Seismic imaging of the crustal structure beneath Lebanon and localization of earthquakes

The Eastern Mediterranean region is characterized by a complex tectonic setting, where the African and Arabian plates converge. This makes Lebanon a seismically active zone that has experienced major destructive earthquakes and could experience similar catastrophic events in the future. Accurate earthquake localization strongly depends on the velocity model used for event location (Gesret et al., 2015). Thus, it is of primary importance to characterize the subsurface structure in order to better locate and understand this seismicity. Characterizing the subsurface crustal structure beneath Lebanon remains challenging due to limited geophysical acquisitions. In this work, we use the P-wave receiver function method to image the subsurface structure beneath two permanent broadband stations, “BHL” and “HWQ”, operational since 2006, which offer a good azimuthal coverage required to characterize dipping interfaces or crustal anisotropy. This study is extended to include several temporary broadband stations distributed across the country to investigate variations in crustal thickness between different regions. Beneath the “BHL” station in western Lebanon, Bayesian inversion of arrival times for PS and PpS phases identifies a Moho boundary at a depth of 33 km, dipping 12° to the southeast. The uncertainties associated to this solution are also estimated thanks to the probabilistic framework. The dip direction aligns with the geological structures of the region. Beneath the “HWQ” station in northern Lebanon, inversion of arrival times and polarities of PS phases from two interfaces suggests an anisotropic lower crustal layer, with a slow axis plunging southwest. This study enabled us to image the crust beneath several broadband permanent and temporary stations and will contribute to the development of a 3D velocity model for Lebanon. In this work, earthquakes are localized using the probabilistic localization method, which will be compared to the deterministic approach commonly used by the National Center of Geophysics in Lebanon. This probabilistic method also provides uncertainty estimates, and allows to compare several velocity models in order to select the best velocity model to be used for event locations in Lebanon.

References

Gesret, A., Desassis, N., Noble, M., Romary, T. & Maisons, C. (2015). Propagation of the velocity model uncertainties to the seismic event location. Geophys. J. Int. (2015) 200, 52–66.