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
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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 symmetricvelocity 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
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