SCEC Award Number 11042 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Shear localization in faulting experiments and implications for source physics
Investigator(s)
Name Organization
Terry Tullis Brown University Nick Beeler United States Geological Survey
Other Participants
SCEC Priorities A7, A8, A11 SCEC Groups FARM, Geology, Seismology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 This work is a pilot study to develop new experimental procedures and to study shear localization and delocalization in fault gouges in which the degree of localization is determined by material properties, stress conditions and strain, rather than influenced by experimental initial and boundary conditions. We are developing procedures for rotary shear deformation under confining pressure for low sliding speeds. Our work will: 1) provide a physical basis for geologic observations of shear localization, 2) produce better understanding of the significance of fault zone characteristics and processes 3) help determine the origin, evolution and implications of on-fault damage and 3) ultimately lead to advances in earthquake rupture simulation and associated calculated ground motions. To date we have successfully designed a complex collection of jigs that will allow us to place powdered Teflon adjacent to the inner and outer boundaries of our rock gouge samples in order to prevent mixing of the Teflon and rock powders during sample assembly. The fabrication of these parts is nearly complete. Soon we will be able to perform the proposed testing of an assembly that should not impose localization on the synthetic rock gouge, even though we have not managed to complete these tests during the time period of this grant.
Intellectual Merit Our particular interest in studying shear localization in the lab is to contribute to understanding the physical basis for geologic observations of shear localization, the relation of degree of localization to the rate dependence of fault strength. This work also addresses the role of localization in shear heating and dynamic weakening. We expect our largest contribution to SCEC will be to the long term research goal to develop a full 3D model of fault-zone structure that includes the depth dependence of shear localization and damage zones, hydrologic and poroelastic properties, and the geometric complexities at fault branches, step-overs, and other along-strike and down-dip variations.
Broader Impacts The broader aspects involve developing an understanding of the causes and effects of shear localization in faults. It has implications for field observations and theoretical modeling of dynamic rupture. Understanding localization has societal implications for the magnitudes of stress drops and the resultant strength of ground motions from earthquakes.
Exemplary Figure Figure 6. CAD assembly drawing of lower sample grip with jigs for loading synthetic rock gouge and Teflon powder while keeping them separate. The lower steel sample grip is in grey with horizontal hashing. The lower rock forcing-block in grey is epoxied into an annular groove in the sample grip. The one-mm-thick layer of rock gouge is in speckled grey. Above it in white is a temporary Teflon compactor/spacer. In red, outside and inside the gouge sample is the Teflon powder. Above and below that in white are the solid Teflon rings similar to those now in use except their axial dimension is only as long as the amount by which the forcing blocks project above the lower and (not shown) upper steel sample grips. Outside and inside of the outer and inner Teflon rings and powder are the Viton O-rings that seal the confining-pressure gas from the sample. Between the rock gouge sample and the Teflon powder are temporary stainless-steel shims, shown in brass color, that separate the rock powder from the Teflon powder. Another pair of shims separate the Teflon powder from the O-rings, and these are attached to outer (shown in violet) and inner (shown in transparent grey) rings. When the rock and Teflon powder is loaded into the annular spaces between the shims, only these shims, the lower forcing-block, and the lower solid Teflon rings are in place. The rock power is compacted with the white Teflon compactor/spacer and the Teflon powder is compacted with the two grey steel compactor/spacer rings presently shown above the upper solid Teflon rings. Once the powders are loaded, the upper solid Teflon rings are installed, followed by the three compactor/spacer rings. Then the three transparent-beige-colored hold-down armature pieces are installed and bolted down via the central grey shaft. Those and the outer and inner green-colored _O-ring hold-down pieces prevent the powders and the O-rings from moving upward when the four annular shims are withdrawn. The withdrawal is accomplished with the transparent-violet-colored clamps and clamp base. Once the shims are withdrawn and the O-ring hold-downs and the three compactor/spacer rings are removed, the top sample grip with its rock forcing block (neither are shown) is lowered into contact with the powders and O-rings and the assembly is ready to be installed in our high-pressure rotary-shear testing apparatus.