SCEC Award Number 15031 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Laboratory Experiments on Fault Shear Resistance Relevant to Coseismic Earthquake Slip
Investigator(s)
Name Organization
Terry Tullis Brown University
Other Participants
SCEC Priorities 3a, 3c, 3e SCEC Groups FARM, Seismology, Simulators
Report Due Date 03/15/2016 Date Report Submitted 03/07/2016
Project Abstract
 After focusing our attention for several years on silica-gel weakening and flash weakening we turned our attention to the widely discussed dynamic weakening mechanism termed thermal pore-fluid pressurization. There are many theoretical and numerical studies of thermal pressurization and it is increasingly used in dynamic rupture and earthquake nucleation models. However, experimental data suggesting the operation of this mechanism is limited, a shortcoming we are working to resolve. The only machine in existence capable of doing the required experiments is our unique high-pressure rotary shear friction machine that combines arbitrarily large slip displacement with independent control of both confining pressure and flow-through pore pressure capability. Ours is the first study in which the thermal pressurization mechanism has been isolated and is beginning to be characterized under controlled conditions on confined samples (e.g., with fully saturated rocks of controlled fluid pressure and proscribed permeabilities of the rock samples). We have conducted experiments on water-saturated rocks with a range of permeabilities and find that varying amounts of weakening occurs. We have developed a successful protocol for thermally cracking our initially low permeability Frederick diabase samples (<10-23 m2) and measuring the resulting permeabilities in the 10-23 m2 to 10-18 m2 range. We have conducted finite element model (FEM) calculations of the temperatures achieved in our experiments. We have been successful in activating thermal pressurization in our experiments as comparison of our mechanical and thermal results with the theoretical predictions of Rice [2006, equations 23a and 23b] make clear.
Intellectual Merit The research contributes to our understanding of the earthquake energy budget, strong ground motions, and accelerations associated with earthquake faulting, by providing fundamental knowledge of the co-seismic shear resistance of faults.
Broader Impacts Results of our experiments are incorporated in coursework at Brown. The experiments have provided new sample fixtures and other enhancements to existing equipment that enhance the infrastructure for research and education. Society benefits from an acquisition of scientific knowledge and in improved understanding of earthquakes and how to mitigate their damage.
Exemplary Figure Figure 1. Fits of decay in shear stress τ(δ) to obtain Rice’s [2006] L* (see our equation (1)), following step increases in velocity V for experiment 319pfp at 3 different velocities. A single set of material parameters fits all three data sets strongly indicating that the weakening is due to thermal pressurization. Fits in red by John Platt.