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Some results for the middle western Mediterranean basin from the study of Pn waves

ISTITUTO GEOFISICO E. GEODETICO UNIVERSITA DI MESSINA, 98100 MESSINA, Italy

Abstract

The most important contributions to our understanding of the structure of the deep layers of the crust beneath the western Mediterranean Sea come from surface-wave dispersion studies. The results obtained in this region from studies of body waves are few, sometimes discordant, and generally applicable only to areas of moderate areal extent.

In this paper, the results of a study of Pn waves are reported. A first solution, based on arrival-time data at Mediterranean coastal stations, gives, for the distance interval 1.78° to 10.70°, the relation

where is expressed in kilometers.

Subsequently, by analysis of the residuals and by taking account of the uncertainty in the value of the apparent velocity, as shown in the previous equation, a further approximation can be attempted in the form of two linear equations. The first one, which applies to the interval 1.78° to 5.59°, takes the form

, giving for the apparent Pn velocity the value of 7.82 km/sec, which is in agreement with the results of a seismic refraction study carried out by Fahlquist in the northeastern area of the Balearic Islands.

The second equation, for the interval of 5.59° to 10.70°, indicates a further refracting horizon, 25 km deeper than the previous one, with a velocity of 8.19 km/sec. This equation takes the form

.

These equations, even with some reservations because of insufficient observational data at about 9° to 10°, confirm the validity of the hypothesis formulated by Berry and Knopoff regarding the longitudinal wave velocity at depths between 29 and 49 km. They suggest a crust of oceanic type in the upper part and of transition type in the deeper part (epicontinental crust) of the western Mediterranean basin.