1 Frequency signal and natural time analyses from acoustic emission monitoring of an arched structure in the Racconigi Castle Gianni Niccolini 1 , Amedeo Manuello 1 , Elena Marchis 2 , Alberto Carpinteri 1 5 1 Politecnico di Torino, Department of Structural, Geotechnical and Building Engineering, Corso Duca degli Abruzzi 24, 10129 Torino, Italy 10 2 Politecnico di Torino, Department of Architecture and Design, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Correspondence to: Gianni Niccolini (gianni.niccolini@polito.it) Abstract. The stability of an arch as a structural element in the thermal bath of King Carlo Alberto in the Royal Castle of 15 Racconigi (on the UNESCO World Heritage List since 1997) was assessed by the Acoustic Emission (AE) monitoring technique with application of classical inversion methods to recorded AE data. First, damage source location by means of triangulation techniques and signal frequency analysis were carried out. Then, the recently introduced method of natural time analysis was preliminarily applied to the AE time series in order to reveal possible entrance point to a critical state of the monitored structural element. Finally, possible influence of the local seismic and micro-seismic activity on the stability of 20 the monitored structure was investigated. The criterion to select relevant earthquakes was based on the estimation of the size of earthquake preparation zones. The presented results suggest the use of AE technique as a tool for detecting both ongoing structural damage processes and micro-seismic activity during preparation stages of seismic events. 1 Introduction 25 Fracture in heterogeneous materials is a complex phenomenon which involves a wide range of time, space and magnitude scales, from microcracking to earthquake ruptures, including structural failures (Omori, 1894; Richter, 1958; Kanamori and Anderson, 1975; Aki, 1981). Thus, acoustic emission (AE) monitoring during loading experiments give an insight into the