Physically-Based Fault Zone Constitutive Responses and Consequences for Earthquake Dynamics

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AbstractNSF EAR Award number 0510193: Physically-based fault zone constitutive responses and consequences for earthquake dynamics(1 July 2005 to 30 June 2008)James R. Rice (principal investigator)Department of Earth and Planetary Sciences and Division of Engineering and Applied SciencesHarvard University, Cambridge, MAEarthquakes occur because fault strength weakens with increasing slip or slip rate. The aim of this work is to identify the physical processes underlying that weakening, and to analyze their consequences for the dynamics of earthquake rupture. The focus is on mature crustal faults, capable of producing large earthquakes. Recent field observations suggest that slip in individual events then occurs primarily within a thin shear zone, 1-5 mm, within a finely granulated (ultracataclastic) fault core. Since energy is dissipated in a narrow zone, the relevant weakening processes in large crustal events might, therefore, be expected to be thermal in origin. Our preparatory work for this study has assembled a strong case that primary weakening mechanisms during significant crustal earthquakes are thermal, and involve the following: (1) Thermal pressurization of groundwater that is resident within the (slightly) porous fault gouge, due to frictional heating of the gouge, and (2) Flash heating at highly stressed frictional micro-contacts which, at high slip rates like during earthquakes, causes them to lose their normally high shear strength even before they have slid out of existence. Elementary modeling of these mechanisms has been constrained with recently determined poroelastic and transport properties of fault core materials, and with recent high-speed friction studies.Predictions are that strength drop should often be nearly complete at slip of order 1 m, and that the onset of melting should be precluded over much of the seismogenic zone, except in large slip events. These are qualitatively consistent with low heat outflow from major faults and a scarcity of glass (pseudotachylyte) that would be left from rapid re-cooling. A more quantitatively testable prediction is of the shear fracture energies that would be implied if actual earthquake ruptures were controlled by those thermal mechanisms. Seismic observations have been processed to allow inference of the fracture energy of large crustal events, including its variation with slip in an event. It is found that the seismic results are plausibly described by the theoretical predictions, thus supporting the possibility that such thermal weakening prevails in the earth. This project focuses on consolidating those new advances, on expanding the modeling of the underlying physics to allow characterization of fault zone response under the highly variable slip rates of natural events, and on applying that understanding, embodied in constitutive relations, along with techniques of dynamic fracture simulation, to revisit a set of important problems in the dynamics of earthquakes, including their nucleation, propagation and arrest. We plan to develop viable methodologies of dynamic rupture analysis which incorporate these thermal mechanisms, in a 2D boundary integral equation formulations and, for more robust 2D and 3D applications, in an explicit-dynamics finite-element formulation. The problem of localization of shearing into a thin zone in otherwise stable granular lithologies will be addressed, as well as its relation to arrest of rupture as the slip zone attempts to penetrate hot regions of inherently stable, rate-strengthening, frictional response. Principles of fault operation under low overall driving stress are to be elaborated for such models of statically strong but dynamically weak fault response.