SCEC Award Number 13011 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Thermally Driven Shear Localization in Fault Zones
Investigator(s)
Name Organization
James Rice Harvard University
Other Participants John D. Platt, Harvard University
SCEC Priorities 3, 4, 6 SCEC Groups FARM, Geology, CS
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 In continuation of our FY 2012 SCEC project, we have focused on identifying physical mechanisms controlling the thermal weakening and related localization of rapid shear in fault gouge.
The major goal is to understand the materials physics and dynamics which allows earthquakes on maturely sheared fault zones to occur at low overall driving stresses, and with the majority of deformation accommodated in a principal shear zone that is generally less than 1mm to 1 cm wide, within a much broader fault core composed of gouge and ultracataclasite, and pervasively cracked rock.
The particular processes identified involve weakening through frictional heating and the consequent pressurization of pore fluids. In our earliest studies, those were in-situ pore fluids, i.e., groundwater, and our major accomplishment in the current grant period was to prepare two papers for publication (now accepted at JGR) on that, as discussed below.
Also, we have continued the studies initiated in the prior year which recognize, based on recently published experiments and field observations, that fluids released by the thermal decomposition of fault gouge components may also be important sources of weakening and localization, and these too are considered in our most recent work.
The studies have also been partly supported, starting 1 July 2013, by a 3-year NSF grant, NSF-EAR Geophysics Program Grant EAR-1315447: "Materials physics of rapidly sheared faults and consequences for earthquake rupture dynamics".
Intellectual Merit Our studies contribute towards the goal of understanding the materials physics and dynamics which allows earthquakes on maturely sheared fault zones to occur at low overall driving stresses, and with the majority of deformation accommodated in a principal shear zone generally less than 1mm to 1 cm wide, within a much broader fault core composed of gouge and ultracataclasite, and pervasively cracked rock.
Broader Impacts An impact, not yet fully realized, is that this level of understanding of the faulting process will help us to interpret, and understand the significance in terms of causative processes, of fault zone observations in the field and in laboratory specimens. The work also takes steps towards understanding how the materials physics of fault zone processes interact with rupture dynamics.
Exemplary Figure The is Fig. 2, which is shown in expanded form (and expanded caption) on the last page of the report. Caption:

This is Figure 8 of Platt, Rudnicki and Rice [2014] (in press), for a layer of thickness h of rate-strengthening fault gouge, with boundaries forced to shear relative to one another at a constant rate V. The plot shows how the strength of the gouge layer evolves, normalized by the initial strength, for localizing shear (in blue, the actual response) and, as a comparison (in red), for non-localized uniform shear - which is shown to be an unstable deformation mode, despite the rate-strengthening. These simulations were produced using path-averaged parameters modeling a damaged material, intended to represent a strike slip fault at ~ 7 km depth. Here V = 1 m/s, a typical average slip-rate in an earthquake, and h = 1 mm. The sudden drop in strength coincides with the onset of localization. The initial deformation, before diffusion and localization have had time to act, is well described by the solution for uniform shear under undrained and adiabatic condition [Lachenbruch, 1980] . At large slips the solution is no longer influenced by the small yet finite width of the shearing zone and the strength is well approximated by the solution for slip on a plane [Mase and Smith, 1987; Rice, 2006]. The two limits for undrained adiabatic deformation and slip on a plane are shown above by the dashed black lines. Note that the undrained adiabatic solution from Lachenbruch [1980] differs from our simulation of a uniformly sheared layer because our numerical simulations allow for diffusion of heat and fluid into the surroundings.