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| Titel |
The intermediate principal stress effect on faulting and fault orientation |
| VerfasserIn |
Bezalel Haimson, John Rudnicki |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250033215
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| Zusammenfassung |
| We conducted true triaxial compression tests on rectangular prismatic specimens (19Ã19Ã38
mm) of siltstone core extracted from a depth of 1252 m, some 140 m below the borehole
intersection with the Chelungpu Fault, Taiwan. Experiments consisted of four series of tests
in each of which Ï3 was kept constant and Ï2 was varied from test to test. The major
principal stress (Ï1), aligned with the long vertical side of the specimen, was raised at
constant strain rate until a through-going, steeply dipping fault was initiated. As
in igneous and metamorphic rocks previously tested, Ï1 required to bring about
faulting rose as Ï2 was set at increasing levels above Ï3. This observation reflects the
significant contribution of Ï2 to the compressive strength, and raises doubt about the
suitability of the Mohr-Coulomb criterion. Rather, a strength criterion in terms of
the invariants octahedral shear stress (Ïoct) as a function of mean stress (Ïoct)
provides a good fit to the experimental data. In all tests fault strike was aligned with
Ï2 direction. The angle (or dip) θ of the fault was also strongly affected by Ï2.
For constant Ï3 the angle rose with Ï2, again departing from the Mohr-Coulomb
criterion, which predicts a fault angle independent of the intermediate principal
stress.
The experimental results, revealing the dependence of fault angle θ on Ï2, were
compared with predictions based on shear localization theory incorporating a yield
surface and plastic potential that depend on the following three stress invariants
(rather than two, as in Rudnicki and Rice, 1975): Ïoct, Ïoct, and the Lode angle θL
(=arctan{[2Ï2 - Ï1 - Ï3]-[-3 (Ï1 - Ï3)]}). Dependences of the yield surface and
plastic potential on mean stress were inferred from the fault angles observed in axisymmetric
compression and deviatoric pure shear. Using these relationships to predict fault
angle θ for deviatoric stress states other than axisymmetric compression and pure
shear, yields good agreement with the experimental observations. The results predict
that for constant mean stress, the fault angle θ decreases as the deviatoric stress
state varies from axisymmetric extension to axisymmetric compression. For fixed
deviatoric stress states, θ decreases monotonically with increasing mean stress. |
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