Earthquake behavior and structure of oceanic transform faults

Abstract

Oceanic transform faults that accommodate strain at mid-ocean ridge offsets represent a unique environment for studying fault mechanics. Here, I use seismic observations and models to explore how fault structure affects mechanisms of slip at oceanic transforms. Using teleseismic data, I find that seismic swarms on East Pacific Rise (EPR) transforms exhibit characteristics consistent with the rupture propagation velocity of shallow aseismic creep transients. I also develop new thermal models for the ridge-transform fault environment to estimate the spatial distribution of earthquakes at transforms. Assuming a temperature-dependent rheology, thermal models indicated that a significant amount of slip within the predicted temperature-dependent seismogenic area occurs without producing large-magnitude earthquakes. Using a set of local seismic observations, I consider how along-fault variation in the mechanical behavior may be linked to material properties and fault structure. I use wide-angle refraction data from the Gofar and Quebrada faults on the equatorial EPR to determine the seismic velocity structure, and image wide low-velocity zones at both faults. Evidence for fractured fault zone rocks throughout the crust suggests that unique friction characteristics may influence earthquake behavior. Together, earthquake observations and fault structure provide new information about the controls on fault slip at oceanic transform faults.Thesis Supervisor: Jeffrey J. McGuire Title: Associate Scientist, Department of Geology and Geophysics, WHOI 4 Acknowledgments I have had the good fortune to work with many scientists who have impacted my experience as a graduate student, and the type of science I will do in the future. Firstly, I'd like to thank Jeff McGuire for sharing his enthusiasm for earthquake swarms, giving me access to the most interesting parts of the QDG dataset, and teaching me how to body surf (not to mention, model surface waves). I am indebted to Mark Behn, Greg Hirth, and Dan Lizarralde for imparting their own elegant strategies for approaching geodynamics, rock mechanics, and marine seismology. I'd also like to thank John Collins for his encouragement and useful discussions, and for trusting me to use the deck-box on the Thompson My family and closest family friends provided me with the confidence, energy, and will to keep persisting, especially through the difficult parts of the past half-decade of work this thesis represents. Equally as important, they have helped me to celebrate the triumphant parts, as I know they will continue to do in the future.Casey Saenger, my husband and best friend, deserves credit for keeping me in graduate school. He has served as my sounding board and singer-of-songs throughout the past many years. I dedicated all of the Love waves to him. that do not conform to spatial and temporal moment release patterns typically associated with large "mainshock" earthquakes. As our understanding of these different styles of fault slip improves, earthquake scientists are tasked ...

Spatial and Temporal Variations in Earthquake Stress Drop on Gofar Transform Fault, East Pacific Rise: Implications for Fault Strength

“…We also imposed a temporal constraint on EGFs by requiring that both the EGF and target event occurred either before or after the M W 6.0 mainshock because strong ground motion from the mainshock jolted some of the OBSs, changing sensor orientations (Roland, 2012). Temporal variations in the S wave coda for events at a few stations within the fault zone also necessitated that EGFs were close in time to the target event.…”