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Abstract:
The Lebanese Restraining Bend (LRB) comprises the NNE-trending Mount Lebanon and the NE Anti-Lebanon ranges which are separated by the Bekaa Valley. Offshore, an east-dipping thrust fault system (the Mount-Lebanon Thrust, MLT) faces the coastal strip between Saida and Arqa. Historical earthquakes with casualties are documented along these active faults in the Eastern Mediterranean. We apply a seismic tomography method to P- and S-wave arrival times retrieved from the ISC seismological bulletins to determine the 3-D seismic velocity structure of the crust and uppermost mantle beneath this region. The seismic data set was generated by carefully selected events which are recorded by at least five stations and all unreliable arrivals have been excluded before the final tomographic inversion. We adopted an initial P-wave velocity model for the tomographic inversion, while the initial S-wave velocity model was computed using an Vp/Vs ratio of 1.725 derived from a wadati diagram. From the obtained P- and S-wave velocity models, we determined the Vp/Vs ratio for a better interpretation of the velocity anomalies. Lateral heterogeneities in the study area are clearly visible. Low velocity anomalies are revealed along the surface traces of the LRB and offshore along the MLT. Average to high Vp/Vs anomalies are clearly detected at lower crustal levels. Results of the checkerboard resolution test and the ray path coverage indicate that the velocity and Vp/Vs anomalies are reliable features. The present observations are compatible with many seismological and geological features detected beneath the Eastern Mediterranean.
