Structure and Anisotropy Beneath Southern Italy

The crust and upper mantle structures beneath southern Italy are often associated with rollback and fragmentation of the Western Mediterranean subduction zone in the past 30 million years. In this study, we utilize broadband records from the Calabria-Apennine-Tyrrhenian/Subduction Collision-Accretion Network (CAT/SCAN) to probe the effect of the past and on-going plate motions beneath this region. Waveforms from two distinct frequency ranges are examined in detail using both forward and inverse waveform modeling approaches, and the resulting 1-D models for each path are subjected to a Monte-Carlo uncertainty test. By analyzing the Love and Rayleigh waves from two regional earthquakes during the temporary deployment between 2003 and 2005, we are able to retrieve information on the anisotropic seismic structure down to 200-km depth. Our study shows that the average seismic structure beneath Calabria/Apulia is significantly faster than that beneath the Apenninic mountain chain. Sharp changes in seismic velocities, regardless of wave polarizations, lend strong support for the distinct geologic histories of the major tectonic units. The difference between Love- and Rayleigh-wave models provides further constraints on the dynamic processes and crust/mantle fabric beneath the study area. Our preliminary results show relatively minor anisotropy beneath the southeastern Tyrrhenian Sea and Calabrian Arc region, which could be partially explained by the rapid changes in the alignment of olivine fast crystallographic-axes due to the complex arc geometry. The ray paths connecting the Ionian Sea and the Southern Apennines reveal strong anisotropy, where transversely polarized waves travel at higher speeds than vertically polarized waves. Both "frozen-in" and flow-induced anisotropy could contribute to the observed waveform differences between the Love and Rayleigh waves sampling this region.