SCEC Award Number 16026 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Laboratory Experiments on Fault Shear Resistance Relevant to Coseismic Earthquake Slip
Name Organization
Terry Tullis Brown University
Other Participants
SCEC Priorities 3a, 3c, 3e SCEC Groups Seismology, Geology, FARM
Report Due Date 03/15/2017 Date Report Submitted 04/28/2017
Project Abstract
We continue to focus 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). This year we have modified our approach by doing experiments on essentially impermeable diabase instead of performing our previous heat-treatment to cause increased and controlled permeability. We have focused on developing techniques and in performing test experiments in which the fault surfaces have realistic roughness and are initially mated. The purpose is to determine whether the dilatancy that is likely to result from sliding such realistically rough surfaces produces enough volume near the fault surface to more than make up for the increase in fluid volume due to shear-heating-induced thermal expansion of the pore fluid and thereby eliminate thermal pressurization. We find significant dilatancy, but less than would occur with rigid rocks.
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 and in public lectures. The experiments have resulted in new methods of sample preparation that allow us and others to conduct experiments on mated fault surfaces and they 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 5. Results from experiment 338pfp, the preliminary experiment done on mated surfaces. The samples were slid 154.6 mm, one full revolution from their initial mated configuration. The observed dilation in blue is the down-ward movement of the lower piston by an external LVDT since the internal LVDT was off-scale. This is smaller than the predicted dilation for rigid samples in green (the curve of Figure 1B reversed due to sense of rotation used). The load-point displacement is based on the driving speed and elapsed time since neither slip-displacement transducer was working.