Modelli delle velocità sismiche di taglio della struttura crostale profonda della Piana Campana mediante cross-correlazione di rumore sismico

Abstract

The aim of this thesis is to define 1D shear wave velocity (Vs) models with depth for crustal structures in the Campanian Plain (southern Italy). This objective is achieved by the non linear inversion with Hedgehog method of Rayleigh wave dispersion data of fundamental mode extracted by FTAN multifilter technique from two station noise cross-correlation (NCF), combined with regional data. The Campanian Plain is a graben like structure mainly filled of Miocene-Pleistocene alluvial, terrigenous and marine sediments deposited on Mesozoic carbonate platform lying at depths greater than 3 km. It is bordered by quiescent volcanoes in the North (Roccamonfina) and in the South (Campi Flegrei and Vesuvio) and seat of past volcanic activity as testified by lava bodies detected in deep drillings. The definition of Vs models in such context is very important to detect high temperatures related to magmatic activities. Vs velocity reductions have been detected beneath Campi Flegrei and the bay of Napoli at ~10 km depth; at a depth of about ~5 km or at 8-10 km depth below Vesuvio; at 6-9 km beneath Roccamonfina; 10-15 km depth, beneath two paths crossing the Campanian Plain. Seismic noise measurements have been performed during the period April 2009 – April 2010, using Kinemetrics Q330 broadband stations equipped with Episensor ES–T FBA (Force balanced Accelerometer). The stations have been set at the borders of the Campanian Plain and noise recordings have been also cross-correlated with those of the INGV-Osservatorio Vesuviano network (OV) at Campi Flegrei. The cross-correlation of a diffused field, recorded at a pair of receivers, is related to the Green function of the medium. According to many researches in the last years, this is true also for one side NCF, coming out from a preferential source-receivers distribution. A total of 19 NCF have been analysed but only 7 average dispersion curves of Rayleigh group velocities have been computed. One-side cross-correlations have resulted being the sea the source of the recorded noise. Stability analysis shows that the computed NCF change daily but preserve monthly, even after some year. Dispersion data from NCF are relative to higher periods than those from earthquake recordings and are in very good agreement at the same periods. Despite some path, only the vertical component of the cross-correlation function has given reliable results. The dispersion data extracted from the NCF and some earthquake signals with T ≥ 3 s, have been combined with regional group and phase dispersion data (7-10 s < T < 150 s) in the Hedgehog inversion, allowing to investigate a maximum depth of 73 km. The main features of the Vs models in the central part of the Campanian Plain are the top of Mesozoic carbonatic sequences at about 4 km and the presence of Vs velocity reductions at 15 km. Such low velocity layer rises to 6-8 km in the south-eastern part of the Plain, towards Vesuvio. The Vs reductions (about 10%) can be explained with the presence of partially melted material, in agreement with the studies at Vesuvio and Campi Flegrei. Moreover, a sharp increase of Vs to 3.85 km/s at about 8-9 km of depth, above the low velocity layer, resembles the models at Roccamonfina and Colli Albani of a cooling magma chamber. These results are in agreement with the noticeable thickness of lava found in deep drillings and can be considered as the seismic signature of a widespread volcanic activity in the Campanian Plain as hypothesized on the basis of volcanological and geochemical studies (Campanian Volcanic Zone).