HR: 0800h
AN: T21B-0479    [Abstracts]
TI: Effect of Hydration State on the Frictional Properties of Montmorillonite-based Fault Gouge
AU: * Ikari, M J
EM: mikari@geosc.psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences 522 Deike Building, State College, PA 16802 United States
AU: Marone, C
EM: cjm@geosc.psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences 522 Deike Building, State College, PA 16802 United States
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences 522 Deike Building, State College, PA 16802 United States
AU: McKiernan, A W
EM: amckiern@geosc.psu.edu
AF: The Pennsylvania State University, Dept. of Geosciences 522 Deike Building, State College, PA 16802 United States
AB: 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 water content. We report on laboratory experiments to examine the effect of hydration state on the frictional properties of simulated fault gouge consisting of mixtures of Ca-montmorillonite powder and quartz sand. Four mixtures were studied (100%, 70%, 50% and 30% montmorillonite) and each was tested at four hydration states: 21 wt% water, 14 wt% water (samples equilibrated to room humidity), 7 wt% water, and 0 wt% water (dry). Samples with water content values approaching 0 wt% were dried in a 105 deg. C oven for at least 24 hours. Time dependent drying curves were constructed in order to reproducibly obtain a given hydration state. Samples reached 14 wt% water when equilibrated with room RH. Higher water content was achieved by equilibrating samples in a sealed environment with saline solutions for at least 24 hours. Experiments were conducted in a servo-controlled apparatus using the double-direct shear configuration. Layers were 3 to 5 mm thick with nominal contact dimensions of 5 cm by 5 cm. For each clay/quartz mixture and hydration state, we measured frictional properties at normal stresses of 5, 15, 25, 40, 70, and 100 MPa. Experiments were conducted under constant shear velocity boundary conditions. Velocity stepping experiments were conducted in the range 1-300 microns/sec. Preliminary results show a systematic decrease in the coefficient of friction with increasing water content and with increasing normal stress and increasing clay content. Friction values for 50/50 mixtures of montmorillonite/quartz range from 0.56 to 0.64 for 0 wt% water and decrease to a range of 0.20 to 0.56 at 14 wt% water. For layers of 100% montmorillonite, friction values are 0.56-0.62 at 0 wt% water and 0.10-0.32 at 14 wt% water. Plotting a-b values obtained from velocity stepping experiments indicate increasingly velocity strengthening frictional behavior (i.e. a-b values become more positive) as water content increases. For all mixtures with 0 and 7 wt% water, there appears to be a critical normal stress value between 25 and 40 MPa below which there is a high variation among a-b values and above which these values become tightly clustered. Ongoing experiments include addition of velocity stepping experiments at higher water content values to further investigate the relationship between water content, clay content, and normal stress as it relates to frictional behavior.
DE: 1859 Rocks: physical properties
DE: 5199 General or miscellaneous
DE: 7209 Earthquake dynamics (1242)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8164 Stresses: crust and lithosphere
SC: Tectonophysics [T]
MN: Fall Meeting 2005