total 97.3% 1.3% 1.2% 0.15% 0.05%

26. Fault Plane Solutions In the G. of Aqaba, lithospheric deformations are concentrated within the uppermost 20km of the continental crust and are characterised by left-lateral shear associated with some tension. In the northern Red Sea including the G. of Suez, focal mechanisms are dominated by normal faulting with left-lateral shear components. This is caused by extensional stresses brought about by divergent environment. In the southern Red Sea fault plane solutions indicate that both normal and strike-slip faults are active, but much higher on the first. Much of the axial seismicity including the 1967 and 1969 swarms have major components of normal faulting, i.e. caused by extensional tectonics. It is concluded that fault plane solutions all along the Red Sea support the regional tectonics along divergent environment with major sea-floor spreading.

27. Magnitude, Number and percentage of the seismicity of the axial trough of the Red Sea M ≤N% 452262% 4.570183% 4.979994.6% 5.983799% 6.9845100% 7 of the 8 earthquakes (M= 6 – 6.9) have depths = 3 – 35 km and one h=56km. The 1967 and 1969 Red Sea swarms appear to occur within the axial trough where NE fractures cross. Depth for most NE earthquakes outside the axial trough region appear to vary in the range 10km – 16km.

28. Characteristics of the Instrumental Seismicity of the Red Sea region Mmax= 6.9 - 7.1; Threshold M = 4 for the whole region; = 4.5 for the axial trough Excellent correlation with the regional geology and tectonics Mostly occur in sequences and swarms. The 1967, 1969 & 1995 swarms account for about 82.5% of the total seismic moment of the whole region. More than 90% of the released moment occurred in sequences and swarms. In the Gulf of Aqaba, the 1983, 1990, 1993 & 1995 swarms account for 97.8% of its total seismicity. As 83% of the R. Sea seismicity (south of latitude 27.8⁰) is restricted to its axial trough, and much of its seismicity occurs in sequences and swarms and are located close to volcanic regions, it is proposed that much of the instrumental seismicity is volcanic related. Shallow, mostly ≤ 40km, lithospheric deformations act partially within the crust and mostly within the upper mantle, but deeper beneath the N. Red Sea than the southern. h=3-56km for crest earthquakes & 10-16km for the fracture earthquakes. Beneath G. Aqaba, lithospheric deformations are restricted almost totally to the brittle upper crust down to 20km depth. Much of the axial seismicity including the 1967 and 1969 swarms have major components of normal faulting, with minor left-lateral shear components i.e. caused by extensional tectonics. Fault plane solutions all along the Red Sea support therefore the regional tectonics along divergent environment with major sea-floor spreading. Relatively moderate hazard.

29. Characteristics of the seismicity of mid-ocean ridges Earthquakes occur mostly as swarms and sequences Swarms concentrate mostly on ridge crests. A few occur near the intersection between crests and fracture zones (strike-slip faults) Crest earthquakes show mostly large component of normal faulting i.e. their origin is related to extensional tectonics Magnitudes of the crest swarms vary mostly in the range >6 to ≤ 4 while relatively larger earthquakes occur on the fracture zones but M is always ≤ 7 Swarms at ridge crests occur at variable depths larger than those of the fracture zones Swarms from ridge crests involve predominantly cataclastic deformation with high b-values, while earthquakes of the fracture zones involve largely brittle behaviour with lower b-values The seismicity of the Red Sea is comparable with mid-oceanic type of seismicity. This is evidenced by the following: sequences & swarm types; 83% axial concentration; mainly normal faulting with minor shear; M≤7, 99% ≤5.9, 94%≤ 4.9; h=3-56km for crest earthquakes & 10-16km for the fracture earthqs.

30. In conclusion: Combining the results of both historical and instrumental sets of seismological data with the previously published bathymetric, heat flow, gravity, magnetic and seismic results, it may be concluded that the axial trough all along the Red Sea appears similar to that of typical well- developed mid-ocean ridges.

32. Characteristics of the seismicity of mid-ocean ridges 1.Earthquakes occur mostly as swarms and sequences 2.Swarms concentrate mostly on ridge crests. A few occur near the intersection between crests and fracture zones (strike-slip faults) 3.Crest earthquakes show mostly large component of normal faulting i.e. their origin is related to extensional tectonics 4.Magnitudes of the crest swarms vary mostly in the range >6 to ≤ 4 while relatively larger earthquakes occur on the fracture zones but M is always ≤ 7 5.Swarms at ridge crests occur at variable depths larger than those of the fracture zones 6.Swarms from ridge crests involve predominantly cataclastic deformation with high b-values, while earthquakes of the fracture zones involve largely brittle behaviour with lower b-values The seismicity of the Red Sea is comparable with mid-oceanic type of seismicity. This is evidenced by the following: 1.Seismicity occurs mostly in the form of sequences and swarms all along the Red Sea region. 2.Not less than 83% of the seismic moment is released from earthquakes and swarms epicentered all along the axial trough with noticeable concentration where NE strike-slip faults cross this trough. 3.Much of the axial seismicity including the 1967 and 1969 swarms have major components of normal faulting, i.e. caused by extensional tectonics. 4.The magnitudes of all known historical and instrumental earthquakes are M ≤ 7.0. More than 94% of the earthquakes of the axial zone have magnitudes M ≤ 4.9 and more than 99% have magnitudes M ≤ 5.9. 5.The seismicity of the axial zone occurs at variable depths down to about 60km. More than 75% of this shallow seismicity occurs in the depth range 20km -40km. Earthquakes of the crest zone have focal depths 3km – 56km, while those associated with the NE trending fractures have shallower depths in the range 10km – 16km. 6.It is unfortunate that the seismicity of the axial trough is incomplete for M≤ 4.5, thus precise b- calculations cannot be made at this stage. 7.In conclusion: Combining these seismological results with the previously obtained bathymetric, heat flow, gravity, magnetic and seismic results, it may be concluded that the axial trough all along the Red Sea appears similar to that of typical well-developed mid-ocean ridges.

33. Sequence and swarm types of seismicity 1.Much of the historical seismicity occurred in the form sequences and swarms. The 1256 volcanic- seismic activity of Al-Madeenah Al-Munawarah is the best typical example. 2.The 1983, 1990, 1993 & 1995 swarms of the G. Aqaba account for 97.8% of its seismicity. At least some of these are related to subsurface volcanic activities. 3.The 1967, 1969 & 1995 swarms account for 82.5% of the total seismic moment of the Red Sea region. Much of the seismicity is observed in the catalogues to occur in sequences and swarms 4.It is concluded therefore that not less than 90% of the seismicity of the Red Sea region occurs in sequences and swarms, and much of it, particularly that of the axial trough (83% of the seismicity of the Red Sea proper) are volcanic related. Fault Plane Solutions 1.In the G. of Aqaba, lithospheric deformations are concentrated within the uppermost 10km of the continental crust and are characterised by left-lateral shear associated with some tension. 2.In the northern Red Sea including the G. of Suez, focal mechanisms are dominated by normal faults with left-lateral shear components. This is caused by extensional stresses brought about by divergent environment. 3.In the southern Red Sea fault plane solutions indicate that both normal and strike-slip faults are active, but much higher on the first. 4.Much of the axial seismicity including the 1967 and 1969 swarms have major components of normal faulting, i.e. caused by extensional tectonics. 5.It is concluded that fault plane solutions all along the Red Sea support the regional tectonics along divergent environment with major sea-floor spreading.

34. Conclusions 1.Some 88 historical earthquakes ( M = 5.6 – 7.0) and many uncounted others with smaller magnitudes have occurred in the study region during the period 627AD – 1955AD. These include only two M=7, three 6.8, and two 6.6, the remaining are of M ≤ 6.4. The overall average recurrence period for M ≥ 6.0 is 28 years. Earthquakes with M ≤ 5.8 appear to be less than expected. Some 1310 instrumental earthquakes ( M ≥ 4.0) and a few thousands with smaller magnitudes have also occurred in the region during the period 1960 – 2010. Only one of these had a magnitude M = 7.1, one 6.9, one 6.7 and the rest are of M ≤ 6.4. 2.All tectonic elements of the Red Sea region are seismically active including its northern, southern and central parts. The latter being of noticeable low activity. No large earthquakes have ever been recorded from the area bound by latitudes 22⁰ & 23⁰ during the last 14 centuries. The seismicity of all elements appears to have varied and fluctuated with time. 3.The seismicity data shows very good correlation with the general tectonics of the study region. In the Gulfs region the regional strike-slip faults are of higher activity than the normal. In the Red sea proper, more than 83% of the seismic moment is associated with the regional faults of the axial trough with noticeable concentration of activity where the NE trending strike-slip faults cross this trough. The published fault plane solutions of major earthquakes all along the Red Sea region are in accord with the characteristics of a divergent environment between two major plates, namely Arabian and African, with sea floor spreading in a regional NW direction. 4.The seismicity of the Red Sea region is shallow. Beneath the gulfs region more than 97% of the seismic moment is associated with earthquakes with depths no more than 10km i.e. lithospheric deformations are restricted to the upper part of the crust. South of latitude 27.8⁰, earthquakes occur at variable depths mostly down to 60km, or slightly more. Lithospheric deformations beneath this region appear to concentrate partially within the thin crust down to 10km depth and much more within the uppermost mantle, mostly from 20.1km to 40km depths. More than 80% of lithospheric deformations appear to concentrate beneath the axial trough all along the Red Sea.

35. 5.Much of the historical seismicity of the Red Sea region has occurred in the form of sequences and swarms and are believed to be volcanic related. The 1256 seismo-volcanic activity of Al Madinah Al Munawarah is a solid evidence. More than 90% of the region’s instrumental seismicity is also of the sequence and swarm types and are mostly volcanic related. The 2009 harrat Lunayyir swarm of NW Saudi Arabia is a typical example. 6.The frequency magnitude analysis show reasonable results from the historical data with a and b- values 8.5 – 8.6 and 1.13 – 1.17 respectively. The instrumental data gave comparable a and b-values. The 1.03 deduced b-value is very close to the expected ideal value for the study region ( b = 1.0 ). The calculated recurrence periods for earthquakes with M = 6, 6.5 and 7 are 30 ± 2 years, 150 ± 10 years and 600 ± 20 years respectively. The data show that the threshold magnitude for the study region is 4.0, and slightly more for the axial trough region. 7.The seismicity of the Red Sea is comparable with mid-oceanic type of seismicity. This is evidenced by the following: a. Seismicity occurs mostly in the form of sequences and swarms. b.Not less than 83% of the seismic moment is released from earthquakes and swarms epicentered along the axial trough with noticeable concentration where NE trending strike-slip faults cross this trough. c.Much of the axial seismicity including the 1967 and 1969 swarms have major components of normal faulting, i.e. caused by extensional tectonics. d.The magnitudes of all known historical and instrumental earthquakes are M ≤ 7.0. More than 94% of the earthquakes of the axial zone have magnitudes M ≤ 4.9 and more than 99% have magnitudes M ≤ 5.9. e.The seismicity of the axial zone occurs at variable depths down to about 60km. More than 75% of this shallow seismicity occurs in the depth range 20km -40km. Earthquakes of the crest zone have focal depths 3km – 56km, while those associated with the NE trending fractures have shallower depths in the range 10km – 16km. f.Combining these results with the previously obtained bathymetric, heat flow, gravity, magnetic and seismic results, it may be concluded that the axial trough all along the Red Sea appears similar to that of typical well-developed mid- ocean ridges.