HR: 1330h
AN: S42C-0183    [PDF]
TI: Fault Zone Drainage, Heating and Melting During Earthquake Slip
AU: * Rempel, A W
EM: rempel@esag.harvard.edu
AF: Division of Engineering and Applied Sciences, Harvard University, 29 Oxford St. Pierce 317, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Division of Engineering and Applied Sciences, Harvard University, 29 Oxford St. Pierce 317, Cambridge, MA 02138 United States
AU: Rice, J R
EM: rice@esag.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Oxford Street, Cambridge, MA 02138 United States
AU: Jacques, L M
EM: laurent.jacques@polytechnique.org
AF: Option de Mecanique, Ecole Polytechnique and Corps des Mines, 60 Boulevard Saint-Michel 75272, Paris, Cedex 06 France
AB: The expansion of pore water caused by frictional heating during large crustal events provides a powerful weakening mechanism (Sibson, 1973; Lachenbruch, 1980). It may explain the magnitude of seismically inferred fracture energy and aspects of its variation with increased slip (Abercrombie and Rice, 2003; Rice {\em et al.}, 2003; Rice, this section, 2003). The weakening is mediated by the effects of fluid transport, which are sensitive to the permeability structure of the fault zone and its modification by damage induced by the passing rupture front (Poliakov {\em et al.}, 2002), as well as by the increase in pore pressure itself. Higher permeabilities allow partial drainage to occur, so that enough strength remains for the heat generated to cause partial melting of the fault gouge at large enough slip. We use recent field and laboratory data for fluid transport through pressurized fault gouge (e.g. Lockner {\em et al.}, 2000; Wibberley and Shimamoto, 2003) to motivate models for drainage and melting during earthquake slip. A dramatic illustration of the role of drainage is provided by an idealized model in which we assume that a freshly damaged, highly permeable region extends right up to a localized shear zone of thickness $h_o=5$ mm, with fixed porosity $n$ and much lower permeability $k$. At $7$ km depth, for $n=0.02$ and $k=10^{-19}$ m$^2$, the slip distance required to reach the onset of melting at $750^\circ$C is approximately $0.4$ m for a constant friction coefficient of $f=0.6$. At $14$ km depth, for $n=0.01$ and $k=10^{-20}$ m$^2$, the same temperature is reached after only $0.1$ m of slip. Yet more efficient drainage might occur due to the permeability increases that accompany reductions in the effective stress, so that even more rapid temperature increases would be predicted. For example, with ten times higher $k$, melting begins after $0.1$ m slip at $7$ km depth and just $0.05$ m at $14$ km. At onset of melting the high melt viscosity impedes further drainage and, with increasing melt fraction, inter-particle contact is soon eliminated (Jacques and Rice, 2002). Then all of the fault-normal stress is supported by pressure in a slurry fluid which, when the normal stress exceeds the least principal stress, can inject into the bordering damage zone. Remaining dissipation in fault slip is dominated by fluid lubrication between the still-solid gouge particles rather than inter-particle friction. The slurry viscosity is thermally activated and reduces dramatically with increasing temperature and decreasing solids fraction. We quantify these effects approximately, and compare our predictions to records of slip behavior as preserved in pseudotachylytes (e.g. Otsuki {\em et al.}, 2003).
UR: http://www.deas.harvard/~rempel
DE: 1832 Groundwater transport
DE: 7205 Continental crust (1242)
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7260 Theory and modeling
SC: Seismology [S]
MN: 2003 Fall Meeting