Fault Zone Structure and Rupture Behavior with Fiber-Optic Sensing and Second Moments

Citation

Atterholt, James W. (2025) Fault Zone Structure and Rupture Behavior with Fiber-Optic Sensing and Second Moments. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/29nc-rd93. https://resolver.caltech.edu/CaltechTHESIS:07232024-193557367

Abstract

The structure of fault zones and the behavior of ruptures are indivisible. Fault structure is molded by the continued overprinting of slip events, and rupture propagation is highly sensitive to fault zone parameters. Observational constraints on both fault zone characteristics and the behavioral response of ruptures to fault variability are thus needed to understand earthquakes. Fault zones are narrow structures that are difficult to image in detail, particularly at depth. This means that fault structure is often oversimplified in rupture models and inversions. Earthquake source descriptions are frequently high dimensional. Fault slip distributions are often complicated and nonunique and seismicity catalogs can contain hundreds of thousands of events. This complexity can be difficult to reduce for the purpose of making clear conclusions on earthquake phenomenology. In this sense, observations of fault structure may benefit from a dimensionality expansion and observations of earthquakes may benefit from a dimensionality reduction. In Chapters 2-5 of this thesis I address the former problem. I show how an emergent technology, distributed acoustic sensing (DAS), that transforms fiber optic cables into dense arrays of strainmeters can be used to resolve fault zone characteristics with astonishing detail. This applies to small and large faults at shallow and deep depths. I define a framework for partitioning the seismic wavefield and show that scattered phases in earthquake wavefields encode information on the location and dimensions of small faults. I then investigate the Garlock Fault with an intersecting DAS array and uncover structural variability across the fault at shallow and seismogenic depths. I also use this array to investigate Moho topography, and find that the Garlock Fault offsets and, by extension, penetrates the mantle over a narrow width. In Chapters 6-8 of this thesis I address the latter problem. I show that second moments, both of source and seismicity distributions, can improve the clarity of observations and make drawing meaningful conclusions about rupture behavior possible. I first develop a framework to probabilistically invert for the second moments of source distributions and use it to investigate all large strike-slip events of the past three decades. These solutions show several patterns between faults and rupture behavior. I also create a seismicity catalog for the Ridgecrest earthquake sequence and use a second moment measure to constrain the evolution of fault orientation and the stress state.

Thesis Files