HR: 14:15h
AN: T43C-03 INVITED     [Abstracts]
TI: Mechanical Role of Fluids in Earthquakes and Faulting
AU: * Rice, J R
EM: rice@esag.harvard.edu
AF: Department of Earth and Planetary Sciences, and Division of Engineering and Applied Sciences, Harvard University, 224 Pierce Hall, 29 Oxford St., Cambridge, MA 02138 United States
AB: Following the contributions of Hubbert and Rubey, the level of ambient pore pressure is of accepted importance for understanding the static frictional strength of faults. There are also important dynamical interactions between pore fluids and faulting. Some of those are addressed here, with examples to be chosen from the following: (1) Pore fluid presence at full saturation promotes strong localization in rapidly shearing granular materials, even in cases for which the friction coefficient increases rapidly with shearing rate [see Rice, Rudnicki and Tsai, this meeting]. (2) Thermal pressurization of earthquake faults during seismic slip may provide the primary weakening process during earthquakes in mature crustal fault zones; it provides a plausible basic explanation, based on geological and laboratory data, of the magnitudes of the fracture energies of earthquakes as inferred independently from seismological data [see web link below]. The process also seems to be active in some large landslides. (3) Pore pressure alterations are induced by rapid mode II slip on fault planes when they have bordering gouge or damage zones which are of dissimilar permeability and/or poroelastic properties. This provides a fuller, new perspective on effects of material dissimilarity across a slip surface on altering the effective normal stress and thus interacting with dynamic rupture [see Rudnicki and Rice, this meeting]. (4) Gouge dilatancy associated with slip-rate increases induces suction in the pore fluid, so as to partially stabilize faults against earthquake nucleation, and also to slow rupture propagation into shallow fault regions. An open question is that of when and if shear heating acts to aid nucleation; the effect seems negligible for nucleation under slow tectonic loading but may be important for nucleation driven by sudden steps in stress. (5) Permeability determines pore pressure gradients for given flow rates, but increases in pore pressure cause increases in permeability. That allows slow solitary waves of pore pressure increase which propagate upwards against gravity in fault zones that are reasonably sealed from their surroundings, following initiation by, e.g., breaching of a pressurized seal at depth. (6) Aseismic slip transients in subduction zones occur in an environment of active compaction and metamorphic fluid release, and fluids seem responsible for associated tremor as well. Recent modeling [see Liu and Rice, this meeting] links elevation of fluid pressure to the speed of along-strike propagation of slip transients. (7) Poroelastic responses to stress transfer have been detected for some earthquakes, and associated transient stress changes may play a role in aftershock sequences, although probably secondary in general. (8) Another type of fluid saturated ``fault zone'', in granulated sediments between dissimilar materials, is the bed of a mobile ice sheet. Some of the concepts in topics 1 to 4 above may have application to surges, ice streams, and glacial earthquakes. These various cases 1 to 8 involve many contributors in the geophysical community, and include collaborative current or recent studies of the author with Massimo Cocco (2), Yajing Liu (4, 6), Alan Rempel (2), John Rudnicki (1,3), Paul Segall (4), and Victor Tsai (1,8).
UR: http://esag.harvard.edu/rice/RicE_heat_weaken_toJGR05.pdf
DE: 1822 Geomechanics
DE: 7209 Earthquake dynamics (1242)
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005