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] A series of laboratory experiments has been conducted in which three-dimensional (3-D) locations of acoustic emissions (AE) were recorded and used to analyze the development of compaction bands in Bleurswiller sandstone, which has a porosity of 25%. Results were obtained for saturated samples deformed under triaxial compression at three different confining pressures (60, 80, and 100 MPa), a pore pressure of 10 MPa, and room temperature. We recorded acoustic emissions, compressional and shear wave velocities, and porosity reduction under hydrostatic condition and under triaxial loading conditions at a constant axial strain rate. Our results show that seismic velocities and their amplitude increased during hydrostatic pressure build up and during initial axial loading. During shear-enhanced compaction, axial and radial velocities decreased progressively, indicating an increase of stress-induced damage in the rock. In experiments performed at confining pressures of 80 and 100 MPa during triaxial loading, acoustic emissions were localized in clusters. During progressive loading, AE clusters grow horizontally, perpendicular to the maximum principal stress direction, indicating formation of compaction bands throughout the specimens. Microstructural analysis of deformed specimens confirmed a spatial correspondence of AE clusters and compaction bands. For the experiment performed at a confining pressure of 60 MPa, AE locations and microstructural observations show symmetric compaction bands inclined to the cylinder axis of the specimen, in agreement with predictions from recent theoretical models.Citation: Fortin, J., S. Stanchits, G. Dresen, and Y. Guéguen (2006), Acoustic emission and velocities associated with the formation of compaction bands in sandstone,

The effect of stress anisotropy on the brittle failure of granite is investigated under uniaxial compression. Non‐standard asymmetric compression tests are performed on cores of Aue granite (diameter 52 mm, length 100 mm), in which 20 per cent of the core top surface remains unloaded. The edge of the asymmetric steel loading plate acts as a stress concentrator, from where a shear rupture is initiated. The propagation of the fracture‐related process zone from top to bottom of the core is mapped by microcrack‐induced acoustic emissions. Compared to standard uniaxial tests with symmetric loading, in the asymmetric tests both a greater quantity and more localized distributions of emission event hypocentres are observed. The maximum event density doubles for asymmetric (20 events per 10−6 m3) compared to symmetric tests. The cluster correlation coefficient, a measure of strain localization in the faulting process, reaches 0.15 for symmetric and 0.30 for asymmetric tests. The clustering of events, however, is found post‐failure only. Three different amplitudes are used to determine b‐values discussed as a possible failure precursor. Focal amplitudes determined at a 10 mm source distance and maximum amplitudes measured at eight piezoceramic sensors lead to b‐values that drop before rock failure. First‐pulse amplitudes automatically picked from emission wavelets show no anomaly. First‐motion polarity statistics of amplitudes indicate that a shear‐crack‐type radiation pattern is responsible for 70 per cent of the failure of granite, irrespective of stress boundary conditions. For type‐S events with an equal percentage of dilatational and compressional first motions, focal mechanisms are determined by fitting measured first‐pulse amplitudes to an assumed double‐couple radiation pattern. While hypocentres of large type‐S events align parallel to the later fracture plane, their fault plane solutions show no coherent pattern. Spatial views of fracture planes reconstructed from X‐ray computed tomograms reveal local small‐scale changes in fracture plane orientation. Nodal planes from average fault plane solutions of the microscopic acoustic emission events coincide with the overall orientation of the macroscopic fracture plane azimuth (strike angle) determined from thin sections and tomograms.

[1] Seismicity clusters within fault zones can be connected to the structure, geometric complexity and size of asperities which perturb and intensify the stress field in their periphery. To gain further insight into fault mechanical processes, we study stick-slip sequences in an analog, laboratory setting. Analysis of small scale fracture processes expressed by acoustic emissions (AEs) provide the possibility to investigate how microseismicity is linked to fault heterogeneities and the occurrence of dynamic slip events. The present work connects X-ray computer tomography (CT) scans of faulted rock samples with spatial maps of b values (slope of the frequency-magnitude distribution), seismic moments and event densities. Our current experimental setup facilitates the creation of a series of stick-slips on one fault plane thus allowing us to document how individual stick-slips can change the characteristics of AE event populations in connection to the evolution of the fault structure. We found that geometric asperities identified in CT scan images were connected to regions of low b values, increased event densities and moment release over multiple stick-slip cycles. Our experiments underline several parallels between laboratory findings and studies of crustal seismicity, for example, that asperity regions in lab and field are connected to spatial b value anomalies. These regions appear to play an important role in controlling the nucleation spots of dynamic slip events and crustal earthquakes.

We present a detailed statistical analysis of acoustic emission time series from laboratory rock fracture obtained from different experiments on different materials including acoustic emission controlled triaxial fracture and punch-through tests. In all considered cases, the waiting time distribution can be described by a unique scaling function indicating its universality. This scaling function is even indistinguishable from that for earthquakes suggesting its general validity for fracture processes independent of time, space and magnitude scales.PACS numbers: 62.20. Mk,91.30.Dk,89.75.Da,05.65.+b The fracture of materials is technologically of enormous interest due to its economic and human cost [1]. Despite the large amount of experimental data and the considerable efforts undertaken [2], many questions about fracture have not yet been answered. In particular, there is no comprehensive understanding of rupture phenomena but only a partial classification in restricted and relatively simple situations. For example, many material ruptures occur by a "one crack" mechanism and a lot of effort is being devoted to the understanding, detection and prevention of the nucleation of the crack [3,4,5,6,7,8]. Exceptions to the "one crack" rupture mechanism are heterogeneous materials such as fiber composites, rocks, concrete under compression and materials with large distributed residual stresses. In these systems, failure may occur as the culmination of a progressive damage involving complex interactions between multiple defects and microcracks.In particular, acoustic emission (AE) due to microcrack growth precedes the macroscopic failure of rock samples under constant stress [9,10] or constant strain rate loading [11,12]. This is an example of the concept of "multiple fracturing" -the coalescence of spontaneously occurring microcracks leading to a catastrophic failure -which is thought be applicable to earthquakes as well [13,14,15]. Due to this and the similarity in their statistical behavior, acoustic emissions can be considered analogous to earthquake sequences. The temporal [16,17], spatial [18] and size distribution [11] of AE events follow a power law, just as it is commonly observed for earthquakes [19,20]. Such power-law scaling can be considered indicative of self-similarity in the AE and earthquake source process [16].The time evolution of AE and earthquake data also display considerable differences. Laboratory rock fracture is dominated by a large number of foreshocks while seismicity in the Earth's crust is characterized by an abundance of aftershocks [21]. Here, we show that despite this difference the probability density function (PDF) for the time interval between successive events is the same in both cases if they are rescaled with the mean waiting time or equivalently with the mean rate of occurrence. In particular, the PDF for laboratory rock fracture neither depends on the specific experiment nor on the specific material. These observations strongly suggest a universal character of the waiting time d...

Abstract. In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 lam/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is -9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be -2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

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We study triggering processes in triaxial compression experiments under a constant displacement rate on sandstone and granite samples using spatially located acoustic emission events and their focal mechanisms. We present strong evidence that event-event triggering plays an important role in the presence of large-scale or macrocopic imperfections, while such triggering is basically absent if no significant imperfections are present. In the former case, we recover all established empirical relations of aftershock seismicity including the Gutenberg-Richter relation, a modified version of the Omori-Utsu relation and the productivity relation-despite the fact that the activity is dominated by compaction-type events and triggering cascades have a swarmlike topology. For the Gutenberg-Richter relations, we find that the b value is smaller for triggered events compared to background events. Moreover, we show that triggered acoustic emission events have a focal mechanism much more similar to their associated trigger than expected by chance.