**Alberto Carpinteri, Gianni Niccolini and Giuseppe Lacidogna \***

Department of Structural, Geotechnical and Building Engineering, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy; alberto.carpinteri@polito.it (A.C.); giuseppe.lacidogna@polito.it (G.N.)

**\*** Correspondence: giuseppe.lacidogna@polito.it; Tel.: +39-011-090-4910

Received: 5 May 2018; Accepted: 14 June 2018; Published: 21 June 2018

**Abstract:** The existence of ongoing damage processes in a masonry wall of the Asinelli Tower in Bologna have been investigated by the acoustic emission (AE) technique. A time correlation between the AE activity in the monitored structural element and the nearby earthquakes has been observed. In particular, the largest cluster of AE signals has been recorded within a few hours after the main shock (4.1 magnitude) occurrence. The presented findings suggest that aging and deterioration of the monitored structural element significantly depend on the action of light earthquakes, even at considerable distance. Trends of two evolutionary parameters, the *b*-value and the natural time variance *κ*1, have been derived from the AE time series in order to identify the approach of the monitored structural element to a "critical state" in relation to the earthquake occurrence.

**Keywords:** structural health monitoring; acoustic emission; time series analysis; *b*-value; natural time; critical phenomena
