SCEC Award Number 16059 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Physical controls of spontaneous and triggered slow-slip and stick-slip at the fault gouge scale
Name Organization
David Goldsby University of Pennsylvania
Other Participants Dr. Behrooz Ferdowsi
SCEC Priorities 2f, 3f, 5c SCEC Groups FARM, SDOT, Seismology
Report Due Date 03/15/2017 Date Report Submitted 11/18/2018
Project Abstract
In this project, we explored the friction dynamics and rate- and state-dependent behavior of a sheared fault gouge, and its relevance for fault mechanics and rupture dynamics. By conducting discrete element models of gouge deformation and dynamics, we sought to link the comparatively mature field of granular physics with fault mechanics. We explored the dynamical and frictional behavior of a granular fault gouge layer with a finite spring stiffness (Figure 1B of the attached Report) and compared the behavior to that of a system with an infinite-stiffness spring. We found that a finite spring stiffness is a necessary condition for exploring the dynamical phase space of friction behavior, from steady-sliding to stick-slip and amorphous creep, and for adding a granular physics perspective to the solid-on-solid friction dynamics phase space as discussed in the attached Report. We measured the variation of local shear stress versus local slip velocity in the granular gouge layer for different regimes of frictional behavior (creep, stick-slip, steady-sliding), as shown in Fig. 3B of the Report. The findings in this figure suggest that creep, slow-slip, and transient stick-slip behaviors are all governed by the same physical mechanisms (granular rheology), and that they might describe a unified frictional dynamics phase space. This finding is in agreement with recent experimental observations from Leeman et al (Nat. Commun, 2016) and Scuderi et al (Nat. Geo., 2016).
Intellectual Merit This research project helped to merge the fields of granular physics and fault mechanics through discrete element modeling of fault gouge. A primary goal of the study was to bring new perspectives and methods from the field of granular physics to understand rate and state friction laws, which, despite their successes, lack a robust physical basis. This goal aligns well with SCEC's goal of developing a physics-based understanding of the earthquake cycle.
Broader Impacts This project contributed greatly to the education and development of Dr. Behrooz Ferdowsi, a physicist, and helped to introduce him to the "earthquake problem". The project helped Ferdowsi adapt discrete element methods from the field of granular physics to studies of fault gouge. Those methods continue to be employed by Ferdowsi in his current research on the frictional behavior of fault materials. The project also introduced PI Goldsby to the field of granular physics.
Exemplary Figure We suggest using Figure 3 from the Final Report.

Figure 3. (A) The evolution of frictional behavior from stick-slip, to transitional stick-slip and steady-sliding as shearing velocity is increased. These friction signals belong to the system with an infinite spring stiffness (Fig. 1C). (B) measurements of local friction coefficient, and local sliding speed for different regimes of frictional behavior. Panels (C) and (D) show the variation of local friction coefficient versus local inertial number defined in the earlier part of this Report, for different regimes of frictional behavior, plotted with semi-log and log-log axes, respectively. This observation suggests that different frictional regimes at the scale of fault gouge all stem from a rheological description of granular materials.