Abstract

We present results of the first paleoseismic study of the Yammoûneh fault, the main on-land segment of the Levant fault system within the Lebanese restraining bend. A trench was excavated in the Yammoûneh paleolake, where the fault cuts through finely laminated sequences of marls and clays. First-order variations throughout this outstanding stratigraphic record appear to reflect climate change at centennial and millennial scales. The lake beds are offset and deformed in a 2-m- wide zone coinciding with the mapped fault trace. Ten to thirteen events are identified, extending back more than ∼12 kyr. Reliable age bounds on seven of these events constrain the mean seismic return time to 1127 ± 135 yr between ∼12 ka and ∼6.4 ka, implying that this fault slips in infrequent but large (M ∼7.5) earthquakes. Our results also provide conclusive evidence that the latest event at this site was the great a.d. 1202 historical earthquake, and suggest that the Yammoûneh fault might have been the source of a less well-known event circa a.d. 350. These findings, combined with previous paleoseismic data from the Zebadani valley, imply that the parallel faults bounding the Beqaa release strain in events with comparable recurrence intervals but significantly different magnitudes. Our results contribute to document the clustering of large events on the Levant fault into centennial episodes, such as that during the eleventh through twelfth centuries, separated by millennial periods of quiescence, and raise the possibility of a M >7 event occurring on the Yammoûneh fault in the coming century. Such a scenario should be taken into account in regional seismic-hazard assessments and planned for accordingly.

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