Earthquake behavior and structure of oceanic transform faults

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2012

Material presented in this thesis is based on work supported by the National ScienceFoundation Division of Ocean Science (OCE) grants #0548785, #0623188, #0649103, and#0242117 and Division of Earth Sciences (EAR) grants #0814513 and #0943480. This workwas also supported by the W. M. Keck Foundation and the Deep Ocean Exploration Institute.

Oceanic transform faults that accommodate strain at mid-ocean ridge offsets represent aunique environment for studying fault mechanics. Here, I use seismic observations andmodels 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) transformsexhibit characteristics consistent with the rupture propagation velocity of shallow aseismiccreep transients. I also develop new thermal models for the ridge-transform faultenvironment to estimate the spatial distribution of earthquakes at transforms. Assuming atemperature-dependent rheology, thermal models indicated that a significant amount of slipwithin the predicted temperature-dependent seismogenic area occurs without producinglarge-magnitude earthquakes. Using a set of local seismic observations, I consider howalong-fault variation in the mechanical behavior may be linked to material properties andfault structure. I use wide-angle refraction data from the Gofar and Quebrada faults on theequatorial EPR to determine the seismic velocity structure, and image wide low-velocityzones at both faults. Evidence for fractured fault zone rocks throughout the crust suggeststhat unique friction characteristics may influence earthquake behavior. Together, earthquakeobservations and fault structure provide new information about the controls on fault slip atoceanic transform faults.