ABSTRACT:

Regularities of transforming different shear deformation regimes of a gouge-filled fault were investigated in laboratory at a slider model. It is shown that slight variations of material content, which have negligible effect on fault shear stiffness, may lead to a radical change of the deformation regime from stable creep to stick-slip and the amount of radiated energy. It is established experimentally that the geomechanical parameter that controls fault shear behavior is the ratio of the shear stiffness of the fault to the one of the enclosing rock mass.

1 INTRODUCTION

According to modern conceptions earthquakes and rock bursts arise not as a result of a sudden emergence and propagation of a new crack in the Earth's crust, but rather due to sudden sliding along already existing faults. Therefore, earthquakes and rock bursts are mainly phenomena of frictional physics than of fracture mechanics. Regularities of fault sliding are governed by frictional interaction of fault sides (Scholz 1998). One of the first laboratory experiments, in which two rock surfaces slide one on another, showed that the motion is rather jerky than smooth (Brace & Byerlee 1966). For a long time stick-slip was the only model of fault behavior used to describe the earthquake cycle.

A noticeable increase of accuracy of seismic and geodetic observations in the last decades allowed to determine reliably a wide range of shear modes on interfaces between structural blocks of the Earth's crust, including earthquakes, stable creep, low frequency earthquakes and slow slip events. Now it is suggested that slip modes span a continuum and are of common occurrence (Peng & Gomberg 2010). Besides it was established that the coseismic displacement is usually localized in the narrow principal slip zone of a fault filled with worn granular material (gouge) (Sibson 2003, Chester & Chester 1998, Sammis et al. 1987). Thus, to understand the mechanics of earthquakes and faulting, it is essential to understand the behavior of gouge-filled faults.

The frictional behavior results from the nature of stress accommodation across the granular layer undergoing shear. During shear specific conglomerates of loaded grains - the so called, stress chains - form across the fault. The evolution of stress chains defines the mechanical properties of the granular medium (Liu et al. 1995, Cates et al. 1998). C. Marone with coworkers have performed numerous series of experiments aimed at investigating the effects of filler particle size and shape, distribution of particles over sizes, thickness of interblock contact, peculiarities of shear loading on the laws of discontinuity shear deformation (Mair et al. 2002, Anthony & Marone 2005, Savage & Marone 2007). However, it is still unclear what exact parameters of a fault govern realization of one or another deformation regime and what are the conditions and reasons of transformation of one regime into the other. Our work is aimed at finding the criterion that defines realization of different deformation regimes.

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