Laboratory Studies of Fault Stability and Slow Earthquakes

Open Access
Author:
Leeman, John Robert
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 17, 2017
Committee Members:
  • Chris Marone, Dissertation Advisor
  • Demian Saffer, Committee Chair
  • Sridhar Anandakrishnan, Committee Member
  • Richard Alley, Committee Member
  • Derek Elsworth, Outside Member
Keywords:
  • slow slip
  • earthquake
  • fault mechanics
  • rock mechanics
  • fault stability
  • ice stream
Abstract:
Fault zones are areas of localized deformation that accommodate strain in the Earth's crust accumulated over time due to tectonic motion or stress transfer from adjacent areas. Faults are traditionally considered to accommodate this strain by either constant slow movement (creep), or by rapid catastrophic failure events (earthquakes). The behavior of faults which produce earthquakes has been extensively studied, including characterization of the time and slip predictability of earthquakes, frequency-magnitude distributions, aftershock decay patterns, dynamic triggering, and frictional processes. Chance observations in global positioning system (GPS) data from the Vancouver Island area in 2001 revealed a new kind of fault slip that had not been considered before, slow-slip events. Since those early observations, slow-slip events have been observed at most major subduction interfaces and even in glacial systems. In this dissertation, I strive to answer some of the fundamental questions about slow-slip systems. Little is known about the dynamics of these systems and how they operate. Scattered laboratory observations have provided clues, but this study is the first systematic examination of slow-slip earthquakes and their frictional behavior in the laboratory. I examine questions such as what controls how a fault zone will fail and what the velocity and normal stress sensitives are, then connect those mechanisms to observations from a natural slow-slip system beneath Whillans Ice Stream in western Antarctica. In chapter 1, I demonstrate how to modify the stiffness of the testing machine to create slow-slip events in the laboratory in an artificial granular material. I also present a method to automatically calculate the stiffness of each slip event in a given experiment. Chapter 2 extends this work into a synthetic fault gouge material and carefully examines the sensitivity of the system to the stiffness of the testing apparatus. Chapter 3 introduces the new parameter of velocity dependence into the test suite and demonstrates that designer frictional laws (those whose velocity dependence of friction is itself velocity dependent) are not necessary to explain observations of slow-slip. Chapter 4 introduces the field area of Whillans Ice Stream, a system that hosts slow-slip events daily at an ice-till interface approximately 1 km below the ice surface. I test samples of the till obtained by piston coring of the ice stream in 1989 and develop a simple hydrologic model to determine the potential stress states of the system and an effective medium model to predict the acoustic velocities under those stresses. The entire system is then examined in the light of stability theory to postulate why it has been able to remain in the slow-slip regime for as long as it has been observed. Finally, in chapter 5 I examine the generation of electrical potential differences on slipping experimental faults and critically evaluate the generation mechanisms proposed in the literature. This dissertation provides insight into the mechanisms and controls of fault slip. I demonstrate that fault failure is not a bifurcation between stable and unstable, but rather a continuous spectrum of failure modes from slow to fast stick-slip. The evidence provided shows that the stiffness of the system is the dominant controlling mechanism and that higher order frictional terms are not required to explain the basic spectrum of behaviors observed.