[S-SS10] Active faults and paleoseismology

[SSS10-10] Discovery of active faults on the coasts of Gulf of Suez and its significance

*Takashi Nakata¹, Steven G. Wesnousky², Hideaki Goto¹ (1.Hiroshima University, 2.University of Nevada, Reno)

Keywords:Gulf of Suez, active fault, ALOS 30 anaglyph

We newly found active faults, which are widely developed across the alluvial fans on both banks of the Gulf of Suez. There are regional differences in the development of faults. In the northernmost part of the bay, NW-SE extending active faults with cumulative displacements across the alluvial fan, develop to the northwest of Suez and east of Ras Sudr. On the east bank of the central part of the Gulf of Suez, the low-lying mountains face the coast, and the alluvial fan is poorly developed. Here, active faults develop in the part of the boundary between low mountains and lowlands located near the coast. A part of the main rift boundary fault east of Al Tur has displaced the alluvial fan, and its activity cannot be completely denied. On the other hand, active faults are well developed on the west bank the central part of the Gulf of Suez. In the central part of the Gharib plain on the west bank, many normal fault-type active faults extending NNW-SSE parallel to the long axis of the Gulf of Suez are widely observed on the alluvial fans. In the southern part of the Gulf of Suez, on the eastern shore south of Al Tur, we find many short active faults across the alluvial fan, but few faults show evidence of cumulative displacement. On the other hand, on the west bank, a remarkable active fault develops to the west of Jemsa Bay. One of the most distinctive active fault developments in the Gulf of Suez area is observed in the Gharib plain on the west bank between Ras Abu Bakr and Ras Gharib. Many active faults extending parallel to each other with northeast-southwest strikes are found in the area. Most of them are normal fault-type active faults with a length less than 10 km long. They displace the old and new alluvial fans, indicating their cumulative activity. We recognized many small grabens that were bounded by fault scarps facing each. The mechanism solutions of 18 shallow earthquakes with magnitudes of 2.8-5.2 in the central and northern parts of the Gulf of Suez between 1983 and 2004 show mostly normal fault components (Morsy et al., 2011) that are consistent with the activity of the active faults in the study area. There is no study regarding the age of alluvial fans in this area, but the correlation of their age with the alluvial fans in the arid regions of the western United States (Hoeft and Frankel 2010) is attempted based on their surface structure characteristics. As a result, the alluvial fan correlated with Q3b (15.5 ± 1.2ka) is vertically displaced by 1-3m, Q2c (92 ± 9ka) 5-6m, and Q2b (137 ± 25ka) 10-15m. The vertical slip rate is roughly estimated at 0.1 mm/year. Since about ten active faults with similar slip rates are recognized on land, the total slip rate of the active faults on land reaches almost 1 mm / year. We have no data regarding the dip of the mapped active faults, but from the source mechanism of the earthquakes in the area, we may consider that faults dip 50-60 degrees. Therefore the expansion rate across the Suez rift may become approximately 1 mm/year if we add the slip of the active faults at the bottom of Suez Bay. The Sharm El Sheikh earthquake (Mw6.6) occurred in 1969 on the seafloor near the west bank at the southern end of the Gulf of Suez, and it is predicted that hazardous earthquakes originating from newly discovered active faults may occur anytime.