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Estimate of the focal depth of deep earthquakes and some structural implications for the deep seismic activity in the Tyrrhenian region

Istituto Geofisico e Geodetico Universita di Messina, 98100 MESSINA, Italy

Abstract

We describe a method for determining the depth of deep earthquakes utilizing the travel-time differences T_S - T_P and T_PdP - T_P relative to P, S, and PdP waves (Pdp indicates P waves reflected off the 20° discontinuity), observed at a single station.

This method has provided valid results and the values calculated for about 20 earthquakes in the Tyrrhenian and Greek areas show differences in the region of ± 10 and ± 15 km, respectively when compared with the corresponding values obtained by statistical methods.

Starting from an analysis of the spatial distribution of 48 intermediate and deep foci in the Tyrrhenian area and from the study of travel-time residuals and of the initial motion at MES, we go on to a critical examination of a number of hypotheses regarding the deep structure of the area examined.

The spatial distribution and the data gathered agree with a certain number of physical parameters characteristic of an underlying lithospheric slab, which appears to be the most valid structural model for the southern Tyrrhenian region, for which one can estimate a frontage of 250 km, an average dip of 46°, and an azimuth of 297°. In the last 40 years the major seismic area has been that between 220 and 340 km in depth, corresponding to the zone under maximum tension. Instead, the slab's terminal zone, beyond a depth of 430 km, appears to be under compression.

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