The absence of significant heat flow from major fault zones, and scarcity of evidence for their seismic melting, means that during earthquake slip such zones could not retain shear strength comparable to the typically high static friction strength of rocks. One line of explanation is that they are actually statically weak, which could be because materials of exceptionally low friction (smectites, talc) accumulate along fault zones, or perhaps because pore pressure within the fault core is far closer to lithostatic than hydrostatic. Without dismissing either, the focus here is on how thermal processes during the rapid slips of seismic rupture can weaken a fault which is indeed statically strong. (The discussion also leaves aside another kind of non- thermal dynamic weakening, possible when there is dissimilarity in seismic properties across the fault, and/or in poroelastic properties and permeability within fringes of damaged material immediately adjoining the slip surface. Spatially nonuniform mode II slip like near a propagating rupture front may then induce a substantial reduction in the effective normal stress \barσ.) The heating and weakening processes to be discussed divide roughly into two camps: (1) Those which are expected to be active from the start of seismic slip, and hence will be present in all earthquakes; and (2) Those that kick-in after threshold conditions of rise of temperature T or accumulation of slip are reached, and hence become a feature of larger, or at least deeper slipping, earthquakes. It has been argued that the two major players of (1) are as follows: (1.1) Flash heating and weakening of frictional contact asperities in rapid slip [Rice, 1999, 2006; Tullis and Goldsby, 2003; Goldsby and Hirth, 2006; Beeler et al., 2007; Yuan and Prakash, 2007]. That gives a strong velocity-weakening character to the friction coefficient, which is consistent with inducing self-healing rupture modes [Noda et al., 2006; Lu et al., 2007]. It is a process for which the details are still poorly understood in presence of substantial fault gouge, almost surely present in some of the large-slip experiments fitting the flash weakening theoretical model. (1.2) Thermal pressurization of pore fluid by frictional heating, a process which reduces \barσ [Sibson, 1973; Lachenbruch, 1980; Mase and Smith, 1987], and is expected to be active wherever the fault wear products, as gouge, retain porosity of a few percent or more. At some depth and temperature they may instead sinter to a coherent solid on the interseismic time scale. Those of category (2) are as follows: (2.1) Macroscopic melting of the shear zone [Tsutsumi and Shimamoto, 1997; Hirose and Shimamoto, 2005; Fialko and Khazan, 2005; Nielsen et al., 2007], a process for which conditions may not be met if (1.1) and (1.2) kill off strength rapidly enough [Rempel and Rice, 2006], or do so with the help of one of the next two items. (2.2) Thermal decomposition like in smectite or serpentine dehydration [Sulem et al., 2004, 2007; Hirose and Bystricky, 2007] or coal devolatilization [O'Hara et al., 2006], leading to a high pressure fluid phase. (2.3) Formation of a weak gel-like layer like in wet silica-rich lithologies [Goldsby and Tullis, 2002; DiToro et al., 2004]. It is argued that some large-slip experiments involving significant weakening of unsaturated specimens in lab air, and others involving dehydration, may exhibit a component of weakening from pressurization of water vapor that is desorbed from mineral surfaces or released by dehydration during frictional heating. The hydraulic diffusivity of water vapor is unexpectedly low at levels of p comparable to the low normal stresses of the experiments involved.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 7209 Earthquake dynamics (1242);
- 7260 Theory;
- 8034 Rheology and friction of fault zones (8163);
- 8118 Dynamics and mechanics of faulting (8004)