Rough faults, distributed weakening, and off‐fault deformation

Abstract

[1] We report systematic spatial variations in fault rocks along nonplanar strike-slip faults cross-cutting the Lake Edison Granodiorite, Sierra Nevada, California (Sierran wavy fault) and Lobbia outcrops of the Adamello Batholith in the Italian Alps (Lobbia wavy fault). In the case of the Sierran fault, pseudotachylyte formed at contractional fault bends, where it is found as thin (1-2 mm) fault-parallel veins. Epidote and chlorite developed in the same seismic context as the pseudotachylyte and are especially abundant in extensional fault bends. We argue that the presence of fluids, as illustrated by this example, does not necessarily preclude the development of frictional melt. In the case of the Lobbia fault, pseudotachylyte thickness varies along the length of the fault, but the pseudotachylyte veins thicken and pool in extensional bends. We conduct a quantitative analysis of fault roughness, microcrack distribution, stress, and friction along the Lobbia fault. Numerical modeling results show that opening in extensional bends and localized thermal weakening in contractional bends counteract resistance encountered by fault waviness, resulting in an overall weaker fault than suggested by the corresponding static friction coefficient. The models also predict static stress redistribution around bends in the faults which is consistent with distribution of microcracks, indicating significant elastic and inelastic strain energy is dissipated into the wall rocks due to nonplanar fault geometry. Together these observations suggest that damage and energy dissipation occurs along the entire nonplanar fault during slip, rather than being confined to the region close to the dynamically propagating crack tip.

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…The model does not account for numerous factors in the faulting process, including opening normal to the fault, nonuniform friction, displacement gradients, influence of fault tips, host rock anisotropy, selfaffine fault geometries, 3-D fault geometries, or pore fluid pressure variability [e.g., Dieterich and Smith, 2009;Griffith et al, 2010;Ritz and Pollard, 2012;Ritz et al, 2015]. As a result, it does not account for changes in elastic moduli in the damage zone resulting from accumulated inelastic deformation [e.g., Faulkner et al, 2006;Cappa et al, 2014;Xu et al, 2014].…”

“…Processes that cause damage zone growth include fault and process zone propagation and linkage [e.g., Cowie and Scholz, 1992;McGrath and Davison, 1995;Vermilye and Scholz, 1998], wear related to fault geometry [e.g., Scholz, 1987;Wilson et al, 2003], and off-fault plasticity accompanying earthquake rupture [e.g., Rice et al, 2005; J. P. Ampuero and X. Mao, Upper limit on damage zone thickness controlled by seismogenic depth, in Fault Zone Dynamic Processes: Evolution of Fault Properties During Seismic Rupture, AGU Monogr., edited by M. Y. Thomas, H. S. Bhat, and T. Mitchell, manuscripts in preparation, 2016]. Fracture modes and attitudes in the damage zone are a function of the stress field when they form, suggesting that fracture characteristics can be useful for constraining off-fault stresses [e.g., Kilsdonk and Fletcher, 1989;Saucier et al, 1992;Chester and Fletcher, 1997;Chester and Chester, 2000;Di Toro et al, 2005;Griffith et al, 2010]. Based on this relation, the types, distributions, and orientations of damage zone structures corresponding to each fault evolution process have been recognized in some cases, including mature faults that have experienced many slip events [e.g., Vermilye and Scholz, 1998;Wilson et al, 2003;Mitchell and Faulkner, 2009].…”

“…Fault friction is a primary control on the stress field around a fault [e.g., Chester and Chester, 2000;Griffith et al, 2010], so damage zones should also be dependent on friction. The influence of fault friction has been addressed by modeling off-fault stress and deformation [e.g., Johnson and Fletcher, 1994;Cooke, 1997;Cooke and Pollard, 1997;Lyakhovsky et al, 1997;Chester and Chester, 2000;Rice et al, 2005;Faulkner et al, 2006;Griffith et al, 2010;Savage and Cooke, 2010;Dunham et al, 2011].…”

Fault roughness and strength heterogeneity control earthquake size and stress drop

“…The observed decrease in average slip per stress drop due to fault roughness corresponds to a decreased frictional breakdown during sliding and thus a decrease in Δμ. Hence, fault roughness may be regarded as a bulk frictional agent [Griffith et al, 2010;Fang and Dunham, 2013], modifying the change in frictional resistance that is associated with a given stress drop (equation (5)). …”

Effect of temperature on the permeability of lava dome rocks from the 2004–2008 eruption of Mount St. Helens

“…Complementary image analysis allowed us to estimate both fracture density and 2D porosity (Fig. 1b) using fracture density analysis (Griffith et al 2010) and an open-source image processing software ImageJ. Fracture networks were traced in Illustrator and analyzed using MATLAB (Griffith et al 2010;Rempe et al 2014) to calculate fracture density, the results of which are shown in Table 1.…”