Shear Induced Pore Pressure Generation in Montmorillonite-Based Fault Gouge
Abstract
Understanding the frictional properties of fault gouge is crucial to understanding the generation and nature of earthquakes and the strength of crustal faults. Montmorillonite is of particular interest because it can exhibit exceptionally low friction and it has been suggested as a candidate source of fault weakness. Previous work shows that the weakness of montmorillonite is associated with its hydration state; a possible mechanism of weakening may be a reduction in effective stress due to increases in pore pressure. We report on laboratory experiments examining changes in shear strength due to increased pore pressure within simulated fault gouge during shearing. Gouge consists of four mixtures of Ca-montmorillonite powder and quartz sand (100%, 70%, 50%, 30%, and 0% quartz). Experiments were conducted in a servo-controlled apparatus within a sealed pressure vessel using the double-direct shear configuration. Confining pressure was 10 MPa and controlled pore pressure ranged from 2-5.5 MPa. Layers were 4 mm thick with nominal contact dimensions of 5 cm by 5 cm. Pore pressure was applied or measured normal to gouge layer contact areas. Experiments were conducted under constant shear velocity boundary conditions of 1-10 μm/s. Preliminary results under controlled pore pressure conditions indicate that the apparent coefficient of friction is lower in gouge that experiences higher sliding velocity. At effective normal stress of 15-40 MPa, friction values of 50/50 mixtures of montmorillonite/quartz range from 0.36-0.26 at 1-2 μm/s and decrease to 0.25-0.19 at 10 μm/s. Under a constant head gradient applied at the upstream and downstream ends of the sample, fluid flow rate across gouge layers decreases with total sliding distance, suggesting build up of excess pore pressure and/or decreased permeability during shearing. Some stick-slip behavior is observed in 50/50 mixtures during shear loading at low velocity. Ongoing experiments include permeability measurements and friction experiments to investigate the effects of sliding velocity, gouge composition, and applied normal stress on shear induced pore pressure changes that may cause gouge weakening.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T21D0455I
- Keywords:
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- 3613 Subduction zone processes (1031;
- 3060;
- 8170;
- 8413);
- 5114 Permeability and porosity;
- 8118 Dynamics and mechanics of faulting (8004);
- 8163 Rheology and friction of fault zones (8034);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)