SCEC Award Number 14094 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Dynamic frictional behavior of SAFOD core materials under relevant normal stress, slip rates, and slip acceleration
Investigator(s)
Name Organization
William Griffith University of Texas at Arlington Vikas Prakash Case Western Reserve University
Other Participants
SCEC Priorities 3a, 3c, 3e SCEC Groups FARM, SDOT, Geology
Report Due Date 03/15/2015 Date Report Submitted 10/14/2015
Project Abstract
 Field observations of mature crustal faults indicate that slip in individual events occurs primarily within a thin shear zone, < 1–5 mm, within a finely granulated, ultracataclastic fault core. Within this nominal shear zone, most of the shearing is accommodated within a zone of extreme shear localization with a thickness of 100–300 μm. The evidence for narrowness of the shear zone suggests that unless we appeal to some mechanism to rapidly diminish fault strength as slip accumulates the temperature rise in the shear zone will far exceed those for the onset of melting. However, evidence for melting in the form of pseudotachylytes has been rare. These observations provide compelling evidence that fault strength cannot remain constant during earthquake slip, at least at a static friction coefficient of ~ 0.6 or higher, but must weaken substantially to explain the apparent absence of melt. Several mechanisms have been proposed as sources of this dynamic weakening including flash heating and/or melting of asperities caused by adiabatic heating. In the present study, a modified torsional Kolsky bar apparatus has been designed and built to investigate the dynamic frictional behavior of confined rock gouge (including gouge prepared from SAFOD cuttings and core materials) under seismically relevant normal stresses slip velocities and accelerations. Moreover microstructural analysis of sheared gouge material is being conducted to reveal principal slip zones as well as the alterations in grain shape and size with slip.
Intellectual Merit Past experimental studies of frictional behavior of SAFOD core and cutting material, including work conducted under the inter-laboratory strength and friction collaboration (e.g., Logan et al., 2010) were conducted at either slow slip rates at realistic normal stress conditions or at fast slip rates at relatively low normal stresses. Activation of various thermal weakening mechanisms may be enhanced at high slip rates AND/OR large normal stresses (e.g., Di Toro et al., 2011) -- and, therefore, knowledge of frictional characteristics of SAFOD materials at relevant interfacial slip conditions (normal stresses, slip rates, AND acceleration) has the potential to yield im-portant results that may be critical in assessing potentially catastrophic fault ruptures.
Broader Impacts This project has supported the participation of one graduate student, Binoy Rodrigues, at CWRU, and one graduate student, Christopher Borjas, at UT-Arlington. Furthermore, the project also dovetails with a simultaneous project sponsored by the NSF to develop the testing apparatus, thus supporting an NSF-sponsored research and education infrastructure.
Exemplary Figure Figure 1. Design of the modified torsional Kolsky bar apparatus (top) with a typical dataset (bottom left, in this case slip velocity and frictional resistance histories for three dry talc gouge samples at constant normal stress of 80MPa), and schematic representations of the new gouge holder (bottom center and bottom right) developed at Case Western Reserve University.