Abstract
This paper demonstrates that, in contrast to relatively soft rocks, intact hard rocks failed in mode II can increase their brittleness dramatically (hundreds of times) with rising confining stress. The brittleness variation in this case follows a typical pattern of initially increasing, reaching a maximum and then ultimately decreasing. The harder the rock, the greater is the effect of embrittlement. A shear rupture mechanism discussed in the paper shows that the embrittlement results from reduction of friction within the rupture zone with rising confining stress. Transient “negative friction”, which can be generated within a certain range of confining stress renders rocks superbrittle. The similarity in variation of rock brittleness with confining stress, and aftershock activity with depth, leads to the supposition that the aftershock process can be caused by generation of new faults in the intact rock mass surrounding the main fault where superbrittle behaviour determines the depth range of earthquake activity.
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