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Effects of elastoplastic material properties on shallow fault slip and surface displacement fields

Johanna M. Nevitt, Benjamin A. Brooks, Todd L. Ericksen, Craig L. Glennie, Christopher M. Madugo, David A. Lockner, Diane E. Moore, Sarah E. Minson, & Kenneth W. Hudnut

Published August 15, 2016, SCEC Contribution #6872, 2016 SCEC Annual Meeting Poster #075

Inadequate knowledge of fault slip and off-fault deformation within the shallow crust (<1 km depth) limits our understanding of fundamental fault physics and seismic hazard associated with near-surface rupture. Recent technological advances in near-field geodesy (e.g., mobile LiDAR), however, provide the displacement data necessary to invert for shallow slip. Such analysis will benefit from a deeper understanding of the rheological properties of surface materials (e.g., soil, gravel) and their effect on partitioning deformation between discrete fault slip and distributed off-fault deformation. We investigate shallow fault physics using co- and post-seismic observations from the Mw 6.0 2014 South Napa earthquake and mechanical models. Detailed mapping reveals secondary echelon mixed-mode fractures rotated ~20° clockwise from the West Napa fault, indicating the primary fault did not rupture the surface. Still, the surface shear zone recorded by displaced vine rows and imaged using mobile LiDAR is remarkably narrow (<5 m), given that the site is underlain by soil and a sedimentary basin up to 2 km deep. In Abaqus, we build 3D finite element models that include a sedimentary basin, defined by Drucker-Prager elastoplasticity, above an elastic basement. Slip on a vertical fault through both layers is governed by the Coulomb criterion. Material properties (elastic moduli, cohesion, internal friction) are systematically varied to assess their impact on the relationship between fault slip and surface displacement. Preliminary results indicate that increased cohesion, representing increased clay content, promotes shear strain localization at the surface. Results from a representative Napa model, based on lab testing of soil samples and the best estimate of fault kinematics, are compared with LiDAR data of displaced vine rows. The results have implications for finite fault models, which may obscure near-surface slip by neglecting plastic deformation in the shallow crust.

Nevitt, J. M., Brooks, B. A., Ericksen, T. L., Glennie, C. L., Madugo, C. M., Lockner, D. A., Moore, D. E., Minson, S. E., & Hudnut, K. W. (2016, 08). Effects of elastoplastic material properties on shallow fault slip and surface displacement fields. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)