Abstract
In this article, P travel-time residuals for the Messina station are analyzed in order to investigate the Tyrrhenian upper mantle, which is considered to be crossed by a lithospheric slab.
A first set of 24 residuals derived from deep earthquakes of the southern Tyrrhenian Sea show early arrivals of, on the average, −1.3 sec at Messina. In addition, these negative residuals are associated with initial motion of the dilatation type. On the contrary, the few deep earthquakes which produce, as first motion, a compression at the Messina station, are associated with late arrivals of about 1 sec. These results are considered and discussed in order to analyze the hypocentral mechanism and P-wave transmission through the lithospheric slab.
A second and wider analysis is then extended to 206 earthquakes which have, with respect to Messina, an epicentral location in the distance range 16° to 103° and azimuthal orientation Z in the interval 180° to 380°. The first conclusion from this analysis is that the P travel times observed at Messina for epicentral distances in the range 20° to 103° and 245° ≦ Z ≦ 380° are generally 0.5 to 3 sec less than those given in the Jeffreys-Bullen tables.
Finally, a further improvement on the foregoing result has been obtained. This gives further confirmation of the consistency of regional variations of the P travel times with a slab model for the Tyrrhenian deep structures. As a matter of fact, the comparison between the travel times of Messina and a standard provided by observations in the stations of Rome and Trieste confirms early arrivals of about 1 sec on the seismic paths which cross the upper mantle in the southern Tyrrhenian region.
