Date of Award:
12-2026
Document Type:
Dissertation
Degree Name:
Doctor of Philosophy (PhD)
Department:
Geosciences
Committee Chair(s)
Alexis K. Ault
Committee
Alexis K. Ault
Committee
Srisharan Shreedharan
Committee
James P. Evans
Committee
Anthony R. Lowry
Committee
Emily H.G. Cooperdock
Abstract
Earthquakes occur when rocks on either side of a fault suddenly slip, releasing stress that has built up over time. But not all faults behave the same way. Some move suddenly in earthquakes, while others slowly creep, releasing stress gradually without producing damaging shaking. Understanding why faults behave differently, and how that behavior changes across space and over time, is critical for anticipating where and how future earthquakes might occur. This dissertation investigates these questions using two approaches: laboratory experiments that simulate fault slip, and a technique called thermochronometry, which uses the cooling history encoded in minerals to reconstruct how rocks have moved toward Earth's surface over millions of years from faulting. The research focuses on two faults within the San Andreas fault system: the Bartlett Springs fault, which creeps, and the San Felipe fault, which does not.
At the Bartlett Springs fault, laboratory experiments show that clay minerals produced by mixing and chemical alteration within the fault are responsible for its creeping behavior. Rock samples containing more clay minerals are consistently weaker and slide more easily, regardless of how much sliding they have experienced beforehand. This suggests the presence and connectivity of clay, rather than the alignment of grains that develops as a fault slips, is the primary factor controlling whether a fault creeps or hosts earthquakes.
At the San Felipe fault, the cooling history of minerals reveals that the fault core experienced a pulse of rapid exhumation when it first formed ~1.5 million years ago, which then slowed over time. Comparing this ancient record with present-day satellite measurements of ground movement suggests this early exhumation was driven by earthquakes, and that faults across the region behaved in a coordinated way in response to plate boundary shifting.
Finally, this work improves a widely used dating method by showing a visible darkening of mineral grains records the internal crystal damage that governs the method, which helps scientists select better samples for reconstructing rock cooling histories. Together, these studies show both the minerals within a fault and its interactions with neighboring faults shape how faults slip over time.
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Recommended Citation
Armstrong, Emma M., "Spatiotemporal Evolution of Fault Deformation Along the San Andreas Fault System Using Friction Experiments and Low-Temperature Thermochronometry" (2026). All Graduate Theses and Dissertations, Fall 2023 to Present. 949.
https://digitalcommons.usu.edu/etd2023/949
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