VS models in the historical centre of Naples (Southern Italy) from noise cross-correlation

Abstract

Ambient noise measurements have been performed in the historical centre of Naples (Southern Italy) along 40 paths (inter-receiver distances) with a length ranging from ~300 m to ~1550 m, for recording time variable from few hours to several months.Synchronous noise recordings at two receivers, 1-bit normalized, have been cross-correlated in order to estimate the empirical Green's function (surface wave train) of the medium between them. The fundamental-mode Raleigh waves have been extracted at periods ranging from 0.15 s to 1 s by using the frequency-time analysis technique. Average group velocity dispersion curves with error bar have been evaluated along 28 paths and inverted with a non-linear approach to get VS profiles extending to depths of ~50–300 m.Comparison of VS with some deep drillings and two down-hole measurements lets to argue important findings. First of all there is a good agreement with shallow (about 35 m) point down-hole measurements. The historical centre of Naples results to be characterized by a cover of man-made ground and pyroclastics of recent activity with thickness of 15–30 m and VS lower than 400 m/s. The products of the Neapolitan Yellow Tuff eruption have a thickness of about 80 m and are characterized by VS of 500–700 m/s in the shallower, altered and fractured part and of 700–900 m/s in the deeper, compact part. Below them, separated by a thin layer of the Campanian Ignimbrite that cannot be resolved by the data, ancient tuffs or marine sands are present which are locally responsible of velocity decrease. VS of 1100–1300 m/s, ascribable to intercalations of volcanic and sedimentary (marls, conglomerates and marly sands) rocks, are detected at ~200 m depth in the ancient part of the historical centre, rising to ~100 m depth, in the north-east of it.This study also gives an important contribution to microzoning studies. The top of the lithoid Neapolitan Yellow Tuff cannot be considered as the top of the seismic bedrock, being characterized by 500 m/s < VS < 800 m/s. The seismic bedrock with VS ≥ 800 m/s is deeper, sometimes at depths >100 m.