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Slip on 'weak' faults by the rotation of regional stress in the fracture damage zone


Slip on unfavourably oriented faults with respect to a remotely applied stress is well documented and implies that faults such as the San Andreas fault1 and low-angle normal faults2 are weak when compared to laboratory-measured frictional strength3. If high pore pressure within fault zones is the cause of such weakness, then stress reorientation within or close to a fault is necessary to allow sufficient fault weakening without the occurrence of hydrofracture4. From field observations of a major tectonic fault, and using laboratory experiments and numerical modelling, here we show that stress rotation occurs within the fractured damage zone surrounding faults. In particular, we find that stress rotation is considerable for unfavourably oriented ‘weak’ faults. In the ‘weak’ fault case, the damage-induced change in elastic properties provides the necessary stress rotation to allow high pore pressure faulting without inducing hydrofracture.

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Figure 1: Field data showing the variation in microfracture density within the fault damage zone.
Figure 2: Variation of measured elastic properties with increasing microfracture damage in Westerly granite.
Figure 3: Variation of elastic properties, principal stress orientation and the mean stress as a function of distance within the fault damage zone.
Figure 4: Mohr diagram illustrating the stress rotation within the damage zone.


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We thank J. Cembrano for field assistance and M. Casey, N. Kusznir and D. Prior for discussions. This work was supported by the Natural Environment Research Council and the Nuffield Foundation.

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Faulkner, D., Mitchell, T., Healy, D. et al. Slip on 'weak' faults by the rotation of regional stress in the fracture damage zone. Nature 444, 922–925 (2006).

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