Delay times and shear wave splitting in the Mediterranean region

We measure SKS splitting parameters and P and S delay times in the Mediterranean region using broad-band recordings from the temporary MIDSEA array and permanent networks. The MIDSEA seismic array has substantially improved data coverage along the European-Mediterranean margins and in northern Africa. Shear wave splitting is observed for 95 per cent of the stations, with splitting times up to 2 s. We propose five anisotropic domains, based on the characteristics of the observations. In the western part of the region we observe an alignment with plate motion, suggesting an asthenospheric origin for anisotropy. In the central and near eastern part, the fast directions are less coherent laterally and influenced by extension and subduction processes. Along the Dead Sea fault the fast directions are parallel to the relative motion between Arabia and Africa, which we attribute in part to coherent vertical deformation. Our measurements of teleseismic, relative P- and S-wave delay times within the Mediterranean region reach peak-to-peak values of 2.5 and 8 s, respectively. Consistent with International Seismological Centre (ISC) delay times, we observe the earliest arrivals along the Hellenic subduction zone and some of the latest along the Dead Sea fault. We find that S-wave delays are roughly three times as large as those of P waves. From a slight frequency dependence of our cross-correlation based delay time measurements, we estimate Mediterranean lateral heterogeneity to have a dominant size of one or more hundred kilometres. We further determine that the anisotropy implied by our SKS splitting measurements does not significantly bias the pattern of teleseismic delay times.