Correlation between Acoustic Emission and Seismicity in the Sacred Mountain of Varallo Renaissance Complex in Italy

In this work, we examine an application of AE technique for a probabilistic prediction of the time and place of earthquakes, in order to preserve the inestimable Italian Renaissance Architectural Complex named “The Sacred Mountain of Varallo”. This historical site is composed of 45 Chapels, some of which are isolated, while others are part of monumental groups. The Chapels contain over 800 life-size wooden and multicoloured terracotta statues, which represent the Life, Passion and Death of Christ. The site is considered the most notable example in the group of Sacred Mounts of Piedmont, a complex that has been included in the UNESCO World Heritage List since 2003. The structure of the Chapel XVII of this Renaissance Complex is at risk for its poor structural health and on account of the intensity of the stresses it is subjected to, due to the level of regional seismicity. For the reliability and safety of this masterpiece of cultural heritage, a life-time assessment should take into account the evolution of damage phenomena. Therefore a continuous AE monitoring is performed to assess the structural behavior of the Chapel. Earthquakes always affect structural stability: the amount of energy released in a seismic event can cause catastrophic damage in a wide variety of ways. Beside this well-known point of view, in this paper it is claimed that the structures of the “Sacred Mountain of Varallo” behave as sensitive earthquake receptors, since the stress propagation through the Earth’s crust, which can be considered as an earthquake precursor, can be effectively monitored by means of the AE technique. In some works, a burst of AE activity is considered as representative of a large amount of stress which is crossing some large crustal area surrounding the AE recording site. Such AE crises are observed some time in advance compared to the earthquake, leading to consider AE records like earthquake precursors. During the monitoring period, a correlation between peaks of AE activity in the masonry of the “Sacred Mountain of Varallo” and regional seismicity is found. These two classes of phenomena, AE in materials and earthquakes in Earth’s crust, though they take place on very different scales, are very similar due to the release of elastic energy from localized sources inside the medium: opening microcracks and hypocenters of earthquakes.