The role of fault fabric in fault stability under different stress conditions
Abstract
Earthquakes are paroxysmal large-scale phenomena. Their nucleation, however, roots in a delicate frictional equilibrium at the asperity-scale. Recent experimental studies show that the macroscopic frictional properties of faults are controlled not only by the material properties, but especially by the architecture of the thin layer (i.e. fault gouge) that accommodates pre-seismic deformation.
We present a study of the frictional behaviour of simulated faults as function of their inner fabric. Mixed anhydrite-dolomite gouges were tested with rock deformation experiments using a double-direct shear configuration apparatus. Initially, samples were sheared for 13 mm at constant displacement rate (1 µm/s) - but different normal load conditions (15, 35, 60 and 100 MPa) - in order to form a stable fabric. After this stage (texturing), the machine was de-stiffened and the normal load brought to 35 MPa. This is the condition where anhydrite-dolomite gouges generally start to show unstable behaviour. Samples were then re-sheared for up to 10 mm. During the texturing phase, deformation is accommodated by damage zones whose thickness roughly increases with the applied stress (~100 to ~400 µm). Their inner fabric is less homogeneous at higher loads (60-100 MPa), presenting anastomotic and foliated patterns. Most of the deformation, however, is localised in principal slip zones (PSZ) with thickness in the order of 100-200 µm and characterised by extreme grain size reduction. Faults with fabrics formed at 15 and 35 MPa experience stable sliding during re-shear but display different textures. While 35 MPa PSZ remains similar to the texturing phase, 15 MPa PSZ localises further to become almost a slip surface. At 60 and 100 MPa we document an initial seismic rupture, which reworks PSZ's fabric. For these two conditions, we recorded an initial shear stress drop of 2.5 and 12 MPa, respectively, paired with fast displacement rates (>> 1 cms-1). These main events are followed by sustained stick-slip events and reduced fault friction. We conclude that fabrics play a major role in the fault slip behaviour. Our results suggest that creeping faults might become unstable while fabrics reorganise to match new stress conditions.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMMR0150015P
- Keywords:
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- 3994 Instruments and techniques;
- MINERAL PHYSICS;
- 7209 Earthquake dynamics;
- SEISMOLOGY;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS;
- 8163 Rheology and friction of fault zones;
- TECTONOPHYSICS