Fault surface geometry as a record of deformation processes
James Kirkpatrick, McGill University
Fault slip surfaces, where displacement is localized during earthquakes, exert a primary control on fault strength, stability, off-fault stresses and rupture characteristics. Slip surfaces are non-planar, or rough, and although distinct features can be identified on the surfaces at outcrop scale they have characteristic geometry, regardless of the tectonic setting or rock type. The self-affine scaling of the surface geometry is controlled by fracturing of the rock during slip, and is direct evidence for scale-dependent strength of rocks. However, at the extremes of observational length scales, the scaling behavior changes. At length scales of tens to hundreds of micrometers or less, the roughness is isotropic, marking a change in wear mechanism. Analysis of the reflector corresponding to the Costa Rica subduction zone megathrust in 3-D seismic data also suggests a transition to isotropy at several km. This dimension matches the spacing of horsts and grabens on the oceanic plate, and indicates processes external to the fault impact the mechanical behavior. These insights highlight the importance of fault geometry to fault mechanics and show how faults can used to constrain the physical controls on slip.