Ambient noise recordings in the Circo Massimo and Vallis Murciae Areas: main results compared with the dynamic response of the XII cen. A.D. Torre della Moletta

In this work we present the results of ambient noise recordings carried out in the Circo Massimo area, within the central archaeological district of Rome. The study area was part of a Quaternary alluvial valley, the Vallis Murciae, a secondary feeder of the Tiber River. Ninetytwo recordings of ambient noise were performed in 2013 on a 20 meters regularly-spaced grid. By applying the Horizontal-to-Vertical Spectral Ratio (HVSR) technique, the first-mode resonance frequency (f0) of the Holocene-Upper Pleistocene alluvial infilling was determined, ranging between 1.2 and 1.8 Hz. The f0 local spatial variation was analyzed and a trend of f0 increase moving south-eastwards was confirmed by a noise recording taken at the eastern termination of the Vallis Murciae (Terme di Caracalla site). To characterize also the dynamic response of local monumental buildings, vibration recordings were done at the Torre della Moletta, a stony-tower built at the south-eastern limit of the Circo Massimo area during the Middle Age (XII cen. A.D.). The vibration pattern features a Hi/Ho peak at a frequency of about 3 Hz, that is close to the ground resonant frequency. This evidence reveals potentially destructive site effects for the Torre della Moletta building during seismic events and set the basis for further studies on seismic hazard assessment in the Rome downtown area.