Upper crustal structure in the Potenza area (Southern Apennines, Italy) using Sp converted wave

Abstract

In this paper we analyse the records of the two Potenza seismic sequences (Southern Apennines, Italy) which occurred in 1990 and 1991, in order to obtain information on the upper crustal structure in this area. The hypocentral depths are mainly concentrated below 10 km which is the supposed upper limit of the crystalline basement. The seismograms recorded at temporary arrays deployed during the two sequences clearly show on the vertical component, an intermediary phase between the P and S waves. For the investigated epicentral distances (less than 30 km) the delay between this secondary phase and the direct S wave arrival is almost constant at each station, suggesting that the observed intermediary phase might be an S to P conversion at a discontinuity shallower than the hypocentral depth. This interpretation has been supported by polarisation analysis and numerical modelling results. Considering the regional geological structure, these latter have also shown that the interface generating strong converted waves could be the top of the Apulian carbonate platform overlaid with recent clay deposits and flysch sediments. 1D inversion of the travel-time data was performed in order to evaluate a local vertical upper crustal profile.