Faulting under High Confinement: Experimental Investigation and Model Development

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Of particular interest (and controversy) in geophysics and seismology are the mechanismscontrolling brittle-like failure at depths of several hundred km or more where deep earthquakesoccur. At these depths frictional sliding, the fundamental failure mechanism operating under thelower confinements present at shallower depths, is nominally suppressed. We propose newexperiments and modeling aimed at elucidating the characteristics and the fundamental physicalmechanism(s) that underlie localized brittle compressive failure under high confinement.Among the mechanisms that have been proposed to explain brittle-like failure under highconfinement, dehydration embrittlement and transformational faulting are the two most oftencited. They act to reduce confinement locally, in effect allowing low-confinement frictionalfaulting to continue into the high-confinement regime. However, changes in earthquake seismiccharacter at greater depths are possibly consistent with a change in the physical mechanism ofrupture for the deepest earthquakes. An alternative mechanism, and the one we propose toinvestigate, is plastic faulting. This mechanism is fundamentally different from the other two: itis a non-frictional shear instability aided by adiabatic heating.Spirited by past success in using ice as a model material for rock--Kirby's [1987] discoveryof transformational faulting, and our own discovery [Renshaw and Schulson, 2001] of whatappears to be a universal mechanism of low-confinement brittle failure--and encouraged by itsmineralogical simplicity and by its marked similarity to the behavior of rocks and minerals, wepropose a series of systematic compression experiments on granular ice to determine the effects,if any, of confinement, grain size and strain rate on both the terminal failure stress and the failuremode, with attention to microstructural detail. Preliminary experiments demonstrate a transitionin brittle-like failure mode with increasing confinement and that the high confinement faultsappear not to be friction controlled. Nor do they appear to be related to either mode II cracking,dehydration embrittlement, or phase transformations. Our working hypothesis is that plasticfaulting is at play. In addition, we propose to develop quantitative models of high confinementcompressive failure, using as inputs direct observations of the physical processes.