Understanding Fault Creep From the Macroscopic to the Microscopic Scale
- Author(s): Swiatlowski, Jerlyn L
- Advisor(s): Funning, Gareth J
- et al.
Fault creep is a behavior of some faults where the two sides of a fault slowly slide past one another in the absence of large earthquakes. This is a form of aseismic slip and can be continuous or episodic. Understanding what drives fault creep is important as it can reduce the amount of strain that accumulates in the crust and can impede future earthquake ruptures. Both of these aspects play an important role in analyzing the seismic hazard of a region, therefore mapping the extent of where fault creep is occurring and identifying common lithology between creeping faults is necessary for accurate hazard assessment.
In this dissertation, we focus on fault creep in northern California using three different methods -- satellite imagery, 3D models derived from photographs, and rock mechanics experiments. In the first project, we focus on mapping the extent of fault creep along on two faults in the North Bay (north of the San Francisco Bay Area), the Maacama and Rodgers Creek faults. Both faults have observations of fault creep at specific locations but the extent and variability of fault creep on each fault is not well mapped. We use Interferometric Synthetic Aperture Radar (InSAR) to map the extent of fault creep along both faults and estimate at least 55% of the Maacama fault and 40% of the Rodgers Creek faults are creeping at the surface.
The second project focuses on offset sidewalk observations along the southern Hayward Fault, known to be creeping at rates up to ~8 mm/yr and how fault creep is being expressed in an urban setting. We construct 3D models from 2D photos taken from 2015 to 2018 to measure 3D displacements of each offset sidewalk. In this way we can monitor how the fault is being expressed in an urban environment due to the creeping fault. We find that on average, individual offset curbs sample < 40% of the overall creep rate measured from nearby alignment arrays (which span a fault-perpendicular distance of 100 m or more). In some locations, multiple adjacent curbs are actively deforming. These findings imply that there is significant off-fault deformation along the southern Hayward fault, and suggests that the `fault trace' can more correctly be considered a zone of deformation, narrower than an alignment array width but wider than one curb length.
The third and fourth projects center on an exposure of the Bartlett Springs fault core near Lake Pillsbury, which is known to be creeping at 3.4 mm/yr from a nearby alignment array. We collected the fault gouge and ran two sets of experiments to investigate the frictional and mineralogical properties needed for a fault to creep. The first set of experiments investigated which of the minerals that we found in the fault gouge, are promoting or not promoting fault creep. We find that the presence of talc has the strongest influence on creep behavior. The second set of experiments explored the frictional properties of the natural fault gouge and compared their frictional properties to samples collected from the creeping section of the San Andreas fault. We found that both fault gouges have similar compositions and frictional properties. In order for fault creep to occur on the Bartlett Springs fault, we estimate that there needs to be at least 50% talc in the gouge, concentrated into layers in which the majority of shear is taking place.
Through the various approaches to understanding fault creep used in this dissertation and the compilation of previous studies using various techniques (e.g. GPS-derived models, seismology, additional geodetic observations), we have a better understanding of fault creep mechanisms and distribution in the North Bay. We estimate a larger extent of the Maacama and Rodgers Creek faults to be creeping than previously observed, supplementing previous estimates from repeating earthquake families, GPS-based models, alignment arrays, and prior InSAR studies in the area. The measurement of offset sidewalks along the Hayward fault shows along-strike variations in the rates of movement but also allows us to quantify how much of the total creep occurs in a narrow zone near the mapped surface trace (3-10 meters wide) versus the wider zone measured by alignment arrays (~100 meters wide). The low frictional strength of talc within the Bartlett Springs fault gouge is the main driving factor of fault creep on the fault, and can promote creep at depths up to 9 km. The mineral assemblage is also similar in its elemental composition to the fault gouge collected within the creeping section of the San Andreas fault, suggesting that the creep in both places is controlled by a similar lithology sampled by each fault. This suggests that the Bartlett Springs fault may also be creeping at deeper depths, up to 9 km, consistent with the depths of repeating earthquakes located on the fault.