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Abstract Field observations of maturely slipped faults show a generally broad zone of damage by cracking and granulation, but nevertheless suggest that shear, and therefore heat generation, in individual earthquakes takes place with extreme localization to a zone < 1-5 mm wide within a finely granulated fault core. Relevant fault weakening processes during large crustal events are therefore likely to be thermal and, given the porosity of the damage zones, it seems reasonable to assume groundwater presence. It is suggested that the two primary dynamic weakening mechanisms during seismic slip, both of which are expected to be active in at least the early phases of nearly all crustal events, are (1) Flash heating at highly stressed frictional micro-contacts, and (2) Thermal pressurization of fault-zone pore fluid. Both will be shown to have characteristics which promote extreme localization of shear. Macroscopic fault melting will occur only in cases for which those processes, or others (thermal decomposition, silica gelation) which may come on line at sufficiently large slip, have not efficiently reduced heat generation and thus limited temperature rise. Elementary modeling of mechanisms (1) and (2), constrained with lab-determined hydrologic and poroelastic properties of fault core material and high-speed friction studies, suggests that, within considerable uncertainties in interpretation, seismic data on the fracture energy of earthquakes and its variation with slip in the events can be plausibly described. The mechanisms suggest that faults may be statically strong but dynamically weak under typical seismic conditions. Progress at more advanced spontaneous dynamic rupture modeling, done with Hiroyuki Noda and Eric Dunham using procedures that embody mechanisms (1) and (2), and explicitly solve diffusion equations for temperature and pore pressure variations at multi-millimeter scales along a fault during rupture, will be described.
Biography James R. Rice, born 1940 in Frederick, Maryland, has been at Harvard University since 1981, where he is Mallinckrodt Professor of Engineering Sciences and Geophysics, jointly appointed in the School of Engineering and Applied Sciences and in the Department of Earth and Planetary Sciences. Previously he was a faculty member of the Division of Engineering at Brown University and, for his education, a student in the Department of Mechanics at Lehigh University. His work of recent years is on geomechanics, especially on the science of earthquakes, including fault friction and the nucleation and propagation of earthquake ruptures, and other problems (landslides, episodic glacial motion) involving pore fluid interactions in deformation and failure of earth materials. His earlier work has also addressed elastic-plastic crack propagation in metals, path-independent integrals in elasticity, wave effects in crack dynamics, microscopic mechanisms of fracture, thermodynamics of interfacial embrittlement, inelastic constitutive relations for solids, deformation localization into shear zones, and finite-element and spectral numerical methodology in solid mechanics. [For fuller details, see a CV as of late 2007, http://esag.harvard.edu/rice/RiceCV.html, or a biography done ~2000 by T.-j. Chuang and J. W. Rudnicki, http://www.imechanica.org/node/111.] |
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