Frictional Mechanics of Stable and Unstable Fault Slip PDF Download

Author: Srisharan Shreedharan
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Languages : en
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Book Description
Tectonic faults fail in a spectrum of slip modes ranging from aseismic creep to fast elastodynamic ruptures. In the laboratory, these slip modes and fault frictional stability can be quantified by second-order changes in friction, and modeled using experimentally-derived designer friction laws known as rate-and-state friction (RSF). Even though RSF has been utilized to study fault slip and stability for many decades, the parameters constituting RSF and their relationship to the underlying grain-scale frictional contact mechanics, particularly in the context of slow and fast ruptures, are poorly constrained. While light intensity-based imaging techniques provide some insights into the evolution of microscopic frictional contacts during shear, their utility is limited in the case of opaque geologic media such as sheared rock and granular fault gouge. Motivated by the successful application of ultrasonic wave monitoring for imaging rock joints and fractures, I use ultrasonic acoustic monitoring for a range of fault slip behaviors in the laboratory, to constrain the micromechanical behavior of deforming load-bearing asperities that make up tectonic faults. In this dissertation, I ask fundamental questions surrounding the deformation of microscopic load-bearing asperity populations that make up frictional interfaces and granular fault gouge assemblages. I dissect the various parameters that make up the RSF constitutive framework, and ask what frictional state and the critical slip distance represent in the context of creeping tectonic faults. I also strive to answer whether the microphysical mechanisms operating across the spectrum of slip behaviors, from stable sliding to fast ruptures, are similar or fundamentally different. I examine the role of normal stress and velocity perturbations on experimental rate-state faults, particularly in the context of contact-scale processes, and use these insights to constrain the potential origins of shallow slow earthquakes, both frictional and mineralogical, at the Hikurangi subduction margin. I start this dissertation by introducing the problem statement broadly and providing some context for the known and unknown aspects of interfacial contact-scale friction in Chapter 1. In Chapter 2, I probe an extended RSF formulation, incorporating the role of normal stress and velocity variations on frictional state, and its application to rough, planar faults using ultrasonic wave amplitudes. In chapters 3-5, I generate a range of slow and fast slip modes on mature faults with simulated wear and jointly characterize precursory creep and ultrasonic wave properties in the context of frictional state evolution. Chapter 3 demonstrates that ultrasonic wave amplitudes have a long, temporal precursory signal strongly related preseismic fault acceleration for the full spectrum of unstable slip modes. I quantify the sensitivities of ultrasonic wave amplitudes and velocities on stress and slip rate in Chapter 4, and demonstrate how they can be used as long- and short-term precursors respectively to seismicity in the lab and, perhaps occasionally, in crustal faults. Chapter 5 leverages results from the previous chapters to provide a framework for laboratory earthquake forecasting using machine learning on the continuous evolution of ultrasonic wave properties over multiple slow and fast stick-slip cycles. Finally, I introduce shallow slow earthquakes in the Hikurangi subduction margin in Chapter 6. I perform RSF experiments and continuous ultrasonic monitoring on input material to the plate interface obtained during an ocean drilling expedition in mid-2018 in order to better constrain the frictional and hydrologic regime facilitating shallow slow slip in this region. This dissertation provides fundamental insights into the microscopic processes that govern fault friction at the laboratory and crustal scales over a range of slip modes. I demonstrate the underlying similarities between these slip modes and provide insights into the microphysical mechanisms that could modulate fault slip behavior. Finally, I introduce time-lapse monitoring of seismic amplitudes and velocities as a viable method to probe transient fault zone processes over multiple scales.