Anisotropy patterns in the subducting lithosphere and in the mantle wedge. A case study: the Southern Italy subduction system.

Abstract

In this study we present a collection of high quality S wave splitting measurements in the southern Italy subduction system. We analyzed 76 deep earthquakes located within the descending slab using the method of Silver and Chan (1991) to determine the splitting parameters ’ and dt. The local deep earthquakes allow us to analyze raypaths primarily sampling the slab and the wedge above it. Mainland Calabria is an outcropping forearc, enabling us to sample rays that propagate up the slab. S wave splitting parameters show a complex pattern of anisotropy with variable fast directions and with delay times ranging from 0.1 s to 2.2 s. We compared local S wave splitting data with SKS results at the same stations, and we found that the average dt is very different (1.8 s for SKS and 0.5 s for S). We found consistency between averaged S fast directions and the SKS splitting measurements, including a pattern of toroidal mantle flow at the SW edge of the slab. The S wave splitting parameters show frequency‐dependent behavior that we attribute to the presence of small‐scale anisotropic heterogeneities. Comparison of S splitting measurements to P wave velocity anomaly at 100–200 km depth shows that where the rays primarily sample the slab the delay times are small. In contrast, where S rays sample the mantle wedge, the delay times are quite high. This dt pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere.