Upper mantle velocity structure in the Mediterranean and surrounding regions from seismological data

Abstract

Upper mantle velocity structure in the Mediterranean and surrounding regions has been studied in detail to a depth of 500 km by analysis of P- and S-wave travel-time data of 229 deep earthquakes. P- and S-wave velocities, obtained “at the depths of foci” by using Kaila's (1969) analytical method, in the intermediate and deep earthquake regions of the Calabrian arc-Tyrrhenian Sea, western Mediterranean-southern Spain, western and southern margins of the Balkan peninsula and the Vrancea region of the eastern Carpathian reveal lateral velocity variations of the order of 3–4% down to 150 km depth, the P velocities in the Balkan margins being the highest. In the Calabrian arc-Tyrrhenian Sea deep earthquakes region, the P velocity increases from 8.01 km/s at 40 km to 8.14 km/s at 210 km depth. The P velocity increases rapidly at deeper depths from 210 km with a gradient of 0.005 s −1, reaching 9.62 km/s at 500 km depth. The S velocity in this region increases from 4.58 km/s at 40 km to 4.73 km/s at 400 km depth without any indication of a zone of rapid velocity increase. However, the S velocity in the Calabrian arc-Tyrrhenian Sea deep earthquakes region is found to be about 6% higher than the average S velocity of 4.34 km/s in the top 150–200 km of the surrounding upper mantle region as deduced from surface waves dispersion studies. Similarly, the S velocities in the intermediate earthquake regions of the western Mediterranean and the Balkan margins are found to be about 5% higher than the corresponding average values in the surrounding upper mantle regions. These variations are essentially due to the presence of a well delineated sub-lid low S velocity channel extending over most of central Europe. The P velocity model for the Calabrian arc-Tyrrhenian Sea deep earthquakes region, determined in the present study, agrees remarkably well with the P velocity models EKW (England et al., 1977) and MR (Mayer-Rosa and Mueller, 1973) which are applicable for the upper mantle beneath south-central Europe. A comparison of the P velocity models in south-central Europe, including that of the present study, with the P velocity models SNE (Hurtig et al., 1979), KCA (King and Calcagnile, 1976) and MA (Masse and Alexander, 1974) which are applicable for the upper mantle beneath the east European platform reveals strong lateral velocity variations in the upper mantle beneath the two regions. The velocities in south-central Europe are about 5% lower than those in the east European platform to a depth of at least 250 km. These results are consistent with the temperature-depth functions for Europe given by Stromeyer (1978) which show 400°–500°C higher temperatures at depths in central Europe than those in the east European platform. The nearly constant S velocity of 4.6–4.7 km/s from 40 to 400 km depth in the Calabrian arc-Tyrrhenian Sea deep earthquakes region, as determined in the present study, is also consistent with the higher temperatures expected in the surrounding upper mantle region beneath central Europe. The “400 km discontinuity” has not been revealed by both P- and S-wave velocity models down to 500 km depth in the present study, possibly due to its larger depth of 520–540 km in south-central Europe, in comparison to its relatively shallow depth of 380–420 km in the east European platform as given by several models. Alternatively, the high P-wave velocity gradient from 210 to 500 km depth may imply absence of a “sharp” 400 km discontinuity which is probably replaced by a broad transition zone in this region.