VS of the uppermost crust structure of the Campi Flegrei caldera (southern Italy) from ambient noise Rayleigh wave analysis

Abstract

Shear wave velocities (VS) are defined in the uppermost 1–2km of the Campi Flegrei caldera through the non-linear inversion of the group velocity dispersion curves of fundamental-mode Rayleigh waves extracted from ambient noise cross-correlations between two receivers. Noise recordings, three months long, at 12 seismic stations are cross-correlated between all couples of stations. The experiment provided successful results along 54 paths (inter-stations distance), of which 27 sampled a depth > 1km.VS contour lines are drawn from 0.06km b.s.l. to 1km depth b.s.l. and show difference between the offshore (gulf of Pozzuoli and coastline) and the onshore areas. At 0.06km b.s.l., the gulf of Pozzuoli and the coastline are characterized by VS of 0.3–0.5km/s and of 0.5–0.7km/s, respectively. Such velocities are typical of Neapolitan pyroclastic soils and fractured or altered tuffs. The inland shows VS in the range 0.7–0.9km/s, typical of Neapolitan compact tuffs. Velocities increase with depth and, at 1km depth b.s.l., velocities lower than 1.5km/s are still present in the gulf and along the coastline while velocities higher than 1.9km/s characterize the eastern sector (grossly coincident with the Neapolitan Yellow Tuff caldera rim), the S. Vito plain and the area between Solfatara and SW of Astroni.Such features are much more evident along two cross-sections drawn in the offshore and onshore sectors by integrating our VS models with literature data. Our models join previous noise cross-correlation studies at greater scale at depths of 0.7–0.8km, hence the picture of the Campi Flegrei caldera is shown up to a depth of 15km. VS of about 1.7km/s, corresponding to compression velocities (VP) of about 3km/s (computed by using the VP/VS ratio resulted in the inversion), are found at depths of 1.1km, in the centre of the gulf of Pozzuoli, and at a depth of about 0.7km b.s.l. onshore. An increment of VS velocity (~1.9–2.0km/s) is locally observed onshore which might be attributed to a layer of tuffs and tuffites interbedded with thin lava beds, according to the correlation of VS with stratigraphies in the deep drillings of S. Vito.