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Fracture energy is a form of latent heat required to create an earthquake rupture surface and is related to parameters governing rupture propagation and processes of slip weakening. Fracture energy has been estimated from seismological and experimental rock deformation data, yet its magnitude, mechanisms of rupture surface formation and processes leading to slip weakening are not well defined. Here we quantify structural observations of the Punchbowl fault, a large-displacement exhumed fault in the San Andreas fault system, and show that the energy required to create the fracture surface area in the fault is about 300 times greater than seismological estimates would predict for a single large earthquake. If fracture energy is attributed entirely to the production of fracture surfaces, then all of the fracture surface area in the Punchbowl fault could have been produced by earthquake displacements totalling <1 km. But this would only account for a small fraction of the total energy budget, and therefore additional processes probably contributed to slip weakening during earthquake rupture.

Abstract. Locally inhomogeneous stress states are expected along faults owing to slip on geometrically irregular fault surfaces. We use an analytical model of elastic deformation along a wavy frictional fault to evaluate the variation in local stress state as a function of surface roughness, elastic modulus, slip, coefficient of friction, and far-field stress. The total stress state along the fault may be described by the sum of a basic stress component resulting from frictional slip on a planar fault surface and a perturbed stress component resulting from the presence of roughness. Roughness produces a variation in normal stress across the fault surface, and assuming roughness and modulus appropriate to crustal faults, the normal stress should be reduced to a near-zero magnitude locally, such that separation of fault walls is likely. The large variation in normal stress along the fault surface resulting from fault roughness may be responsible, in part, for complexity in moment release during large earthquakes and for lateral variation in seismic coupling along faults. The variation in principal stress orientations and magnitudes along a fault increases with a decrease in the coefficient of friction of the fault. The location and size of regions with a high likelihood for brittle failure depend on the orientation of the far-field principal stress and fault friction. The average orientation of the principal stresses in the region of likely failure is not the same as the far-field principal stress orientation. Although inversion of earthquake and fabric data for stress orientation along a fault may be possible, the model results suggest that inversion results are insufficient to determine far-field stress states and fault friction without additional independent data.

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[1] To understand the frictional behavior of natural faults at seismic slip rates, high-speed rotary shear experiments were conducted on disaggregated ultracataclasite from the Punchbowl fault. The experimental gouge layers were sheared at normal stresses of 0.2-1.3 MPa and velocities of 0.1-1.3 m/s to total displacements of 1.3-84 m. We employ thermomechanical FEM models and microstructural observations to consider spatial and temporal variation of normal stress and temperature in the samples and understand microprocesses. Four distinct gouge units form during shear. A slightly sheared starting material (Unit 1) and a strongly sheared and foliated gouge (Unit 2) are produced when frictional heating is insignificant and the coefficient of sliding friction is 0.4-0.6. A random fabric gouge with rounded prophyroclasts (Unit 3) and an extremely fine, microfoliated layer (Unit 4) develop when significant frictional heating occurs at greater velocity and normal stress, and the coefficient of sliding friction drops to approximately 0.2. Unit 3 forms at the critical temperature for vaporization of water and is associated with localization of slip to Unit 4 and elevation of temperature. The critical displacement for dynamic weakening in the rotary configuration can be understood as a consequence of the progressive inward migration of the friction-generated thermal front and the weaker localized slip surface and associated fluidized zone.Citation: Kitajima, H., J. S. Chester, F. M. Chester, and T. Shimamoto (2010), High-speed friction of disaggregated ultracataclasite in rotary shear: Characterization of frictional heating, mechanical behavior, and microstructure evolution,

Characterizing the structure of fault zones is necessary to understand the mechanical, fluid flow and geophysical properties of the lithosphere. This paper provides a detailed characterization of two large-displacement, strike-slip fault zones of the San Andreas system in southern California, the Punchbowl and North Branch San Gabriel faults. The faults cut crystalline and well-lithified sedimentary rocks, and consist of broad zones of fractured and faulted rock (damage zone) containing one, or more, narrow, tabular zones of highly sheared rock (fault core). Subsidiary faults and fractures of the damage zone formed in response to stress cycling associated with slip. The fault core is composed of very fine-grained, altered fault-rocks that reflect high shear strain, extreme comminution, and enhanced fluid-rock reactions. The characteristic and relatively ordered structure of these large-displacement faults is consistent with progressive damage accumulation over the lifetime of the fault. Layers of ultracataclasite containing mesoscopic slip surfaces within fault cores record extreme localization of slip at the macroscopic and mesoscopic scale. Progressive accumulation of ultracataclasite from abrasive wear along a slip surface throughout faulting history can explain the particle size distribution, layered structure, and sharp boundaries of the ultracataclasite layer. The longevity and relative stability of the slip surface is evidenced by the accumulation of progressively younger ultracataclasite towards the slip surface. Although not a common feature, the incorporation of slivers of wall rock into the ultracataclasite layer document occasional branching of the slip surface within fault cores. The damage-zone and fault-core characterization may be used to describe the geophysical, mechanical, and fluid-flow properties of brittle fault zones in crystalline and well-lithified sedimentary rocks of the continental crust. fault zones is necessary to understand the mechanical, fluid flow, and geophysical properties of the lithosphere.Our current understanding of the mesoscopic structure of large-displacement, strike-slip fault zones primarily results from study of uplifted and exhumed faults (e.g., Little 1995;Chester and Logan 1986; Flinn 1977) and faults exposed in mines and drill core (e.g., Ohtani et al. 2000;Wallace and Morris 1986). Early petrographic study of mylonites and cataclasites initiated considerable research into the classification and origin of fault rocks (for review, see Snoke and Tullis 1998). The most commonly used fault-rock classification schemes are based on particle size, fabric and cohesion. For example, in Sibson's 1977 classification, mylonites and cataclasites both are distinguished by primary cohesion and tectonic grain size reduction, but mylonites have a well-developed foliation with some evidence of syntectonic recovery and recrystallization, whereas cataclasites have random fabrics and microstructures indicative of cataclasis. The breccia-gouge series is distinguished by lack of primar...

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