The Role of Fluid Pressure and Fluid Drainage on Frictional Strength and Stability in Fault Wear Material
Open Access
- Author:
- Affinito, Raphael
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 13, 2025
- Committee Members:
- Donald Fisher, Program Head/Chair
Chris Marone, Chair & Dissertation Advisor
Derek Elsworth, Outside Unit & Field Member
Tushar Mittal, Major Field Member
Donald Fisher, Major Field Member
Marco Scuderi, Special Member - Keywords:
- Faults
Friction
Frictional Stability
Earthquakes
Fluid Pressure
Fault Drainage
Fault Healing
Ultrasonic Fault Monitoring
Slow Slip
Frictional Strength
Dilatancy Strengthening - Abstract:
- Understanding the processes that govern frictional strengthening and the occurrence of seismic cycles remains a key challenge in characterizing the earthquake potential of faults. Although pore fluids - and more specifically, fluid pressure distribution - are frequently cited as key mechanisms controlling the mode of fault failure, relatively few experimental studies have directly correlated fluids to the dynamic aspects of earthquake rupture. Laboratory friction experiments provide important insights into these mechanisms. Rate-and-state friction (RSF) laws offer a useful framework for understanding a fault’s seismic potential, but they often struggle to incorporate the effects of fluids and fault drainage on stability. The objective of this dissertation is to elucidate how the mechanical and hydraulic properties of fault zone wear material (commonly referred to as fault gouge) interact to influence earthquake dynamics, and highlight the limitations of current frictional models in fluid-saturated fault systems. This document comprises five main chapters and two additional collaborative works, detailing research on the fundamental processes governing earthquakes and faulting. Chapter Two focuses on how fluid pressure influences stick-slip initiation and evolution, with particular emphasis on fluid diffusion timescales and the validity of stability criteria in fluid-pressurized systems. We demonstrate that frictional stability predictions from dry experiments also apply under conditions of constant pore fluid pressure (drained conditions in poromechanics). However, rather than merely the presence of fluids, our results suggest that fault-drainage is the dominant mechanism governing slip breakdown in fluid-saturated faults. Chapter Two is currently published in Geophysical Research Letters. Chapter Three examines the rate dependence of fault strength in gouges collected from the DOE Utah FORGE geothermal project. We show that even small changes in slip rate can induce locally undrained behavior, and that fluid boundary conditions control frictional strength and the spatial-temporal evolution of fault stability. This chapter introduces a state-of-the-art RSF model integrating fault drainage, plastic, and poroelastic deformation, demonstrating the necessity of multiple effective permeability terms for modeling fault gouges. We employ simplified forward modeling and inversion to explain phase delays in porosity and fluid pressure recharge, concluding that standard RSF formulations alone are insufficient for fluid-saturated fault systems. Chapter Three is currently accepted in the Journal of Geophysical Research: Solid Earth with revisions. Chapter Four presents collaborative work with Dr. Giuseppe Volpe on fluid-assisted fault healing, with particular attention to fault porosity evolution. We show that fluid-mediated processes significantly influence re-strengthening in anhydrite fault gouge, linking re-strengthening rates to hydration kinetics. A coupled frictional-chemical healing model is proposed to explain ultra-high healing rates observed in natural environments. Because chemical healing does not adhere to traditional velocity-dependent friction, we introduce a time-dependent cohesion term within the RSF framework, illustrating how even velocity-strengthening materials can host earthquake-like ruptures. Chapter Five builds on Chapters Two and Three by generating stick-slip cycles at geologically relevant fluid pressures while documenting porosity and fluid pressure evolution. We highlight the role of dilatancy throughout the laboratory seismic cycle and identify both `active' and `passive' regions of fault behavior. These findings provide a foundation for implementing the fluid drainage--plastic--poroelastic model proposed in Chapter Three. Here, we investigate the role of fluids and effective normal stress on the onset of fictional instabilities. We argue that poroelastic evolution is critical for predicting the mode of failure, though it remains elusive in most laboratory studies. We suggest that ultrasonic measurements can illuminate the evolution and distribution of fault elastic properties. Chapter Six addresses a long-standing gap in experimental literature: the role of fluids in precursory changes to wave speed and acoustic transmissivity prior to failure. Motivated by the goal of earthquake prediction in laboratory studies, we developed specialized acoustic sensors for our pressure vessel to continuously monitor the fault throughout the laboratory seismic cycle. Comparing 100% humidity and fluid-pressurized conditions (from Chapter Five) reveals how fluids influence precursory signals to failure in the laboratory. Overall, this dissertation underscores the interplay between fluid flow, poroelastic properties, and frictional behavior in dictating fault stability and precursory seismic signals, offering new perspectives for both laboratory research and the interpretation of natural fault processes.