Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of Pwave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of doublecouple events indicating shear, suggests shear cracks linking previously formed tensile cracks.

[1] We observe the nucleation phase of in-plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi-static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi-static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi-static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.

We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow basalt from Etna (Italy) and columnar basalt from Seljadur (Iceland). Measurements were made in a servo-controlled steady-state-flow permeameter at effective pressures from 5-80 MPa, during both increasing and decreasing pressure cycles. Selected samples were thermally stressed at temperatures up to 900 1C to induce thermal crack damage. Acoustic emission output was recorded throughout each thermal stressing experiment.At low pressure (0-10 MPa), the P-wave velocity of the columnar Seljadur basalt was 5.4 km/s, while for the Etnean lava flow basalt it was only 3.0-3.5 km/s. On increasing the pressure to 80 MPa, the velocity of Etnean basalt increased by 45%-60%, whereas that of Seljadur basalt increased by less than 2%. Furthermore, the velocity of Seljadur basalt thermally stressed to 900 1C fell by about 2.0 km/s, whereas the decrease for Etnean basalt was negligible. A similar pattern was observed in the permeability data. Permeability of Etnean basalt fell from about 7.5 Â 10 À16 m 2 to about 1.5 Â 10 À16 m 2 over the pressure range 5-80 MPa, while that for Seljadur basalt varied little from its initial low value of 9 Â 10 À21 m 2 . Again, thermal stressing significantly increased the permeability of Seljadur basalt, whilst having a negligible effect on the Etnean basalt. These results clearly indicate that the Etnean basalt contains a much higher level of crack damage than the Seljadur basalt, and hence can explain the low velocities (3-4 km/s) generally inferred from seismic tomography for the Mt. Etna volcanic edifice. r

We perform a new analysis on the dissipative Olami-Feder-Christensen model on a small world topology considering avalanche size differences. We show that when criticality appears the Probability Density Functions (PDFs) for the avalanche size differences at different times have fat tails with a q-Gaussian shape. This behaviour does not depend on the time interval adopted and is found also when considering energy differences between real earthquakes. Such a result can be analytically understood if the sizes (released energies) of the avalanches (earthquakes) have no correlations. Our findings support the hypothesis that a self-organized criticality mechanism with long-range interactions is at the origin of seismic events and indicate that it is not possible to predict the magnitude of the next earthquake knowing those of the previous ones.PACS numbers: 05.65.+b, 91.30.Px, 05.45.Tp In the last years there has been an intense debate on earthquake predictability [1] and a great effort in studying earthquake triggering and interaction [2][3][4][5]. Along these lines the possible application of the Self-Organized Criticality (SOC) paradigm [6][7][8][9][10][11][12][13][14] has been discussed. Earthquakes trigger dynamic and static stress changes. The first acts at short time and spatial scales, involving the brittle upper crust, while the second involves relaxation processes in the asthenosphere and acts at long time and spatial scales [15][16][17][18][19][20][21]. In this letter, by means of a new analysis, we show that it is possible to reproduce statistical features of earthquakes catalogs [22,23] within a SOC scenario taking into account longrange interactions. We consider the dissipative OlamiFeder-Christensen model [12] on a small world topology [24,25] and we show that the Probability Density Functions (PDFs) for the avalanche size differences at different times have fat tails with a q-Gaussian shape [26][27][28][29] when finite-size scaling is present. This behaviour does not depend on the time interval adopted and is found also when considering energy differences between real earthquakes. It is possible to explain this result analytically assuming the absence of correlations among the sizes (released energies) of the avalanches (earthquakes). This finding does not allow to predict the magnitude of the next earthquake knowing those of the previous ones.The Olami-Feder-Christensen (OFC) model [12] is one of the most interesting models displaying Self-Organized Criticality. Despite of its simplicity, it exhibits a rich phenomenology resembling real seismicity, like the presence of aftershocks and foreshocks [14]. In its original version the OFC model consists of a two-dimensional square lattice of N = L 2 sites, each one connected to its 4 nearest neighbours and carrying a seismogenic force represented by a real variable F i , which initially takes a random value in the interval (0, F th ). In order to mimic a uniform tectonic loading all the forces are increased simultaneously and uniformly, until one of them...