Safeguard of Cultural Heritage: Evaluation of Ground Motion at Palazzo Te (Mantova, Italy)

Built on the first half of the XVI century, under Federico II Gonzaga, Duke of Mantova, Palazzo Te is one of the most relevant Italian monuments and it belongs to the UNESCO World Heritage List; the Camera dei Giganti (Chamber of the Giants) hosts a famous XVI century Giulio Romano fresco. Palazzo Te raises concerns in particular for the statics of the Fruttiere external wall and for some fissures that cross the above mentioned fresco. Therefore a monitoring survey has been undertaken to detect and qualify the vibrations caused by earthquakes and anthropic activity. Palazzo Te is situated in a low seismicity area, nevertheless even weak earthquakes could result in significant damage due to the poor soil conditions. A seismic station deployed inside the Palazzo Te recorded, on July 17, 2011, 2 cm/sec2 PGA for the ML 4.7 earthquake located at approx 51 km distance: some light damage has been observed. As far as vibrations due to anthropic origin, the station has evidenced that the major threat is due to the trains at the nearby railway: inside the Palazzo Te PGA reached 5 cm/sec2. The seismic survey led to the following considerations: 1. even low magnitude earthquakes can be a threat, given the intrinsic vulnerability of some parts of the Palazzo Te; 2. the main and possibly unique significant dangerous vibrations of anthropic origin come from the trains running nearby; 3. the response spectra evidenced that the earthquake had a consistent energy content till T₀=2 seconds, while the spectrum of the trains decay abruptly at around 10 Hz. In contrast the train response spectrum is significantly higher in the high frequency range. In conclusion, despite the relatively low hazard, seismic risk is still relevant, because of the poor soil conditions coupled with the high vulnerability. The train vibrations are not a threat for the Palazzo Te structure but can cause some concerns to the frescoes.