Thermodynamics of Fault Slip at Seismic Velocities

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Thermal perturbations associated with seismic slip may significantly affect the dynamic friction and the mechanical energy release during earthquakes. Previous theoretical work and field observations suggest that a number of temperature-dependent mechanisms, including micro-and macroscopic melting, enhanced plasticity, and pore fluid pressurization may dramatically modify the effective fault strength at high slip velocities. However, the magnitude, the spatio-temporal patterns, and even the sign of the thermally-induced variations in the effective fault friction are not well known. Recently, experimental measurements of the rock friction have been extended to slip rates approaching the seismic range of meters per second. The experimental data reveal a complex dependence of the effective rock friction on temperature, slip rate, and fault-normal stress; both weakening and strengthening behavior is reported. Perhaps the most intriguing finding of the high-speed laboratory experiments is an apparent increase in friction upon the onset of macroscopic melting. This increase may indicate a change in the basic physics from the asperity-contact sliding to a viscous-like flow at high slip velocities. This project investigates effects of the co-seismic heating on the dynamic fault strength by developing a hermodynamically consistent model of the fault zone deformation that explicitly includes heat transfer, phase transitions (e.g., melting and freezing), and realistic rheology of the fault zone rocks. Observables used to validate the model predictions include field observations of the "fine structure'' of the exposed fault zones (in particular, the degree of slip localization), morphology of the pseudotachylite veins, and new results from the laboratory high-speed sliding experiments. This work may contribute to the on-going discussion about the magnitude of stress at which the major crustal faults operate. --