13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- events such as earthquakes. Furthermore, the limited ductility of the masonry, combined with the large size of this type of construction, provides a structural behavior rather fragile [6]. For these reasons, the damage evolution in time can be effectively evaluated by means of the Acoustic Emission technique [7,8,9,10]. Moreover, the statistical distribution of earthquakes shows a complicated behavior space–time, that reflects the complexities of the Earth's crust. Despite this complexity, there is a scaling law universally valid: the statistical distributions of frequency-magnitude of an earthquake provided by the Gutenberg-Richter (GR) law [11]. On the other hand, AE in materials and earthquakes in the crust are very similar and correlated in time, even if they are phenomena that occur at different scales [12]. In both cases there is a release of elastic energy from a source located in the medium: respectively the opening of micro cracks and the seismic hypocenter [13]. This similarity suggests an interpretation in which the seismic events and the AE events can be related in space and time. In this view is therefore possible to search for a correlation between the parameters related to AE and the regional seismicity. As a matter of fact, this approach can be used to identify the warning signals that precede a catastrophic event for a structure since, in many cases, these warning signals can be detected well in advance with respect to the time at which the undesired event will occur [14,15]. Most earthquakes have precursors, ie phenomena that in the short or long term change their activity before the earthquake. In the past, many precursors have been proposed but it is still not clear what is really reliable. Surely any operative scheme of prediction must be based on a combination of more clues. Recently, major efforts in the field of earthquake prediction have focused on the fluctuations of the physical parameters of the crustal rocks of the seismically active continental areas, and on regular intervals in the space-time distribution of earthquakes [16]. The variation in the rate of the regional seismicity is considered as a precursor in the long term. A region which had a small earthquake activity for a remarkable number of years is called "seismic gap". The "seismic gaps" are considered as potential sites for major earthquakes. On the other hand, the increasing pressure in the rock surface in the region of the epicenter produces numerous cracks before the final collapse and, as a result, it causes changes in the properties of rocks. Therefore, the drop in speed of seismic waves caused by the expansion of the rock becomes a significant precursor. Other precursors linked to the expansion of the rocks and the opening of cracks are the crustal tilt and elevation changes, the decrease of the electrical resistivity of the rocks, and the release of radon gas in the atmosphere, which requires small pores to propagate. As the process of damage develops, the water diffuses from the surrounding rocks in pores and micro cracks of increasing size, which in the meantime are forming. The moment the water fills the cracks, the speed of seismic waves grows, the soil lifting stops, the emission of radon from the new cracks is relieved, and the electrical resistivity decreases. The next stage is the beginning of the earthquake, which is immediately followed by several aftershocks in the surrounding area [7,16]. When a crack in the Earth's crust increases, the corresponding AE show progressively lower frequencies, up to abandon eventually the ultrasound field reaching the sonic range: this represents the well known phenomenon of seismic roar. Thus, AE techniques have the potential for effectively monitoring the spread of tensions through the earth's crust. In fact, Italian researchers collected continue AE recordings for many years in the Gran Sasso massif. It was noted a peak of the AE about 400 km away from the epicenter, many months before the occurrence of the earthquake in
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