Electronic Theses and Dissertations

Date

2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Earth Sciences

Committee Chair

Thomas Goebel

Committee Member

Chris Cramer

Committee Member

Eunseo Choi

Committee Member

Mitchell Withers

Committee Member

Randel Cox

Abstract

Earthquakes occur across multiple spatial scales depending on fault zone characteristics and mechanics. Fault failure, the mechanism responsible for earthquakes, is driven by the complex interaction of stress, material properties, fault structure, and fluids. Understanding how these factors impact fault stability is key to seismic hazard assessment. Seismological observations provide crucial information about fault mechanics. However, directly measuring the influence of fault properties on earthquake nucleation and fault rupture remains a significant challenge. This dissertation investigates the multi-scale processes of earthquake nucleation and fault failure. This involves integrating high-precision locations of acoustic emission events (AEs) in laboratory fault gouge experiments with analysis of induced seismicity and ground deformation in the Blue Mountain Geothermal, Nevada. This multiscale approach aims to understand fault behavior as a function of stress state, structural maturity, and gouge composition. We first analyzed ground deformation and induced seismicity in the geothermal reservoir using seismic, geodetic, and hydraulic data from 2016 to 2020. High resolution InSAR mapping and high-quality earthquake locations reveal long-term surface subsidence above the reservoir, punctuated by short-lived seismicity spikes during abrupt shutdowns of operational wells. We show that these rapid seismic transients are driven by short-term poroelastic stress changes, which temporarily increase Coulomb stressing rates on the faults. On the other hand, the long-term deformation is induced by aseismic fault slip and thermal contraction. These findings demonstrate the complex interactions between fluid flow and fault failure in geothermal reservoirs. Next, to explore the microscale processes associated with earthquake nucleation and fault failure, we conducted frictional sliding experiments on fault gouge layers between corrugated polymethyl methacrylate (PMMA) blocks under controlled shear and normal stresses. The AE events produced during the stick-slip and stable sliding experiments were located with millimeter accuracy. The spatiotemporal AE evolution on stick-slip fault gouge exhibits long-term strain localization followed by a brief period of delocalization immediately prior to fault failure, coinciding with rapid slip acceleration. In contrast, stable sliding fault gouge shows strain localization without the precursory delocalization phase before failure. Furthermore, we compared AE distributions from two different gouge zones: homogeneous gouge between PMMA blocks and heterogeneous gouge in granite. The results reveal that heterogeneous fault zones with off-fault damage exhibit prolonged strain partitioning from the damage zone into the gouge layer before fault slip. The findings from laboratory and geothermal reservoir demonstrate that the seismogenic potential of a fault is governed by stress state, structural maturity, and gouge composition. Together, these factors influence strain localization and earthquake nucleation. Our study emphasizes the significance of a multiscale approach to understanding fault mechanics, combining the lab and field observations. The integrated investigation of induced seismicity and labquakes under diverse conditions revealed a broad spectrum of fault behavior and may contribute to improving seismic hazard assessment and earthquake prediction.

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