**1. Introduction**

Fracture precursors in metallic [1,2] and quasi-brittle materials like rocks [3,4], concrete [5–9], and masonry [10,11] can be experimentally investigated focusing on the statistical properties of an acoustic emission (AE) time series from growing micro-fractures, where the discovery of underlying scaling laws suggests a description of fracture as a critical phenomenon [12–21]. Within this context, finding fracture precursors means identifying critical scaling exponents and early indicators of the approach to a critical state [22].

The relevance of the AE applications in civil engineering for structural health monitoring is widely recognized. In this regard, it is increasingly necessary to give the highest priority to seismic risk mitigation in large portions of the Italian territory. Minor and light earthquakes can drive invisible damage processes in buildings and monuments, which eventually result in catastrophic collapses during stronger earthquakes. It is worth noting that AE can be exploited as a diagnostic tool in geophysics as well, since recent experimental evidences and theoretical studies support the hypothesis that increased AE and electromagnetic activities may be signature of crustal stresses redistribution in a large zone during the preparation of a seismic event [23–34]. Laboratory studies have been motivated by the need to provide tools for the earthquake prediction [35].

Therefore, the AE structural monitoring might potentially provide twofold information: one concerning the structural damage and the other concerning widespread micro-seismic activity, propagating across the ground-building foundation interface, for which the building foundation represents a sort of extended underground probe [10,11,21,23,24,30,33].

The presented research study was initially motivated by the debate about the alleged incompatibility between heavy vehicle traffic and Bologna's historical center, which concerns also the structural stability of two medieval towers, the Garisenda and the Asinelli (the taller) [36]. As reported in a previous publication [30], the influence of environmental phenomena on the AE activity in the Asinelli tower, such as the flux of surrounding vehicle traffic, unusual anthropogenic activities, and wind action, has been excluded. Actually, the nearby seismicity apparently has some influence on the AE activity detected in a masonry wall of the tower [30]. Here, the focus is on the time correlation observed between the largest AE cluster and the strongest (magnitude 4.1) local earthquake, with an epicenter 100 km far from the monitoring site.
