Analyses of ambient noise recorded in the Colfiorito Basin, Central Italy

The amplification of earthquake motion by shallow sediments is an important parameter in determining seismic hazard. Furthermore, in presence of a complex subsurface structure, variation of ground motion can be significant over very short distance. In this work we analyse the ambient noise recorded in the Colfiorito Basin during two experiment carried out in May and July 2002. The Colfiorito Basin is a tectonic depression in the Southern part of the Northern Apennine Chain filled by Quaternary alluvial deposits. The structure of the basin results to be very irregular, with narrow and deep sags, the deepest one (up to 180 and 150 m) are located at the NW corner of the basin. A relatively flat and shallower (60-70 m) area occupies the centre of the basin. Previous studies performed in the Colfiorito basin have shown the role of the basin structure on the propagation of seismic waves. Noise measurements were carried out in May 2002 along four profiles, investigating 45 different sites characterized by different sediment thickness over the basement. A reference-site was located on the outcropping rock-basement, at the South-Eastern border of the basin, recording continuously for all the time of the experiment. In July 2002 a second experiment was carried out in the Colfiorito Basin, installing 4 arrays in the sites. This work is aimed at analysing the predominant frequency variations in the ambient noise recorded in the Colfiorito Basin, which could be correlated to the basin structure, through application of the Nakamura’s method. The correlation method applied to data collected by the B array allows to study the dispersive properties of the noise wavefield, from which a 1-D velocity model is derived.