The seismic monitoring of a medieval tower in the Circus Maximus (Rome, Italy) during pop and rock concerts

Between May and September 2023 we recorded the environmental seismic vibrations in the archaeological area of the Circus Maximus (Rome, Italy) before, during and after four live concerts of Italian and international musicians: Springsteen, Mengoni, Scott and Pezzali (an audience of around 70,000 people for each concert).

The Circus maximum is an ancient Roman chariot-racing elliptical shaped stadium (621 m x 118 m) whose first construction dates back to 329 BC. 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 the structure of the hemicycle can be seen. It is here that the archaeological area, managed by the Sovrintendenza Capitolina, is separated and protected from the public area by a metal fence. Apart from the Roman ruins, in this area there is also a 3-floors medieval tower - known as Torre della Moletta - placed at about 500 m away from the main stage.

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 metres from the tower. Here, as well as in other points of the archeological area, we were asked to install some seismometers during the concerts, being the archeologists worried about the observed oscillations of the Tower during previous live shows.

For all concerts, we deployed one seismic station on the top and one on the bottom level of the Tower, and some other seismic stations in the surrounding archeological area. The equipment used consisted of six-channels high-resolution digitizers (i.e. with velocimeter and accelerometer sensors) or nodal stations. Among the four concerts, the live show of Travis Scott was the one with the highest amplitude level; the seismic signals were almost entirely clipped during the performance. Many alarmed Roman citizens claimed to have felt an earthquake for the vibrations caused by the concert.

In this study we present the analysis of the environmental vibrations recorded before, during and after the concerts, in terms of velocity and acceleration time series, spectral analysis and variation of the resonance parameters of the Tower. In particular, even if the emitted sound should be focused at frequency beyond 20 Hz, the spectrograms clearly highlight very distinct frequencies related to each song also in the seismic bandwidth (1-5 Hz). This effect can be both due to the transmission of the sound waves to the soil but also to the dancing and jumping of the public during the songs. Before the Scott concert, the Tower had a resonant frequency peaking at about 3 Hz. After the Scott concert, the resonant peak drops by about 0.2 Hz, as confirmed by the subsequent observation for the Pezzali concert.