Tarasov, BG 2008, 'New Insight into the Nature of Shear Rupture Propagation in Pristine Rocks and Pre-Existing Faults', in Y Potvin, J Carter, A Dyskin & R Jeffrey (eds), Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium
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This paper proposes a new insight into the role of fault structure in the determination of fault properties which explains a number of enigmatic aspects of fault behaviour. It is shown that hard rocks which failed at high confining pressure (σ3) exhibit specific properties that distinguish them markedly from common rock behaviour. They become extremely brittle with brittleness increasing with σ3 and lose shear resistance within a certain range of shear rupture displacement. The behaviour is caused by the intrinsic nature of the fault structure, which is an echelon of blocks operating as hinges, essentially eliminating friction at high confining pressure of a certain displacement range. Such rock properties result in increasing instability with depth and make rupture abnormally violent, both of which are well-established experimentally from studies of earthquakes and rockbursts at high stress level.
The paper demonstrates that, while the same block structure may be found at a different scale in primary fractures and general faults, significantly different mechanisms are responsible for the formation of the structure in each case. A new approach is proposed for understanding fault segmentation and the role of junctions in fault propagation. It is argued that segmentation is a result of advanced triggering of new fractures that propagate both towards the current fracture and in the opposite direction. This mechanism triples the fault propagation speed. Special joining shear fractures formed at the meeting of the approaching segments help to accommodate the fault displacement and can significantly decrease the fault strength – thus contradicting the general belief that junctions represent strength barriers impeding the fault motion.
It is shown also that the specific block structure involved in pre-existing natural faults can cause repeatable fault instability. A new stick-slip mechanism based on the frictionless concept is proposed in this paper. Improved understanding of the fracture process is important for better prediction and mitigation of dynamic events such as earthquakes and rockbursts.
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