I'm going to stick my neck out and make a few remarks in response to
Steve's comments concerning frictional sliding theory and fault stress
analysis. I'm sure someone will correct me if I'm wrong!
Frictional sliding theory is just one way of calculating stress from
fault displacement data. As Rob Twiss said in an earlier email, to
calculate stress from strain requires knowledge of the constitutive
relationship for the material in question. For the case of a discrete
fault you might decide to treat it as a frictional sliding surface and
calculate the stress that caused the displacement on the basis of that
assumption; alternatively, you might treat the fault as a body of rock
having finite thickness, the behaviour of which is defined by some
constitutive law (e.g. Mohr-Coulomb). The stress that you calculate will
only be "correct" if the constitutive relationship is "correct".
Quoting from the abstract of Sibson and Xie (1998):
"Assuming horizontal trajectories for maximum compressive stress, the
observed dip range is consistent with reactivation of faults possessing
rock friction coefficients within Byerlee's (1978) range...."
So, what they show is that the observed fault slip is consistent with
frictional slip on surfaces subject to a horizontal maximum compressive
stress. They have not proved that the maximum compressive stress is
horizontal.
I agree that frictional sliding is a useful approximation of the
behaviour of discrete, brittle faults, and that stresses inferred from
frictional sliding theory are probably quite accurate in many cases. But
you would have to resort to a continuum model (such as mohr coulomb
constitutive behaviour) in order to understand the orientation of the
stress field that caused the fault to form in the first place. And then
you would have to account for strain weakening in the fault rock, and
quantify this in the constitutive model.... etc etc
Regards,
Heather
Heather Sheldon
Computational Geoscience Group
CSIRO Exploration and Mining
ARRC, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
Email [log in to unmask]
Tel. +61 (0)8 6436 8915
Fax. +61 (0)8 6436 8555
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