Preliminary Shear Wave Velocity Models in the Scotia Sea Region, Antarctica

Phase and group velocity of the fundamental mode of the Rayleigh waves in the period between 10s to 30-40s are used to obtain the shear wave velocity versus depth along paths crossing the Scotia Sea region. Data recorded by two IRIS (Incorporated Research Institutions for Seismology) stations (PMSA, EFI) and the stations of the OGS-IAA (Osservatorio Geofisico Sperimentale - Istituto Antartico Argentine) network (ESPZ, USHU) are used. The Frequency-Time Analysis (FTAN) technique is applied to more than 40 earthquakes to measure the dispersion properties. A non linear inversion procedure is performed to retrieve the shear wave velocity models from the dispersion data. Our preliminary results show that three distinct models can be isolated from the most frequently sampled paths: the South Scotia Ridge (SSR), the western Drake Passage zone (DP) and the extreme tip of the Antarctic Peninsula (TAP). We estimate shear wave velocity ranging between 3.0 and 3.6 km/s for the thinner (9-13 km) and faster crust of the DP channel. In contrast, shear wave velocities ranging between 2.5 and 3.6 km/s were found for the thicker (13-16 km) and slower crust of SSR. The TAP model can be summarised as consisting of two layers with a high velocity gradient (2.3-3.0 km/s) in the upper crust (6 km) and a low velocity gradient (3.0-4.0 km) in the lower crust (14 km). The resulting models constrain the average Moho depth within acceptable uncertainties. In practice we have found that the Moho depth can vary between 15 km and 18 km and between 11 km and 15 km for the paths sampling SSR and DP, respectively. Conversely we have found no evidence of sharp changes in shear wave velocity for the TAP model consistent with a smooth ocean-continent transition.