Time-dependent evolution of off-fault deformation

Abstract:

Within the seismogenic zone deformation occurs by frictional slip on faults and folding of the surrounding rock volume. To realistically model crustal deformation over the earthquake cycle, it is necessary to understand how these modes of deformation operate together. There is currently no agreed-upon constitutive relationship to describe off-fault deformation that is consistent with geodetic observations of distributed folding, geological observations of off-fault topography, and earthquake recurrence intervals. To reconcile these spatiotemporally variable observations and their rheological implications, we analyze instantaneous and permanent off-fault strains in the form of subtle but ubiquitous folding (10-10⁴ m wavelength). We use differential lidar and insar to measure off-fault deformation in coseismic datasets and high-resolution altimetry data to measure distributed deformation accrued over multiple earthquake cycles in the form of off-fault topography. We measure folding wavelength, amplitude, and 3D shape for 3,000+ faults to compile coseismic and permanent catalogs of off-fault deformation spanning a range of strain rates, slip magnitudes, and crustal properties. Instantaneous folding strains are ~10^-3 and approach the elastic failure limit without exceeding it. Over time, these strains grow by factors of 10-100. Evidently, because finite deformation accrued over multiple earthquakes exceeds the elastic limit, incremental coseismic folding produced during each slip event must be transformed into permanent (viscous/plastic) deformation during the interseismic period. This transformation is recorded by a change in the 3D shape of the folds, where permanent folds have larger peak-to-peak distances and lower curvatures, and by a change in scaling relationship between folding amplitude and fault length from instantaneous to finite strains. We investigate the responsible mechanism for the time-dependent evolution of off-fault deformation, and discuss its implications for the treatment of off-fault plasticity and interseismic stress dissipation in numerical models.