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Crustal stress can be monitored by acoustic emissions (AE, ultrasound), which give an indication of whether a physical system is subject to stress, either of tectonic or endogenous origin. AE intensity critically depends on the damping of the signal; however, AE signals are clear indicators of the fatigue state of the crustal structures constituting the AE source. This aspect can be studied by fractal analysis of AE time series; these are, however, not suited for earthquake forecasting, as they only denote a changing state involving large lithospheric volumes. Several case histories from Italy show that an increased high-frequency AE activity (200 kHz) occurs approximately seven to eight months in advance of large earthquakes that affect areas of a few hundred kilometres radius, and an increased low-frequency AE activity (at 25 kHz) is observed several weeks in advance. Low-frequency AE also correlate with soil exhalation (water-well chemistry) and CH4, whereas fractal analysis of AE signals recorded close to a ‘future’ epicentral area gives a clear indication of the evolution of the system from about two months before the mainshock. This suggests that systematic monitoring of crustal stress variations may be used for assessing the time evolution of seismic activity.

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