13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- monitoring period (July-September): at 60 km there was a tendency for AE to anticipate earthquakes even though they are effectively correlated only in the long term; at 80 km they are precursive signals only in the short term; at 100 km we see immediately the reversal outlined above, from precursor behavior to aftershock. In any case, it is worth of the further investigations distinguishing environmental contributions to AE activity on the Chapel XVII due to crustal trembling (external source) from contributions due to structural damage (inner source). To investigate the nature of these results, true progress can be realistically achieved by discriminating the signals recorded on the basis of a predetermined threshold both in frequency and amplitude, consistently with the physical nature of Acoustic Emissions detected by the sensors. Looking at the USAM data stored, a good choice to discriminate the signals nature, is setting a threshold frequency −for example 30 kHz, which divides the field VLF (Very Low Frequency) from the field LF (Low Frequency)− and a purposeful threshold signal amplitude. It seems reasonable even from a theoretical point of view, that low frequencies allow the diffusion of the elastic waves in the masonry bulk, both intact or damaged. On the other side, low amplitudes are reasonably related to the fact that an event captured by AE sensors on the monitored structure may have originated from a source physically distant from the site monitored (seismic hypocenter) and therefore is subject to the laws of amplitude damping. 6. Conclusions Besides the canonical use in Non-destructive Tests, the heuristic potential of AE monitoring of civil structures for earthquakes prediction appears very intriguing. Starting from the assumption that any structure should not be regarded as separated from its environoment, a method of correlating AE activity on the Renaissance Complex of the Sacred Mountain of Varallo subjected to a long-term monitoring with regional seismicity is investigated. Two qualitatively very similar phenomena such as Acoustic Emission and earthquakes become two aspects of a unique phenomenon which looks self-similar. Furthermore, in this work by applying the modified Grassberger-Procaccia correlation algorithm −with the aim of explaining the correlation between regional seismicity and Acoustic Emission emerging from the Chapel XVII of the Sacred Mountain of Varallo− it is observed that the structure is a good sensitive receptors for earthquakes occurring within a radius of about 60 km. In any case, it is worth of the further investigations distinguishing environmental contributions to AE activity on the Chapel XVII due to crustal trembling (external source) from contributions due to structural damage (inner source). Acknowledgements The financial support provided by the Piedmont Region (Italy) to the Project ‘‘Preservation, Safeguard and Valorisation of Masonry Decorations in the Architectural Historical Heritage of Piedmont” (RE-FRESCOS) is gratefully acknowledged. The assistance of student Roberto Sodano in the processing of the data is gratefully acknowledged.
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