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Microfracturing and rock deformation as effects of tectonic stress generates sounds called acoustic emission. Many reporting on the earthquakes say that first people hear a noise and then they feel the two shocks (P and S waves). How is this possible as long as the speed of sound is less than the P wave speed? This article gives an answer for these question. An earthquake is an effect of high tectonic stress in an area (hypocenter) and the last part of this process is the loss of elasticity of rocks followed by breaking and release of a great energy. Basically the seismic wave generates and carries on the acoustics. We record earthquakes using seismic equipment and pressure sensors (air and ground) in Vrancea (bending area of Romanian Carpathians Mountains). In both cases you can see the P and S waves but the sounds generated by cracks contain high frequencies that are attenuated rapidly and appear to several hours prior to the event. The sound depends on where you are registered (geological structure, seismic activity, soil, elasticity). Acoustic emission (AE) forecast earthquakes but we cannot evaluate the magnitude. We could have a big one or several small earthquake. AE is part of a multidisciplinary network that analyzes precursor phenomenon (atmospheric aerosols, ions, CO2, radon and clouds in relationship with temperature, humidity, atmospheric pressure, wind speed and direction, variations of the telluric currents, local magnetic field, infrasound, atmospheric electrostatic field, electromagnetic and seismic activity, radio waves propagation, and animal behavior). Our records indicate an intensity of sounds before earthquakes greater than 4.5R with 8 - 10 hours. [ABSTRACT FROM AUTHOR]

We simulate rock fracture using ESyS_Particle, which is a 3-D Discrete Element Model developed for modeling geological materials. Two types of simulations are carried out: Unconfined Compressive Test (UCT) and Brazilian Tensile Test (BTT). The results are compared to laboratory tests. Model parameters are determined on the basis of theoretical studies on the elastic properties of regular lattices and dimensionless analysis. The fracture patterns and realistic macroscopic strength are well reproduced. Also the ratio of the macroscopic strength of compression to the tensile strength is obtained numerically. [ABSTRACT FROM AUTHOR]

A hydrothermal flow-through experiment was performed for an artificially created single tensile fracture of granite. Water that had dissolved granite (Si concentration: 250 ppm) was injected into the fracture under hydrothermal conditions (effective normal stress: 10–13 MPa, temperature: 150 °C) during 450 hours. Non-monotonic changes of fracture permeability were observed. Fracture permeability decreased significantly only during first 150 hours. Si concentration of water produced from the fracture was increased monotonically. However, the Si concentration was smaller than that of water injected into the fracture. A numerical modeling using experimental data was also performed for hydrothermal flow in the fracture. Aperture structures for the beginning and the end of the experiment and resulting hydrothermal flow were determined using experimentally obtained fracture surface geometries and fracture permeability. For the beginning of the experiment, developments of preferential flow paths (channeling flow) in heterogeneous aperture structure were observed obviously, while the developments of preferential flow paths were not observed obviously for the end of the experiment. It was also observed that local apertures around the preferential flow paths for the beginning of the experiment tended to become smaller at the end of the experiment, while other local apertures for the beginning of the experiment tended to become greater at the end of the experiment. Although both increase and decrease of local apertures were observed between the beginning and the end of the experiment, arithmetic mean values of local apertures for the end of the experiment (71 μm) was smaller than that for the beginning of the experiment (84 μm). [ABSTRACT FROM AUTHOR]

We report on results of fluid flow in a single fracture obtained from combination technique of physical experiments and flow simulation for investigating effects of asperity degradation due to sliding of fracture on flow properties of a fracture. We conducted sliding experiments of a rock fracture using cylindrical granite specimens (60 mm in diameter, 140 mm in axial length) containing a single tensile fracture. Sliding along the fracture was made by controlling a loading piston at constant rate of 0.001 mm/s under constant confining pressure of 5 MPa. Sliding displacement was applied up to approximately 6 mm along the surfaces. During the sliding, fluid flow was made within the fracture by applying a constant fluid pressure differential of 0.1 MPa through boreholes of 3 mm diameter drilled in the specimens. Fracture permeability was evaluated at every 0.5 mm in axial displacement by measuring the pressure differentials between inlet and outlet of the fluid and mass flow at the outlet. Flow field within the fracture is calculated with flow simulation in which based on the measured fracture permeabilities, which shows that major flow paths can significantly change with sliding even at the almost constant permeability during sliding. Our result suggests that the change in flow field with shearing may have important roles in material transport due to advection, which could affect physical strength of a fracture by water-rock interactions on fracture surfaces such as precipitation and/or dissolution. [ABSTRACT FROM AUTHOR]

Anisotropic fluid flow in single sheared fractures in granite is investigated numerically under normal stresses up to 90 MPa that are the extended stress conditions of the previous studies. Aperture distributions of the sheared fractures under the normal stresses generated numerically on the basis of direct measurements of contact area suggest anisotropy of connectivity in fracture aperture even under the normal stresses. Numerical simulation of fluid flow using the generated aperture distribution shows not only anisotropy in fracture permeability and that in preferential flow paths, but also the normal stress dependency in the anisotropy of fracture permeability. An investigation on contact ratio reveals the anisotropy and the normal stress dependency in aperture connectivity, which supports the results of flow simulation. These results suggest importance of the flow anisotropy in discrete fractures for investigating three-dimensional flow properties in fractured rocks at a greater depth. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]

In this paper, we describe a numerical method to model aperture structures and flow paths in single rock fractures based on actual fracture surface geometries and actual fracture permeability. Fracture surfaces were measured using a CCD laser displacement sensor (resolution: 10 μm) with 250 μm square mesh for 100 mm × 150 mm single tensile fractures in granite samples. Fracture permeability was also measured under 10–100 MPa confining pressure conditions. Aperture structures and flow paths were modeled numerically using the actual fracture surface geometries so that the model’s permeability consistent with actual fracture permeability. Channeling flows were clearly observed at all conditions because of heterogeneous aperture structures. The results also suggested that fracture permeability could be overestimated if based on the conventional parallel plate model using an arithmetic mean value of local apertures. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]