Holocene faulting and earthquake recurrence along the Serghaya branch of the Dead Sea fault system in Syria and Lebanon

Abstract

SUMMARY The Serghaya fault, located approximately along the Syrian–Lebanese border, is a prominent structure within the 200 km restraining bend in the left‐lateral Dead Sea fault system. This study documents palaeoseismic and geomorphic expressions of Holocene movements on the Serghaya fault based on trench excavations and radiocarbon dates. Trenches were excavated across and parallel to a 4.5 m fault scarp where Late Pleistocene sediments are faulted against Holocene alluvium and colluvium. Locally oblique slip on the Serghaya fault has produced a sequence of fault‐derived colluvial wedges that distinguishes individual palaeoseismic events. In addition, the trench excavations also depict a sequence of buried and displaced channels. Our palaeoseismic study reveals evidence for five surface‐rupturing events within the past ∼6500 yr. The last event involved 2–2.5 m of primarily left‐lateral displacement and may correspond to one of two historically documented earthquakes during the 18th century (in 1705 and 1759). The displaced channels provide an estimated slip rate of approximately 1.4 ± 0.2 mm yr− 1 during the Holocene. The chronological relationships between the colluvial wedges and faulted channels demonstrate an average left‐lateral displacement of about 2 m per event, suggesting that such events correspond to earthquakes of M≳ 7 with a mean return time of about 1300 yr. These results demonstrate that the Serghaya fault may present a previously overlooked earthquake hazard for populations in the vicinity of the AntiLebanon Mountains, including the cities of Damascus and Beirut. In a regional context, the inferred slip rate along the Serghaya fault accounts for about 25 per cent of the total expected motion of Arabia relative to Africa along the Dead Sea fault system. The fact that the Serghaya fault accounts for only a fraction of the expected plate motion implies that the remaining strike‐slip and shortening must be accommodated by other active fault branches within the large restraining bend of the Dead Sea fault system. These results contradict suggestions that the northern Dead Sea fault system in Lebanon and Syria is presently inactive as a result of an evolving regional stress field in the eastern Mediterranean region.

Late Cenozoic uplift along the northern Dead Sea transform in Lebanon and Syria

“…Recent plate tectonic models have begun to suggest that the northern portion of this transform may be characterized as transpressional in nature. Recent studies have identified clear evidence of active tectonism along the DSFS, including paleoseismic indicators, for the main fault branches along the northern 500 km of the DSFS (i.e., north of approximately 32.5° N) [1][2][3][4][5]. Results of these studies firmly refute recent assertions that the northern DSFS and the strike-slip faults in the Bekaa Valley are presently inactive [6,7].…”

Different styles of faulting deformation along the Dead Sea Transform and possible consequences for the recurrence of major earthquakes

“…It is one of the largest continental strike-slip faults in the world with more than 1000 km length and links the Red Sea seafloor-spreading center to the collision zone in SE Turkey. According to Butler et al (1997) and Gomez et al (2003) the DST can be subdivided into a southern and northern part, joined by a ~200 km long restraining bend (Lebanese segment). The DST has been active since the middle Miocene with a total displacement of 105 km in the south (Bartov et al, 1980;Eyal et al 1981;Garfunkel et al, 1981) and 70-80 km in the north (e.g.…”

“…However, the analysis of the structural displacement has been proved controversial (Westaway, 2004;Mart et al, 2005). The estimated slip rate varies between 1 and 10 mm/a following different measurement techniques (field studies, GPS, e.g., Garfunkel et al, 1981;Klinger et al, 2000;Gomez et al, 2003;Pe'eri et al, 2002;Meghraoui et al, 2003).…”

“…Previous structural studies which took place along several fault branches of the DST, such as the Arava fault (AF; e.g., Klinger et al, 2000;Diabat et al, 2004); the Serghaya fault (SF; e.g., Gomez et al, 2001;Gomez et al, 2003) and the Ghab (or Masyaf) fault (GF; e.g., Meghraoui et al, 2003;Chorowitz et al, 2005) suggest notable variations in the amount of displacement and the level of seismicity. Therefore, the question arises to what extent fault-related observations based on geophysical and geological investigations of individual fault segments are valid for large-scale strike-slip faults systems as a whole.…”

Using ambient vibration measurements for risk assessment at an urban scale: from numerical proof of concept to Beirut case study (Lebanon)

“…The most recent large earthquake is the double shock of March 16, 1956 (Ms = 4.8-5.1) which killed 136 people, destroyed 6000 houses and damaged 17,000 houses (Brazee and Cloud 1984;Khair et al 2000). Seismological trenches have shown that the return period of devastating earthquakes is about 1100 years along the Yammouneh Fault ), 1500-1750 years along the Mount Lebanon Thrust ) and ~1300 years along Serghaya Faults (Gomez et al 2003).…”