LABORATORY STUDIES OF FAULT RESTRENGTHENING: The Role of Shear Stress and Mineralogy Inferred from Mechanical and Ultrasonic Amplitude Measurements
Open Access
- Author:
- Ryan, Kerry
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 02, 2018
- Committee Members:
- Chris Marone, Dissertation Advisor/Co-Advisor
Chris J Marone, Committee Chair/Co-Chair
Demian Saffer, Committee Member
Charles James Ammon, Committee Member
Parisa Shokouhi, Outside Member - Keywords:
- fault healing
frictional healing
earthquakes - Abstract:
- During the seismic cycle, faults repeatedly fail and regain strength. The gradual strength recovery is often referred to as frictional healing, and existing works suggest that healing can play an important role in determining the mode of fault slip ranging from dynamic rupture to slow earthquakes. Laboratory studies can play an important role in identifying the processes of frictional healing and their evolution with shear strain during the seismic cycle. These studies also provide data for laboratory-derived friction constitutive laws, which can improve dynamic earthquake models. In this dissertation, I aim to understand how shear stress impacts frictional restrengthening processes in a variety of fault materials. Specifically, how does stress dependent healing differ between Westerly granite bare surfaces and Westerly granite gouge and how does the presence of phyllosilicates influence shear stress dependent healing behavior. To investigate these questions, I utilize both traditional frictional analysis and work to understand if and how ultrasonic amplitude measurements can add to our understanding of healing processes. In Chapter 1, I investigate the difference in healing mechanisms between Westerly granite bare surfaces and Westerly granite gouge. I find that the shear stress dependence previously observed in granular quartz holds true for Westerly gouge and likely arises from the granular nature of gouge material as this behavior is not observed in bare surface experiments. In Chapter 2, I investigate how mineralogy impacts shear stress dependent frictional behavior with the addition of phyllosilicates by analyzing the frictional healing behavior of synthetic quartz and smectite mixtures along with several natural fault gouges. The interplay between frictionally strong materials (quartz, feldspar, and calcite) and frictionally weak materials (smectite, illitie, and kaolinite) is complex but I find that phyllosilicates can significantly reduce the overall frictional strength and restrengthening of synthetic quartz gouge mixtures. The presence of 30% smectite in synthetic mixtures can entirely remove the shear stress dependence observed in quartz. Finally, Chapter 3 focuses on ultrasonic amplitude measurements and understanding how they vary as a function of normal stress and layer thickness. In Chapters 1 and 2 we use ultrasonic amplitudes to continuously probe the frictional state of laboratory faults and in Chapter 3 we work to understand how we can interpret the changes in amplitude we observe during biaxial experiments in the earlier chapters. This dissertation provides insight to the complex healing behaviors that may arise based on stress state and mineralogy of faults. It also presents lab scale relationships for amplitude based on normal stress and layer thickness which help us to understand the conditions in which ultrasonic amplitude can be used to monitor frictional state.