Slip heterogeneity on a corrugated fault

摘要

Slip heterogeneity reflects the fundamental physics of earthquake rupture and has been attributed to strong fault patches termed asperities or barriers. We propose that variations in fault-surface orientation due to slip-parallel corrugations may act as geometric asperities and barriers, generating variations in incremental (i.e. due to a single earthquake) slip across a fault surface. We evaluate this hypothesis using observations from the Arkitsa normal fault exposure in central Greece. A scan of the Arkitsa fault surface with 1-m spatial resolution and mm-scale precision reveals corrugations made up of 1–5 m wide synforms, antiforms, and nearly planar fault sections with long axes that extend in the slip direction across the entire exposed surface. The surface is thus more than an order of magnitude smoother in the direction of slip than in the perpendicular direction. Slip-perpendicular profiles exhibit nearly self-similar scaling across the range of observed wavelengths (~2–50 m), whereas slip-parallel profiles are significantly smoother at shorter wavelengths (<~5 m). Millimeter-scale striations, indicators of incremental slip direction, have heterogeneous orientations across the corrugated surface. Using spherical statistics we demonstrate a statistically significant correlation between striation orientation and the local orientation of the fault surface. These results are consistent with the hypothesis that corrugations have acted as asperities and/or barriers and thus play a fundamental role in earthquake mechanics. We propose that slip heterogeneity associated with corrugations may play an important role in the evolution of fault-surface morphology both by smoothing fault surfaces in the slip-perpendicular direction and by generating wall-rock flow perturbations that may act to enhance corrugation topography with slip. Slip heterogeneity is likely to further impact fault zone mechanics by generating significant variations in near-fault stress. Patterns of wall-rock fracturing and localization of aftershocks in the hanging walls of normal fault earthquakes may reflect these stress effects. Slip-parallel corrugations may be particularly important to the mechanics of large-slip faults where faults are smoothed significantly in the slip direction.