Regional seismic phases across the Ligurian Sea: Lg blockage and oceanic propagation

The Ligurian Sea (West Mediterranean) is an example of a narrow oceanic basin which stops Lg wave propagation. Locating the exact point of Lg extinction was one of the goals of the SISBALIG II seismological experiment, which deployed both inland stations in Provence and Corsica and Ocean Bottom Seismometers (OBS) in the Ligurian Sea. A precise analysis of all available regional seismological data using energy diagrams reveals the role of the Provence margin in the Lg phase extinction for seismic events occurring in the southwestern Alps. The extinction is not a progressive phenomenon along oceanic travel path but occurs in a narrow zone of about 20 km width in the vicinity of the Provence margin. The extinction is accompanied by the generation of two types of diffracted waves: the mantle phase Sn (V-s > 3.5 km s(-1)), and very low velocity S waves appearing both near the Provence margin and in the middle of the Ligurian basin. We performed numerical simulation of SH wave propagation in two-dimensional (2D) media to investigate the mechanisms of Lg extinction. The simulations confirm that simple crustal thinning at the continent-ocean boundary alone cannot cause the observed extinction of Lg waves, although the change in Moho depth does induce the diffraction of a part of the incident Lg wavetrain into mantle waves, giving rise to the Sn phase on the other side of the basin. The existence of soft oceanic sediments is a very important factor in explaining the extinction, as a large part of the Lg energy is transferred into slow S waves trapped in the sediments. Moreover, comparison of synthetic seismograms and observed data shows that the observed two packets of low-velocity waves are an indication of the asymmetry of the Ligurian basin, with the existence of two shallow basins with very low S velocity (1.0 km s(-1)) near the Provence margin and in the middle of the Ligurian Sea. However, the attenuation of Lg waves in the vicinity of the Provence margin is more abrupt in the data than in the simulations. This discrepancy could result from the complex geometry of the continent-ocean transition zone and its possibly very diffractive nature, two factors that could add to Lg extinction at the margin and are impossible to take into account in models.