New Insight of Northern Apennines (Italy): SKS Splitting Measurements Reveal a Complex Anisotropic Structure

The multidisciplinary RETREAT project (REtreating-Trench, Extension and Accretion Tectonics) is focused on the development of a 3D self-consistent dynamic model of the syn-convergent extension in the Northern Apennines. The seismological deployment of the Project started on 2003 and closed on September 2006, using 10 instruments lent by the GFU and 25 instruments lent by PASSCAL, added to the permanent stations of the Italian National Network. Many of the stations were deployed along a NE-SW transect across the Apennine chain. We present here new results of seismic anisotropy analysis obtained from SKS core-refracted shear waves. The study of SKS splitting is applied on twenty teleseismic earthquakes; for all of them we calculate the anisotropic parameters (delay time and fast polarization direction) by minimizing the energy in the transverse component. Our analysis assesses uncertainty by testing the parameters for stability to noise. Previous studies of splitting analysis have found in the study region evidence for tectonic domains in which a coherent splitting signal can be found. The Tuscany domain (Tyrrhenian side) shows homogeneous NW-SE fast axes directions; the Po-Plain domain (Eastern side of the Apennines) shows a N-S to NE-SW directions, here strongly influenced by backazimuth. To better define the complex structure that may exist below the Northern Apennines and Po Plain we apply the cross convolution method of Menke and Levin (2003) to discriminate whether a two-layer anisotropic model fits the splitting pattern more convincingly that a simple one-layer model. Previous analysis suggested that structure beneath the Tuscany side is simpler; a single anisotropic layer with a NW-SE fast polarization direction is in agreement with all the dataset. Beneath the Po Plain the complexity of the structure is confirmed in the analysis of most stations.