REGIONAL SEISMIC PHASES ACROSS THE LIGURIAN-SEA - LG BLOCKAGE AND OCEANIC PROPAGATION

The Ligurian Sea (West Mediterranean) is an example of a narrow oceani c basin which stops Lg wave propagation. Locating the exact point of L g extinction was one of the goals of the SISBALIG II seismological exp eriment, which deployed both inland stations in Provence and Corsica a nd Ocean Bottom Seismometers (OBS) in the Ligurian Sea. A precise anal ysis of all available regional seismological data using energy diagram s reveals the role of the Provence margin in the Lg phase extinction f or seismic events occurring in the southwestern Alps. The extinction i s 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 ma rgin. 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 l ow 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 mec hanisms of Lg extinction. The simulations confirm that simple crustal thinning at the continent-ocean boundary alone cannot cause the observ ed extinction of Lg waves, although the change in Moho depth does indu ce 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 exp laining the extinction, as a large part of the Lg energy is transferre d into slow S waves trapped in the sediments. Moreover, comparison of synthetic seismograms and observed data shows that the observed two pa ckets 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 vicin ity of the Provence margin is more abrupt in the data than in the simu lations. This discrepancy could result from the complex geometry of th e continent-ocean transition zone and its possibly very diffractive na ture, two factors that could add to Lg extinction at the margin and ar e impossible to take into account in models.