HR: 1340h
AN: T13C-1382    [Abstracts]
TI: Fresh Submarine Seismic Breaks due to Historical Thrust Earthquakes Offshore Lebanon
AU: * Elias, A
EM: elias@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris - Laboratoire de Tectonique, 4 place Jussieu - case 89, Paris, 75252 France
AU: * Elias, A
EM: elias@ipgp.jussieu.fr
AF: Lebanese National Centre for Geophysics, Bhannes, B.P. 16-5432, Beyrouth, 1100-2040 Lebanon
AU: Tapponnier, P E
EM: tappon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris - Laboratoire de Tectonique, 4 place Jussieu - case 89, Paris, 75252 France
AU: King, G C
EM: king@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris - Laboratoire de Tectonique, 4 place Jussieu - case 89, Paris, 75252 France
AU: Singh, S
EM: singh@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Laboratoire de G\'{e}osciences Marines, 4 place Jussieu - case 89, Paris, 75252 France
AU: Sursock, A
EM: asursock@cnrs.edu.lb
AF: Lebanese National Centre for Geophysics, Bhannes, B.P. 16-5432, Beyrouth, 1100-2040 Lebanon
AU: Briais, A
EM: anne.briais@cnes.fr
AF: Observatoire Midi-Pyr‚n‚es, 14 avenue Edouard Belin, Toulouse, 31400 France
AU: Carton, H
EM: carton@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Laboratoire de G\'{e}osciences Marines, 4 place Jussieu - case 89, Paris, 75252 France
AU: Jacques, E
EM: Eric.Jacques@eost.u-strasbg.fr
AF: Institut de Physique du Globe de Strasbourg, 5 rue Descartes, Strasbourg, 67084 France
AU: Da\"eron, M
EM: daeron@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris - Laboratoire de Tectonique, 4 place Jussieu - case 89, Paris, 75252 France
AU: Jomaa, R
EM: rjomaa@cnrs.edu.lb
AF: Lebanese National Centre for Geophysics, Bhannes, B.P. 16-5432, Beyrouth, 1100-2040 Lebanon
AB: The M$>$7 earthquake that struck the shore of Lebanon in 551 AD was associated with a large tsunami that destroyed Beyrouth and other seaports, and thus likely originated offshore. Onshore in the north, the Tripoli thrust, whose 70m-high cumulative scarp cuts the city in half, is responsible for the Plio-Quaternary growth and current uplift of the Turbol-Qualhat anticline. Similarly, the growth of the Aabd\'{e} anticline, and the tilt and emergence of the Ramkine islands group result from slip on the Aabd\'{e} thrust. We have inferred both thrusts to continue at sea. The EM300 multibeam bathymetric data obtained during the SHALIMAR cruise establishes the existence of the active, submarine Mount Lebanon thrust system, which connects the Tripoli and Aabd\'{e} thrusts with the Roum fault. Between Beyrouth and Tripoli the Levantine margin shows its steepest near-shore bathymetric gradient (100 to 1500 m in $<$5 km). The base of this topographic front is bounded by slope breaks characteristic of emergent active faults. Up to 30 km from shore, the seafloor is uplifted and warped by anticlines. Flat-floored turbiditic channels fed by the deep river canyons onshore have been tilted and abandoned, or deviated by folding. Most of the anticlines appear to grow on blind ramps, and show spectacular, active extensional hinge faulting. The strikes of fold axes imply WNW-ESE shortening, consistent with the ongoing rise of Mount Lebanon and uplift of flights of coastal shorelines. The largest submarine shortening faces the steepest relief, where the flexed western limb of the Mt Lebanon mega-anticline tangents the coastline. This is where the highest marine cut terraces are observed. South of Saida, and north of Tripoli offshore the Akkar plain, by contrast, the overall topography of the continental edge is typical of passive margins with little evidence of active uplift. Profiles stretching as far as the Latakia ridge confirm that extensions of the Roum fault do not continue to Cyprus, but act as lateral ramps of the offshore Mt Lebanon fold and thrust belt. Deep-towed SAR side-scan sonar images confirm that the most prominent bathymetric slope-breaks are cumulative, active fault scarps. "Fresh" seismic ruptures are found on 2-3 km-long segments of these scarps, 8 to 18 km from shore between Beyrouth and Batroun. The seismic scarplets face west and have sinuous, stepping traces, consistent with dip-slip faulting. They likely mark the surface trace of past coastal thrust earthquakes, among which the 551 AD event. The multiple vermetid constructed benches seen along this part of the coastline thus likely reflect incremental coseismic uplift ($<=$80 cm). Near bottom studies are in urgent need to date seismic events and better assess seismic hazard along this most populated region of Lebanon.
DE: 7221 Paleoseismology
DE: 7230 Seismicity and seismotectonics
DE: 8005 Folds and folding
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting