SCEC Award Number 14074 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Defining asperities on faults from spatial variations in fault gouge thickness: Implications for fault strength and rupture mechanics
Name Organization
James Kirkpatrick Colorado State University
Other Participants
SCEC Priorities 4b, 3b, 3c SCEC Groups FARM, Geology, CS
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
This project aimed to measure the geometry of the Boyd fault, southern California, to establish the dimension of contact asperities and how the fault roughness evolves with displacement. We mapped the internal structure of the fault using the structure-from-motion methodology. Photographs were taken of extensive cross-sectional exposures of the fault and integrated into a 3-D model using Agisoft’s PhotoscanPro. The resulting models were rotated to view the fault down the slip direction and high-resolution images were exported and used to map the edges of the fault core and the principal slip zone, and the boundaries between abandoned principal slip zones within the gouge-filled fault core. Spectral analysis of these contacts showed that the fault slip surfaces smoothed as the fault evolved. However, the smoothing trend is non-linear and scale dependent due to competition between smoothing and reroughening processes. Clasts present within the fault gouge provide a record of the asperities sheared off during deformation. Scale dependent reroughening arises as clasts are sheared off, the fault surface roughens at a scale smaller than the clast dimension, but smoothes at a scale larger than the clast dimension. The maps of the fault core also define the scale of asperities across the slip zone. Geostatistical analysis shows a characteristic length scale of around 3 m where the thickness of the principal slip zone diminishes to sub-millimeter. Together, these results suggest a change in the physical process controlling fault slip occurs after displacements of the order of a few meters.
Intellectual Merit The intellectual merit of this study is that the results provide a new definition of wear processes on fault surfaces and how they may contribute to fault and rupture mechanics. Using spatial variations in fault core thickness, we have found a characteristic length scale in the fault system defining the length scale of asperities for the first time from field observations. The results also show that fault geometry evolves as faults mature. These observations may explain differences in the source parameters between small and large magnitude ruptures. Our results all contribute to SCEC objective d: Structure and evolution of fault zones and systems: relation to earthquake physics. The insights into rupture mechanics and quantification of asperity dimension are relevant to objective c: Evolution of fault resistance during seismic slip: scale appropriate laws for rupture modeling.
Broader Impacts The broader impacts of the work include field training and experience for a graduate student at Colorado State University, who has now successfully defended her master’s and is teaching at the State University of New York this year, and an undergraduate student who is now a graduate student at Colorado School of Mines. Their work in developing and testing the structure-from-motion methodology for this application should prove to be transferrable across the geosciences.
Exemplary Figure Figure 1. Example of the field workflow used in this study: a.) Photograph of an exposure of the Boyd fault, CA. Around 150 photos similar to this were used to construct the outcrop model. b.) Model generated with Agisoft’s PhotoscanPro shown from the same perspective as a. Boxes in a and b show the extent of c. c.) Rectified image exported from PhotoscanPro after the model was rotated to view the exposure down the slip vector. Lines show traces mapped in the field that were used to calculate roughness. White lines are the edges of the principal slip zone and green lines define the extent of the fault core.
Credit: This figure was prepared by Kate Shervais and will be submitted as part of a manuscript presenting the results in the next few weeks.