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Active thrusting offshore Mount Lebanon: Source of the tsunamigenic A.D. 551 Beirut-Tripoli earthquake

¹ Institut de Physique du Globe de Paris, 4 place Jussieu, case 89, 75252 Paris, France, and National Center for Geophysical Research, P.O. Box 11-8281, Riad EL-Solh, 1107 2260 Beirut, Lebanon
² Institut de Physique du Globe de Paris, 4 place Jussieu, case 89, 75252 Paris, France
³ Laboratoire de Dynamique Terrestre et Planetaire, CNRS UMR 5562, Observatoire Midi-Pyrenees, 14 avenue Edouard Belin, 31400 Toulouse, France
⁴ Tectonics Observatory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
⁵ Institut de Physique du Globe de Paris, 4 place Jussieu, case 89, 75252 Paris, France
⁶ National Center for Geophysical Research, P.O. Box 11-8281, Riad EL-Solh, 1107 2260 Beirut, Lebanon
⁷ Institut de Physique du Globe de Strasbourg, UMR CNRS 7516, 5 rue René Descartes, 67084 Strasbourg Cedex, France
⁸ National Center for Geophysical Research, P.O. Box 11-8281, Riad EL-Solh, 1107 2260 Beirut, Lebanon
⁹ Institut de Physique du Globe de Paris, 4 place Jussieu, case 89, 75252 Paris, France

On 9 July A.D. 551, a large earthquake, followed by a tsunami, destroyed most of the coastal cities of Phoenicia (modern-day Lebanon). Tripoli is reported to have "drowned," and Berytus (Beirut) did not recover for nearly 1300 yr afterwards. Geophysical data from the Shalimar survey unveil the source of this event, which may have had a moment magnitude (M_w) of 7.5 and was arguably one of the most devastating historical submarine earthquakes in the eastern Mediterranean: rupture of the offshore, hitherto unknown, 100–150-km-long active, east-dipping Mount Lebanon thrust. Deep-towed sonar swaths along the base of prominent bathymetric escarpments reveal fresh, west-facing seismic scarps that cut the sediment-smoothed seafloor. The Mount Lebanon thrust trace comes closest (8 km) to the coast between Beirut and Enfeh, where, as 13 ¹⁴C-calibrated ages indicate, a shoreline-fringing vermetid bench suddenly emerged by 80 cm in the sixth century A.D. At Tabarja, the regular vertical separation (1 m) of higher fossil benches suggests uplift by three more earthquakes of comparable size since the Holocene sea level reached a maximum ca. 7–6 ka, implying a 1500-1750 yr recurrence time. Unabated thrusting on the Mount Lebanon thrust likely drove the growth of Mount Lebanon since the late Miocene.

Key Words: Lebanon • thrusts • submarine seismic breaks • tsunami • vermetid benches