Ultrasound monitoring of applied forcing, material ageing, and catastrophic yield of crustal structures

Abstract

A new kind of data analysis is discussed &ndash; and a few case histories of actual application are presented &ndash; concerning the physical information attainable by acoustic emission (<i>AE</i>) records in geodynamically active or volcanic areas. The previous analyses of such same kind of observations were reported in several papers appeared in the last few years, and here briefly recalled. They are concerned with the inference of the forcing ("<i>F</i>") acting on the physical system, and on the ageing ("<i>T</i>") or fatigue of its "solid" structures. The new analysis here discussed deals with the distinction between a state of applied stress ("hammer regime"), compared to state of "recovery regime" of the system while it seeks a new equilibrium state after having been perturbed. For instance, in the case of a seismic event &ndash; and according to some kind of almost intuitive argument &ndash; the "hammer regime" is the phenomenon leading to the main shock, while the "recovery regime" deals with the well known aftershocks. Such same intuitive inference, however, can be investigated by a much more formal algorithm, aimed at envisaging the minor changes of the behaviour of the system, during its history and during its present dynamic evolution. As a demonstrative application, detailed consideration is given of <i>AE</i> records &ndash; each one lasting for a few years &ndash; collected on the Italian peninsula vs. records collected on the Kefallinìa Island (western Greece). Such two areas are well known being characterised by some great comparative difference in their respective tectonic setting. When considering planetary scale phenomena, they appear comparatively very close to each other. Hence, they are likely being presumably affected by similar large-scale external actions, although they ought to be expected to respond in some completely different way. Such facts are clearly manifested by some substantially different <i>AE</i> responses of the local crustal structures. However, a full understanding of such entire set of geodynamic and tectonic details ought to require several year data series of <i>AE</i> records, and/or (maybe) also simultaneous <i>AE</i> records collected within some suitable array of <i>AE</i> stations. Such understanding ought to permit the inference of the spatial features of the crustal stress propagation &ndash; including its diagnosis and "forecasting" &ndash; in addition to the temporal diagnosis and "prevision" that can be attained by isolated point-like <i>AE</i> recording stations. Additional analyses are in progress.