A. A. Ostapchuk

Mesomechanics of shear resistance along a filled crack

The paper reports on laboratory experiments with the aim of studying the effect of microstructural and macromechanical properties of a crack filled with discrete material on the formation of a sliding mode. It is shown that the spectrum of possible deformation events on the discontinuity is governed by both the macroscopic characteristics of the gouge and its mesoscale structure. The evolution of force bridges which are formed and collapsed in shear along the crack, their length and number fully control the type of deformation—stable sliding, stick-slip, and intermediate modes with low-velocity motion of the crack edges. The variation of the Coulomb strength affects mainly the stress drop value in dynamic failure or a slip event with low displacement velocity and little affects the deformation mode. Consideration is also given to the regularities by which the macroscopic characteristics of contact vary in shear.

Acoustic Emission during Different-Type Inter-Block Movements

The article reports the lab experimentation on seismic/acoustic emission during different-type inter-block movements of rock mass. The fact that co-seismic displacement under induced earthquakes occurs along the existing interfaces is the basis for relatively simple tests on a slip-model plant. Using different materials as fracture fillers allowed modeling entire range of probable deformation modes. The deformation modes are conditionally grouped as creep or steady-state slip, unsteady-state slip and regular discontinuous slip or stick-slip. The authors show that statistics of acoustic emission during slip is described using the Gutenberg–Richter law. The strongest “representative” events under shearing occur quasiregularly, with probability much higher than follows from G–R law. The functional relation is found between the acoustic emission energy and the shear velocity.

Some questions of geomechanics of the faults in the continental crust

We present the results of laboratory experiments on studying the formation of different slip modes on the interfaces in a rock massif such as aseismic creep, stick-slip, and periodic slow-slip events. It is shown that the way of releasing the accumulated elastic energy is determined by the mesoscale structure of the gouge rather than by its macroscopic strength characteristics. The evolution of the stress chains which are formed and broken during the displacement on the fracture, as well as the length and number of these chains, completely determines the regularities of the deformation. The role of these load-bearing elements in nature can be played, e.g., by the “contact spots,” which determine the regularities of stress concentration near the interblock boundary. We consider the effects of low-amplitude vibrations on stressed fractures. It is shown that, depending on the mode of deformation, the vibration impact can either reduce or boost the amplitude of separate events and the fraction of energy that is released dynamically. In the conclusion of the paper, we discuss the possibility of using the shear strength of the fault zone as a geomechanical parameter controlling the mode of deformation.

Study of acoustic emission signals during fracture shear deformation

We study acoustic manifestations of different regimes of shear deformation of a fracture filled with a thin layer of granular material. It is established that the observed acoustic portrait is determined by the structure of the fracture at the mesolevel. Joint analysis of the activity of acoustic pulses and their spectral characteristics makes it possible to construct the pattern of internal evolutionary processes occurring in the thin layer of the interblock contact and consider the fracture deformation process as the evolution of a self-organizing system.

Effect of deformation properties of discontinuities on intensity of induced seismicity sources in rocks. Part II: laboratory and numerical experiments

Variations in strength and stress state of rocks fail to explain the difference observed in efficiency of seismic radiation from separate sources within the limits of the same mine field. The laboratory and numerical experiments show that insignificant variation in ultimate strength of a fracture and, thus, different shear fracture stiffness results in radical change in the seismic event efficiency. The experimentally obtained relations between the key parameters should be taken into account in geomechanical modeling of large-scale objects.

Traces Of Laboratory Earthquake Nucleation In The Spectrum Of Ambient Noise

The short-term forecast of earthquakes associated with fault rupture is a challenge in seismology and rock mechanics. The evolution of mechanical characteristics of a local fault segment may be encoded in the ambient noise, thus, converting the ambient noise to an efficient source of information about the fault stress-strain conditions. In laboratory experiments we investigate micro-vibrations of a block-fault system induced by weak external disturbances with the purpose of getting reliable evidence of how the system transits to the metastable state. We show that precursory changes of spectral characteristics of micro-vibrations are observed for the complete spectrum of failure modes. In the course of experiments we systematically change the properties of interface to perform the transition from stick-slip to steady sliding and observe the characteristics of micro-vibrations of the laboratory block-fault system. Detected were systematical alterations of the system natural frequency and those alterations were determined by the evolution of fault stiffness. The detected regularities suggest that the final stage of seismic event preparation can be revealed in analyzing the spectral characteristics of ambient noise. The detection of natural oscillations of a block-fault system can be a new useful tool to monitor active faults in real time.

Experimental study of different modes of block sliding along interface. Part 2. Field experiments and phenomenological model of the phenomenon

The paper reports the results of field experiments on studying different modes of gravitational sliding of a block on the natural fault surface. Various materials were used as interface filler to model the whole range of deformation events that can be arbitrarily divided into three groups: accelerated creep, slow slip, and dynamic slip. The experiments show that the type of modeled deformation events is defined by both structural parameters of contact between blocks and material composition of the contact filler.Foundations for a new geomechanical model of occurrence of different-type dynamic events were developed. The model is based on the idea that “contact spots” form subnormally to the crack edges during shear deformation; the “spots” are clusters of force mesostructures whose evolution governs the deformation mode. The spatial configuration of “contact spots” remains unchanged during the entire “loading-slip” cycle but changes after the dynamic event occurrence. The destroyed force mesostructures can be replaced by similar structures under intergranular interaction forces when the external influence is fully compensated. Unless “contact spots” are incompletely destroyed, the deformation process dynamics is defined by their rheology. The migration of “contact spots” during deformation of a crack filled with heterogeneous material causes changes in deformation parameters and transformation of the mode itself due to changing rheology of local contact areas between blocks.It is found by fractal analysis that in order for dynamic slip to occur, spatially structured “contact spots” characterized by low fractal dimension must be formed; slow slip events can exist only in a certain parametric domain called the “dome of slow events”. It is found that the probability of slow slip occurrence is higher on fault regions characterized by maximum fractal dimension values: fault tips, fault branching and fault intersection zones.

Alteration of fault deformation mode under fluid injection

First experimental results on fluid injection influence on a mode of interblock contact deformation are reported. It is found that deformation energy consumed to radiate seismic waves largely depends on parameters of a fluid injected into a fracture. Dilatant fluid appears to be the most efficient to decrease seismic energy as viscosity of such fluid grows with increasing deformation rate. In laboratory tests the slip-stick motion mode transforms into a quasi-stable creep in a single deformation cycle after fluid injection is ceased.

Precursors of dynamic failure on a tectonic fault

A new method for controlling the dynamic shear stiffness of interblock contact during stick-slip is proposed. This method enables us to reveal changes in the mechanical properties of the contact long before the macroscopic slip will be recorded. In the experiments conducted, the time of precursor manifestation was about 1/3 of the duration of the “seismic cycle.”