Superbrittleness of rocks and earthquake activity

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.

► We examine hard rock embrittlement due to rising confining pressure.
► The embrittlement is caused by reduction of friction in the shear rupture head.
► Transient “negative friction” renders rocks superbrittle.
► Superbrittleness of rocks can determine the depth range of earthquake activity.