HR: 12:05h
AN: T42C-08    [Abstracts]
TI: Frictional and Hydrologic Properties of Clay-Rich Fault Gouge
AU: * Ikari, M J
EM: mikari@geosc.psu.edu
AF: Pennsylvania State University, Geoscience Dept. 521 Deike Building, University Park, PA 16802, United States
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Pennsylvania State University, Geoscience Dept. 521 Deike Building, University Park, PA 16802, United States
AU: Marone, C
EM: cjm@geosc.psu.edu
AF: Pennsylvania State University, Geoscience Dept. 521 Deike Building, University Park, PA 16802, United States
AU: Niemeijer, A R
EM: arn3@psu.edu
AF: Pennsylvania State University, Geoscience Dept. 521 Deike Building, University Park, PA 16802, United States
AB: Understanding the frictional and hydrologic properties of fault gouge is crucial to understanding the generation and nature of earthquakes and the strength of crustal faults. Clay minerals are a major constituent of fault gouge and are of particular interest because they may exhibit exceptionally low friction and low permeability. Previous work has shown that different clay minerals exhibit differences in shear strength, thus it has been suggested that different clay minerals may also exhibit stability differences. We report on laboratory experiments examining the frictional and hydrologic properties of saturated fault gouges dominated by the minerals illite, chlorite, and Ca- montmorillonite. Gouge consists of naturally occurring illite shale and chlorite schist (grain size < 106 μm), and a synthetic mixture of 50% montmorillonite-50% silt-sized quartz. Experiments were conducted in a servo-controlled apparatus within a sealed pressure vessel using the double-direct shear configuration. Confining pressure was 40% of the applied normal stress, which varied between 15-100 MPa. Pre- and post shear permeability was measured by controlling pore pressure normal to both gouge layer contact areas. During shearing, pore pressure was controlled at 5 MPa on one end while changes in pore pressure were measured on the other end. Layers were initially 4 mm thick with nominal contact dimensions of 5 cm by 5 cm. Experiments were conducted under constant shear velocity boundary conditions of 1-300 μm/s. Preliminary results at effective normal stresses of 15-30 MPa indicate that montmorillonite gouge is slightly weaker (coefficient of friction = 0.19-0.23) than illite and chlorite gouge (0.29-0.32). Montmorillonite, chlorite, and illite gouges are consistently velocity strengthening, and (a-b) tends to increase with increased sliding velocity. Under a constant head gradient, layer-normal permeability decreases ~3 orders of magnitude in montmorillonite gouge (from 3x10^-16 m²), ~2 in illite gouge (from 5x10^-17 m²), and ~1 in chlorite gouge (from 4x10^-16 m²) after 13-34 shear strain. Post-shear, chlorite gouge is the most permeable followed by illite and montmorillonite. In all gouges, permeability drops ~1 order of magnitude for each increase of 15-20 MPa of effective stress. Gouges with measured post shear permeabilities below 1x10^-19 m² develop significant overpressure (>550 kPa) after 10-20 shear strain. Ongoing experiments include permeability and friction measurement at higher effective stresses to further investigate frictional and hydrologic variations among different clay minerals and natural fault gouges.
DE: 5114 Permeability and porosity
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8163 Rheology and friction of fault zones (8034)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting