SCEC Award Number 19117 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Experimental Study of Thermal Pressurization and the Role of Fault Roughness
Investigator(s)
Name Organization
Terry Tullis Brown University
Other Participants Nir Badt
Nick Beeler
SCEC Priorities 3f, 2c, 2d SCEC Groups FARM, Seismology, Geology
Report Due Date 04/30/2020 Date Report Submitted 04/18/2020
Project Abstract
 This project investigates the high-speed frictional weakening mechanism of thermal pore-fluid pressurization using laboratory experiments. One goal has been to determine whether faults with roughness similar in amplitude to natural faults could cause enough dilatancy to inhibit the theoretically predicted weakening. We have shown that on initially flat surfaces thermal pressurization weakening does occur, although the behavior is more complex than is included in current theoretical models. Our experiments on surfaces with initially realistic roughness characteristics show that the behavior differs from that seen for initially flat surfaces. It is too early in our study of these rough surfaces to be able to characterize their behavior with confidence. Our initial expectation is that thermal pressurization weakening will be decreased due to dilatancy that should counteract the thermal pressurization. However, the results of the two initial experiments show unexpected weakening which may or may not be due to thermal pressurization. We hope that the series of experiments we are currently undertaking as a continuation of this project will allow us to understand the role that fault roughness may play on thermal pressurization weakening. This is intended to help determine whether this weakening mechanism is a viable one to use in understanding dynamic weakening during earthquakes.
Intellectual Merit The project increases our understanding of the physics of the earthquake process, one of the prime goals of SCEC. This high-speed weakening mechanism has been investigated in many theoretical studies but our research represents the most definitive experimental study of the process and is showing that some important effects are not yet included in the existing theoretical analysis.
Broader Impacts One of the major contributions of SCEC is to perform complex numerical calculations of dynamic rupture during an earthquake and thereby increase our understanding of the origins of strong damaging coseismic ground motions. The stress resisting slip during an earthquake, together with the ambient stress prior to the earthquake, determine the earthquake’s dynamic stress drop and through that the intensity of strong ground motions. Some of the most advanced of these dynamic rupture calculations employ extreme frictional weakening due to the process of thermal pressurization. If this weakening did not occur or were less extreme it would be one factor that would decrease strong ground motions in such models. Our lab measurements are intended to determine whether the theoretical predictions of the amount and rate of weakening due to this mechanism are in fact found experimentally, and are therefore realistic to include in rupture simulations, or whether the predictions need to be altered.
Exemplary Figure Figure 4. Results of an experiment on rough mated surfaces with 9 wavelengths, each having an amplitude to wavelength ratio of 0.0014. The sample has the same permeability and the conditions of the experiment were the same as for experiment 340. See text for discussion of the various curves and the implications of the results.