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Geometric and Rheological Asperities in an Exposed Fault Zone, J. Geophys. Res.

Amir Sagy, & Emily E. Brodsky

Published 2009, SCEC Contribution #1607

Earthquake dynamics are strongly affected by fault zone structure and fault surface geometry. Here we investigate the interplay of bulk deformation and surface topography using detailed structural analysis of a fault zone near Klamath Falls, Oregon, combined with LiDAR measurements of the fault surface. We find that the fault zone has a layered damage architecture. Slip primarily occurs inside a 1–20 mm wide band that contains principal slip surfaces with individual widths of 100 mm. The slip band sits atop a cohesive layer which deforms by granular flow. Several fault strands with total slips of 0.5–150 m also have cohesive layers with thicknesses increasing monotonically with slip. The thickness added to the cohesive layer per unit slip decreases with increasing displacement indicating that slip progressively localizes. The main fault is a continuous surface with 10–40 m long quasi-elliptical geometrical asperities, i.e., bumps. The bumps reflect variations of the thickness of the granular cohesive layer and can be generated by a pinch-and-swell instability. As the granular layer is rheological distinct from its surroundings, the asperities are both geometrical and rheological inhomogenities. Modeling slip along wavy faults shows that slip on a surface with a realistic geometry requires internal yielding of the host rock. Our observations suggest that the internal deformation processes in the fault zone include ongoing fracture, slip along secondary faults, and particle rotation. Granular flow is an important part of faulting in this locale. Slip surfaces localize on the border of the granular cohesive layer. The ongoing slip smoothes the surfaces and thus the structural and geometrical evolution of the granular layer creates a preference for continued of slip on the same surface. There is a feedback cycle between slip on the surface and the generation of the granular layer that then deforms and controls the locus of future slip

Sagy, A., & Brodsky, E. E. (2009). Geometric and Rheological Asperities in an Exposed Fault Zone, J. Geophys. Res.. Journal of Geophysical Research, 114, B02301. doi: 10.1029/2008JB005701.