Travel-time tomography of the European-Mediterranean mantle down to 1400 km

Abstract

The 3-D P-wave velocity structure of the mantle below Europe, the Mediterranean region and a part of Asia Minor is investigated. This study is a considerable extension of an earlier tomographic experiment that was limited to imaging upper-mantle structure only. Here, the Earth's volume under study encompasses the mantle to a depth of 1400 km, and we increase the number of International Seismological Centre (ISC) data for inversion by a factor of four by taking more years of observation, and by including data from teleseismic events. The most important departure from the earlier study is that we do not use the Jeffreys-Bullen model as a reference model, but an improved radially symmetric velocity model, the PM2 model, which is appropriate for the European-Mediterranean mantle. Our inversion procedure consists of two steps. First, the radial model PM2 is determined from the ISC delay times by a nonlinear trial-and-error inversion of the data. As opposed to the Jeffreys-Bullen model, the new reference model has a high-velocity lithosphere, a low-velocity zone, and seismic discontinuities at depths of 400 and 670 km. Next, the ISC data are corrected for effects related to the change in reference model and inverted for 3-D heterogeneity relative to the PM2 model. We follow this two-step approach to attain a better linearizable tomographic problem in which ray paths computed in the PM2 model provide a better approximation of the actual ray paths than those computed from the Jeffreys-Bullen model. Hence, the two-step scheme leads to a more credible application of Fermat's Principle in linearizing the tomographic equations. Inversion results for the 3-D heterogeneity are computed for both the uncorrected ISC data and for the PM2 data. The data fit obtained in the two-step approach is slightly better than in the inversion of ISC data (using the Jeffreys-Bullen reference model). A comparison of the tomographic results demonstrates that the PM2 data inversion is to be preferred. To assess the spatial resolution an analysis is given of hit count patterns (sampling of the mantle by ray paths) and results of sensitivity tests with 3-D synthetic velocity models. The spatial resolution obtained varies with position in the mantle and is studied both in map view and in cross-section. In the well-sampled regions of the mantle the spatial resolution for larger-scale structure can (qualitatively) be denoted as reasonable to good, and at least sufficient to allow interpretation of larger-scale anomalies. A comparison is made of the results of this study with independent models of S-velocity heterogeneity obtained in a number of investigations, and with a prediction of the seismic velocity structure of the mantle computed from tectonic reconstructions of the Mediterranean region. In the context of this comparison, interpretations of large-scale positive anomalies found in the Mediterranean upper mantle in terms of subducted lithosphere are given. Specifically addressed are subduction below southern Spain, below the Western Mediterranean and Italy, and below the Aegean. In the last region a slab anomaly is mapped down to depths of 800 km.