SCEC Award Number 13155 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Laboratory Experiments on Fault Shear Resistance Relevant to Coseismic Earthquake Slip
Investigator(s)
Name Organization
David Goldsby Brown University Terry Tullis Brown University
Other Participants Dr. Tom Mitchell, INGV-Rome
SCEC Priorities 3, 4, 6 SCEC Groups FARM, Seismology
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 During recent years we have conducted research to understand three potential dynamic fault-weakening mechanisms: silica gel weakening, flash heating and thermal pore-fluid pressurization. This year we have focused on thermal pore-fluid pressurization. We are alone in possessing the unique experimental capabilities and experience gained in the last few years to effectively study this mechanism in the laboratory. Our high-pressure rotary-shear apparatus is the only machine in existence that combines arbitrarily large slip displacement with independent control of both confining pressure and pore pressure. In other words, ours is the first study in which this mechanism has been isolated and is beginning to be characterized under controlled conditions on confined samples.
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. Furthermore, we demonstrated the critical role played by dilatancy in determining whether or not fluid pressurization is effective, and designed an experimental protocol for limiting the negative effects of dilatancy on thermal pressurization. During 2013, we followed that protocol and conducted experiments on bare surfaces of Frederick Diabase. Based on finite element model (FEM) calculations of the temperatures achieved in our experiments, we conducted tests at higher normal stresses (~100 MPa) to increase the degree of heating and hence the magnitude of thermal pressurization. We have been successful in activating thermal pressurization in our experiments as comparison of our mechanical results with theoretical predictions 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 coseismic 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 3 – LEFT- Plot of inferred pore-fluid pressure for diabase experiment assuming that all changes of shear stress were due to changes in fluid pressure, i.e., assuming the effective stress law holds for the experiment. Inferred temperatures calculated using our FEM model are also shown in the data plot. RIGHT – a fit of the initial part of the shear stress decay to equation B19 of Rice [2006] that predicts the decay of shear stress τ from its initial value τ in the case of slip on a plane. Although the theoretical prediction continues to decline, after about 28 mm of slip the experimental data level out, which we infer is due to the steel sample grip preventing the temperature from rising as it would for the rock half-space assumed by the analysis of Rice [2006]. The right half of the figure is courtesy of John Platt of Harvard, and shows his estimate of the slip at which the steel becomes important estimated from diabase thermal properties and the distance from the fault to the steel.