SCEC Award Number 17242 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Ultrasonic investigation of role of fault zone mineralogy and maturity on frictional processes occurring in natural fault gouges
Investigator(s)
Name Organization
Ahmadreza Hedayat Colorado School of Mines
Other Participants One PhD student and two undergraduate students
SCEC Priorities 3d, 3c, 1d SCEC Groups FARM, SDOT, SAFS
Report Due Date 06/15/2018 Date Report Submitted 06/13/2018
Project Abstract
 The main objectives of this one year research project were to (a) develop a novel direct shear apparatus for ultrasonic investigation of shear behavior of granular fault gouges and (b) investigate the fault gouge’s mineralogy and maturity impact on stability of slip and the associated signatures of compaction and grain-scale shearing on the geophysical data (SCEC research priorities P3.d; P3.c; and P1.d). A direct shear apparatus instrumented with a fast ultrasonic imaging system was built and used to perform velocity-step shear tests on gouges made of quartz. During the compression stage of the test, an increasing trend was observed for P-wave velocity, transmitted amplitude and dominant frequency with normal stress. The changes in the transmitted amplitude were linked to the changes in the true contact area between the particles with a transitional point in the slope of normalized amplitude, coinciding with the yield stress of the granular soil layer. The amount of grain crushing as a result of increase in the normal stress was experimentally measured and a linear correlation was found between the degree of grain crushing and the changes in the normalized dominant frequency of compressional waves. During the shearing stage of the tests, the variations in ultrasonic waves were monitored and compared during both velocity weakening and velocity strengthening phases. Our results indicated that changes in transmitted amplitude are related to changes in contact area between the particles, while the velocity variations are related to changes in contact stiffness between the particles.
Intellectual Merit This project allowed for developing a unique direct shear apparatus for direct observation of micromechanical processes at the grain scale in gouge materials. Such capability has the potential to illuminate the processes at the grain scale that are otherwise invisible to traditional characterization techniques. Our measurements of gouge materials have direct implications for the natural faults and the seismic cycle. Our laboratory experiments directly measure the fault strength and its frictional evolution with slip and therefore can contribute to better understanding and interpretation of field data despite the differences in level of applied normal stress, material, temperatures, and time scales over which changes are measured. As identified in SCEC5’s research priority P3.d, determining how fault zone mineralogy governs the stability of slip, interseismic strength recovery and rupture propagation is critically important and this research documented both stable sliding and stick-slip modes of failure for gouge materials as a function of the applied shearing velocity.
Broader Impacts This SCEC project provided funding for training of one graduate student, Amin Gheibi, who is currently conducting his PhD at the Colorado School of Mines on geophysical investigation of friction in granular media. This project also provided an opportunity for a minority student, Sydney Slouka, to join our research team and participate as an undergraduate student researcher in conducting the experiments, analyzing the results, and publishing the work in the form of a conference proceeding. This activity also supported the PhD student’s and PI’s travel to attend SCEC annual meeting in 2017 to continue their interaction with the SCEC community. The direct shear apparatus developed as part of this project has significantly increased the experimental capabilities in our soil and rock mechanics laboratories and now allows us to conduct shear experiments with simultaneous non-destructive monitoring of state of contact between the grains during both compaction and shearing. Due to the presence of gouge materials in faults, the frictional behavior of the fault is dominantly controlled by the gouge and the underlying processes that occur in the gouge materials during shearing are still poorly understood. Thus, there is a critical gap in the knowledge that pertains to the evolution of fault frictional strength and the time-, slip-, and velocity-dependent processes that occur in natural gouge materials and result in different slip modes. The research demonstrated the potential in using ultrasonic technique in illuminating the key microphysical processes occurring in granular gouges. This research effort has important societal implications for seismic hazard assessment and improved fundamental understanding of earthquake processes and nucleation.
Exemplary Figure Figure 3. (a) Shear stress-displacement results for three identical gouge layers of quartz No. 1 with different normal stresses; (b) velocity strengthening and weakening behavior as a function of normal stress and sliding velocity; (c) variations of shear stress, normalized transmitted amplitude, and sliding velocity with time in a velocity step from 5 to 1 µm/s under 25 MPa normal stress; and (d) variations of shear stress, normalized transmitted amplitude, and sliding velocity with time in a velocity step from 20 to 40 µm/s under 25 MPa.