Observations of ambient vibrations at the archaeological site of Circus Maximus (Rome, Italy) induced by the rock and pop concerts held in the adjacent Circus Maximus Arena in the summer of 2023.

In order to evaluate the response of the archaeological structures to the dynamic load induced by concerts, a group of INGV researchers involved in the ST-Predict Pianeta Dinamico project, recorded the environmental vibrations in the archaeological area of the Circus Maximus (Rome, Italy) before, during, and after the concerts of several famous artists between May and September 2023: Bruce Springsteen, Marco Mengoni, Travis Scott and Max Pezzali (70000 tickets sold, i.e. sold out for each concert). During the Scott performance, many alarmed Roman citizens claimed to have felt an earthquake, presumably due to the vibrations caused by the concert. The Circus Maximus is an ancient Roman chariot-racing stadium whose first construction dates back to 329 BC, but it was Julius Caesar who built the first masonry seat stadium starting in 46 BC. With a length of 621 m and a width of 118 m, the Circus had (and still has) a narrow elliptical shape with a NW-SE orientation that could accommodate an audience of 150,000. The Circus filled the space of the ancient Valle Murcia, which ran between Colle Palatino to the north and Colle Aventino to the south. At present, the main structure of the stadium is buried under a green lawn, which is open to the public, and only on the south-eastern side can the structure of the hemicycle be seen. It is here that the archaeological area, managed by the Superintendence of Cultural Heritage of Rome, is separated and protected from the public access by a metal fence. In the centre of the archaeological site there is a tower called the "Torre della Moletta", which rises to three floors. It was built after the fall of the Western Roman Empire and restored to its current form by the Frangipane family in 1145 AD. The concert stage is located at the north-western tip of the Circus Maximum area, facing south-east towards the archaeological area, at a distance of less than 500 meters from the tower. Seismic stations were only installed inside the archaeological area. For the purpose of structural monitoring, we selected two sites in the Torre della Moletta, one on the top level and one on the bottom level, and two sites in the hemicycle structures, one located towards the Palatino hill and the other towards the Aventino hill. These four sites were occupied during each concert. Other "free-field" sites within the archaeological area were only occupied during the Mengoni and Pezzali concerts.  We also considered two stations belonging to the Italian strong motion network operated by the Presidency of the Council of Ministers - Department of Civil Protection (https://doi.org/10.7914/SN/IT), located at a distance of about 1 km from the Circus Maximus stage. We present an analysis of the environmental vibrations in terms of time series of acceleration and velocity, as well as a spectral analysis (Fourier amplitude spectra and spectrogram, HVSNR) to study the dynamic behaviour of the tower before, during, and after the concerts and to extract the resonance parameters.