We performed the statistical analysis of acoustic emission time series in the ultrasonic frequency range, obtained experimentally from laboratory samples subjected to external uniaxial elastic stress. We found a power law scaling behavior in both the acoustic emission amplitude distribution and time correlation function, with exponents very close to those found in fracturing processes occurring at different time and space scales. These facts strongly suggest the existence of a critical dynamics underlying the process, which might be related to the idea of a self-organized critical state based on the energy dissipation through all the length scales.PACS numbers: 62.20.Mk, 05.40.+j, 91.60.Lj Power law behavior in physical phenomena is usually the fingerprint of temporal and spatial critical fluctuations of which well known examples are Ising-like systems, fractal growth phenomena, turbulence, etc. Unlike the usual second order phase transitions, some of the previous examples exhibit a critical behavior without the need to fine tune any control parameter; i.e., the critical state is an attractor of the dynamics. A few years ago Bak, Tang, and Wiesenfeld [1] termed this kind of situation "self-organized criticality" (SOC) and introduced a simple model of a dynamically driven system, inspired by the dynamics of sandpiles, that evolves spontaneously to a stationary critical state. This model is an example of SOC phenomenon in which a system with short range coupling self-stabilizes in a stationary state characterized by avalanches (activity) with power law distribution functions. Hence, the system has no characteristic length (and is therefore self-similar) and is in this sense critical. The SOC concept has been proposed also as a possible mechanism for the generation of the so-called 1/ f noise; however, it has been shown successively [2,3] that the spatialtemporal scaling in the SOC state does not necessarily manifest itself in nontrivial exponents for the power spectrum.Because of the importance of the SOC concept as a possible unifying framework for a wide range of physical phenomena, a lot of work has been devoted to studying these systems through computer simulations, theoretical approaches, and experimental findings [4]. In particular, the SOC framework has been proposed as a possible interpretation for the empirical observation of the energy release in earthquakes [5]. In fact, existence of statistical self-similarity in seismic processes is a well established fact, which has its strongest evidence in the power law behavior of the well known Gutenberg'S [6] and Omori's [7] empirical laws. Power law behavior was observed by Mogi [8] in the distribution of the maximum trace amplitude of audio signals emitted from samples subjected to various forms of stress, in analogy with the Ishimoto Iida 0031-9007/94/73(25)/3423(4)$06.00 empirical relation [9]. Hirata [10] observed self-similarity in the time frequency distribution of aftershock signals due to fracturation of basalt under constant stress. More recently, ...

Abstract. Intense crises of crustal stress appear to cross large regions, and to precede by several months the eventual occurrence of some strong earthquake within them. The phenomenon is not linear, and the stress control reflects some wide scale-size rather than local effects. The stress propagation through the crust can be effectively monitored by means of acoustic emission (AE) techniques (ultrasounds). The correlation is here investigated between crustal stress crises and the total release of seismic energy within some space domain around the AE recording site. Some clear inferences can be envisaged, although a significant diagnosis of the state of the crust within a given region ought to request arrays of simultaneously operated AE recorders. Some case histories are described dealing with the Italian peninsula and with the Cephallonia Island.

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The blades are crucial components of the Virgo super attenuators. The material used for their construction is maraging steel, a low-carbon-content alloy with high ultimate tensile strength and low creep under stress. Young's modulus, the shear modulus, the Poisson ratio and the corresponding elastic energy-loss coefficients have been measured. The measurements have been performed on specimens subjected to the same thermal treatments as those of elements for the Virgo interferometer realized with maraging steel. In addition, anelastic properties of the material subjected to different thermal treatments have been measured. It has been found that, for a maraging-steel structure (one free of plastic deformation), which undergoes an excitation with flexural vibrations, the elastic energy-loss coefficient can vary over a wide range as a function of the thermal treatment of the material and it is dominated by the thermo-elastic effect. The main reason for such a great alteration is supposed to be the dependence of the thermal conductivity on the average sizes of the precipitate particles and their relative separations.

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