Space-time correlation of earthquakes and acoustic emission monitoring of historical constructions

This PhD thesis is primarily concerned with the evaluation of seismicity in regional areas in order to analyse the correlation between earthquakes and Acoustic Emissions (AE) detected by monitoring historical constructions. Acoustic emissions transmitted in materials and earthquakes in the Earth's crust are similar phenomena. Despite the fact that they take place at different scales, in both cases they involve a release of elastic energy from a source located in a medium: micro cracks in materials and earthquakes' hypocenters in the Earth's crust. Considering the similarity between the two phenomena and also that earthquakes may have precursors, i.e. phenomena of change in the Earth's physical conditions that take place prior to an earthquake, it was observed that acoustic emission can anticipate the occurrence of certain earthquakes. This is due to micro-cracking of the Earth's crust that anticipates the more properly catastrophic phenomenon. The research into these precursors was carried out through the analysis of correlation between acoustic emissions and seismic events. The monitoring sites, where this experimentation has been applied, are two important historical constructions of the Italian cultural heritage: the "Sacred Mountain of Varallo" in Piedmont region and the "Asinelli Tower" in Bologna, Emilia-Romagna region. The proposed statistical method of analysis is based on the Grassberger-Procaccia (GP) correlation integral. It gives the cumulative probability occurrence of couples of events, AE and seismic events, that occur in the same time windows nearby the monitored site. The analyses conducted for the historical constructions allowed us firstly to confirm the precursory behaviour of acoustic emissions and secondly to verify that between 24 hours and 10 minutes before the main shocks the highest density of acoustic emissions appears. In the second part of the PhD thesis the space correlation between only the earthquakes was studied, in particular those starting from the significant earthquake of the 6th April 2009, with its epicentre close to L'Aquila (Abruzzi region) in Italy, and those starting from the catastrophic and recent earthquake of the 24th August 2016, with its epicentre close to Amatrice (Lazio region), also in Italy. By using the b-value of the Gutenberg-Richter (GR) Law and the Gutenberg-Procaccia (GP) algorithm, the correlation between earthquakes in specific areas has been performed. The aim of the analyses was to recognize the seismic events itself as possible statistic precursory of significant earthquakes. Moreover, starting from the (GP) correlation integral the fractal distribution of events in space, Ds, was evaluated, finding the fault lines activated by the analysed seismic sequence. The analyses conducted for the L'Aquila seismic sequence allowed us to recognize precursory events in the months before the earthquake of April 2009; in fact in the months before April there was a continuous decrease of the b-value trend and of the fractal dimension of seismic events in space, Ds. A relation between the fractal dimension in space of the active fault system during seismic activity, Ds, and the b-value can be established considering the concepts of geometrical self-similarity. The applicability of a fractal relation implies that the seismicity is scale invariant over the range of applicability of the relation, therefore the Gutenberg-Richter relation for earthquakes defining the b-value is a fractal relation with Ds=2b. The analyses, b-value analysis and correlation integral, carried out separately allowed us to verify that Ds≅2b in the month of April 2009. In the case of the Amatrice seismic sequence, a sharp variation in the trend of b-value between July 2016 and August 2016 was recognized; the b-value changes quickly over a short time span. Therefore, in this case, a not so clear precursory behaviour can be noticed. Also in this case, the b-value analysis and the correlation integral, carried out separately, allowed us to verify that Ds≅2b in the month of August 2016. In conclusion, this PhD thesis allows us to evaluate whether there are statistical precursors of significant earthquakes by means of the b-value and the correlation analysis between seismic events, or between seismic events and acoustic emissions by monitoring of the historical constructions. This work represents a first step for the realization of a monitoring and analysis network to assess the behaviour of seismic sequences in order to understand if there are possible precursory events of significant earthquakes, as in the case of L'Aquila, and also to evaluate acoustic emission events as a precursory phenomenon useful for earthquake prediction. The improvement of seismic monitoring, together with structural consolidation and reinforcement, could lead to significant benefits for the mitigation of the devastating effects of earthquakes.