Assessing energy balance via seismic and mechanical observations in high temperature induced crack damage in marbles.

Assessing the rock physical and mechanical behavior under different temperatures has become of utmost importance. It is well known that thermal stresses induce significant crack damage in rocks due to thermal expansion or phase transformation and volume changes. Quantifying and forecasting the evolution of rock physical and mechanical parameters with temperature is thus crucial for evaluating rock integrity in many applications such as geothermal fields, nuclear waste storage, wildfire or volcanic processes. In marbles the degree of previous exposure to temperature and the chemical composition (i.e. calcite vs dolomite) plays a key role for controlling the mechanical evolution under temperature. Moreover separating out the energy contribution provided by anelastic processes driving crack damage and elastic reversible deformation under increasing temperature remains an open challenge. With these aims, three sample sets of marbles with different contents of calcite and dolomite from two Brazilian quarries were tested under different temperature conditions (from room temperature up to 600 °C). A marked increase of thermal cracking was observed after 400 °C, accompanying mass loss up to 1% and porosity increase. Moreover, a significant drop in seismic wave velocities, uniaxial compressive strength and electrical resistivity, in wet conditions, was also detected. Spectral behavior from seismic traces and energy dissipation from stress-strain curves were analyzed. A dominance of the dissipated energy compared to the elastic one was observed and related to the generation of new fracturing surfaces. This hypothesis was supported by the spectral behavior showing multiple scattering effects in the high frequency components, with an increase in attenuation. The results suggested that the percentage of dolomite has a high influence on the mechanical behavior even at low temperature, mirroring the prevalence of brittle processes in dolomitic marbles. This study represents a comprehensive benchmark for the study of effect of temperature on rocks because of its multidisciplinary and multimethod approach and the demonstrated sensitivity to subtle textural changes. Moreover, it provides a reliable tool for crack damage analysis at each thermal stress.

Acoustic Emissions in Rock Deformation and Failure: New Insights from Q-Statistical Analysis.

We propose a new statistical analysis of the Acoustic Emissions (AE) produced in a series of triaxial deformation experiments leading to fractures and failure of two different rocks, namely, Darley Dale Sandstone (DDS) and AG Granite (AG). By means of q-statistical formalism, we are able to characterize the pre-failure processes in both types of rocks. In particular, we study AE inter-event time and AE inter-event distance distributions. Both of them can be reproduced with q-exponential curves, showing universal features that are observed here for the first time and could be important in order to understand more in detail the dynamics of rock fractures.

Genesis and mechanisms controlling tornillo seismo-volcanic events in volcanic areas.

Volcanic activity is often preceded or accompanied by different types of seismo-volcanic signals. Among these signals, the so-called tornillo (Spanish for "screw") events are considered to belong to a unique class of volcano-seismicity characterised by a long-duration coda, amplitude modulation and high-quality factor. These data constitute important evidence for the gas fraction inside magmatic fluids. However, the mechanism behind this unique signal remains not fully understood. Here we report new laboratory evidence showing that two different processes have either scale-invariant or scale-dependent effects in generating tornillo-like events. These processes are respectively the gas pressure gradient, which triggers the event and regulates the slow decaying coda, and the fluid resonance into small scale structures which, in turn, control the frequency content of the signal. Considering that the gas pressure gradient is proportional to the fluid flow, these new findings, as applied to volcanoes, provide new information to better quantify both gas rate and volume, and the dimension of the resonator.

The brittle-ductile transition in active volcanoes.

Contrasting deformation mechanisms precede volcanic eruptions and control precursory signals. Density increase and high uplifts consistent with magma intrusion and pressurization are in contrast with dilatant responses and reduced surface uplifts observed before eruptions. We investigate the impact that the rheology of rocks constituting the volcanic edifice has on the deformation mechanisms preceding eruptions. We propose a model for the pressure and temperature dependent brittle-ductile transition through which we build a strength profile of the shallow crust in two idealized volcanic settings (igneous and sedimentary basement). We have performed finite element analyses in coupled thermo-hydro-mechanical conditions to investigate the influence of static diking on the local brittle-ductile transition. Our results show that in active volcanoes: (i) dilatancy is an appropriate indicator for the brittle-ductile transition; (ii) the predicted depth of the brittle-ductile transition agrees with the observed attenuated seismicity; (iii) seismicity associated with diking is likely to be affected by ductile deformation mode caused by the local temperature increase; (iv) if failure occurs within the edifice, it is likely to be brittle-dilatant with strength and stiffness reduction that blocks stress transfers within the volcanic edifice, ultimately damping surface uplifts.

Laboratory simulation of volcano seismicity.

The physical processes generating seismicity within volcanic edifices are highly complex and not fully understood. We report results from a laboratory experiment in which basalt from Mount Etna volcano (Italy) was deformed and fractured. The experiment was monitored with an array of transducers around the sample to permit full-waveform capture, location, and analysis of microseismic events. Rapid post-failure decompression of the water-filled pore volume and damage zone triggered many low-frequency events, analogous to volcanic long-period seismicity. The low frequencies were associated with pore fluid decompression and were located in the damage zone in the fractured sample; these events exhibited a weak component of shear (double-couple) slip, consistent with fluid-driven events occurring beneath active volcanoes.

Power-law time distribution of large earthquakes.

We study the statistical properties of time distribution of seismicity in California by means of a new method of analysis, the diffusion entropy. We find that the distribution of time intervals between a large earthquake (the main shock of a given seismic sequence) and the next one does not obey Poisson statistics, as assumed by the current models. We prove that this distribution is an inverse power law with an exponent mu=2.06+/-0.01. We propose the long-range model, reproducing the main properties of the diffusion entropy and describing the seismic triggering mechanisms induced by large earthquakes.