Date of Award:

5-2020

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

Alexis K. Ault

Committee

Alexis K. Ault

Committee

Dennis L. Newell

Committee

James P. Evans

Abstract

Earthquakes occur on faults, or rock that has experienced displacement at depth. Experimental work on a range of rock types reveals that >90% of earthquake energy on fault surfaces is given off as heat. Heat weakens rock and promotes earthquake rupture propagation. Thin (<0.5mm), high-gloss, "polished", light-reflective exposed fault surfaces are called fault mirrors (FMs). Fault mirrors may record rapid thermal, textural, and chemical changes that occur during an earthquake event.

The Wasatch Mountains are a N-S trending mountain range in Northern Utah that are the backdrop for Salt Lake City, Provo, Ogden, and Brigham City. This mountain range is currently built by the active Wasatch fault, and includes the exposed Wasatch fault damage zone. Many segments of the Wasatch fault are overdue for a potentially catastrophic earthquake (> M 6.7) in the next 50 years. This is significant because over 80% of Utah’s population lives in the shadows of the Wasatch mountain range and are presently underserved in the recognition and mitigation of earthquake hazards.

The goal of this thesis was to investigate exposed silica-rich FMs that cut granite to better understand the earthquake deformation processes at the nano- and microscales occurring on small faults in the Wasatch fault damage zone. This study integrates a variety of field and laboratory techniques to characterize the textures and chemistry of silica-rich FMs to infer high temperatures during fault slip. The results herein document variable FM particle morphologies, textures, and chemistries, which are consistent with observations from previous work that are linked to seismicity and heat. We interpret that silica-rich FMs are derived from the adjacent granite and experienced transient high temperatures (>1000 ̊C) due to shear heating at the fault surface during past earthquakes. The abundance of silica FMs, together with hematite FMs, suggests that these thin, localized slip surfaces likely accommodated ancient earthquake aftershocks in the Wasatch fault damage zone.

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