SCEC Award Number 12180 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Laboratory Experiments on Fault Shear Resistance Relevant to Co-Seismic Earthquake Slip
Investigator(s)
Name Organization
David Goldsby Brown University Terry Tullis Brown University
Other Participants Giulio Di Toro, University of Padua, Italy
Tom Mitchell, INGV, Rome
SCEC Priorities 3a, 3e, 3d SCEC Groups FARM, Seismology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 We have undertaken a systematic study of thermal pore fluid pressurization, a dynamic fault-weakening mechanism for which numerous theoretical analyses exist but for which no unequivocal experimental confirmation is available. Experiments to date underscore the critical effects of slip-induced compaction or dilation on pore-fluid pressure, and therefore the effective normal stress, and the tendency for pore fluids to pressurize within the fault zone. Experiments have not yet yielded significant weakening due unequivocally to thermal pressurization, due to 1) slip-induced dilation of the fault zone, and 2) insufficient shear heating. Dilation of the fault within the first 0.5 m of slip drastically lowers the fluid pressure and increases the effective normal stress (and therefore the shear stress), such that significant thermal pressurization does not occur. The observed dilation trends over the first 0.5 m of slip, followed by compaction with further slip, suggest a protocol that will maximize our ability to activate the thermal pressurization mechanism. Calculations of the rise in average fault surface temperature indicate that the heat generated in experiments conducted thus far may be too small to yield significant fluid pressurization. These thermal calculations suggest that higher normal stresses, easily achievable in our rotary shear apparatus, are required to effectively activate thermal pressurization. Using these two new approaches, we expect within the coming year to obtain the first laboratory evidence for thermal pore-fluid pressurization and begin to quantify the effects of dilation and/or compaction, normal stress, slip, and slip rate on its operation.
Intellectual Merit All of the dynamic fault weakening mechanisms we are studying, including thermal pressurization, may have profound implications for the magnitude of stress drops during earthquakes and consequently for the magnitude of strong ground shaking. The manner in which fault strength varies with displacement and slip velocity, as well as the rate at which healing occurs as slip velocity decreases behind the rupture tip, can control the mode of rupture propagation, i.e., as a crack or as a self-healing slip pulse. Furthermore, these data can be important for resolving questions concerning stress levels in the crust. If coseismic friction is low, and the magnitudes of dynamic stress drops are constrained to modest values by seismic data, then the tectonic stress acting on faults must also be modest. We may have a strong crust that is nevertheless able to deform by faulting under modest tectonic stresses if the strength is overcome at earthquake nucleation sites by local stress concentrations and at other places along the fault by dynamic stress concentrations at the rupture front. Thus, understanding high-speed friction is important not only for practical matters related to predicting strong ground motions and resulting damage, but also for answering major scientific questions receiving considerable attention and funding, e.g., the strength of the San Andreas Fault / the Heat-Flow Paradox, the question that ultimately was responsible for the SAFOD project.
Broader Impacts Discussion of dynamic fault-weakening mechanisms and their implications for earthquake source physics has been incorporated into the rock mechanics canon at Brown. That material is taught to the entire rock physics group (which includes three female grad students), in formal classes (e.g., Greg Hirth's Deformation Mechanisms course) as well as in various reading seminars. Tullis continues to give general audience talks to Brown alumni and other community groups about earthquakes. Goldsby gave a general audience talk at Penn in 2012 on frictional mechanics relevant to earthquakes. These activities serve to help educate the next generation of scientists, a benefit to our society.
Exemplary Figure Figure 2 and caption.
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