Poor knowledge of how faults slip and distribute deformation in the shallow crust hinders efforts to mitigate hazards where faults increasingly intersect with the expanding global population at Earth's surface. Here we analyze two study sites along the 2014 M 6.0 South Napa, California, earthquake rupture, each dominated by either co-or post-seismic shallow fault slip. We combine mobile laser scanning (MLS), active-source seismic tomography, and finite element modeling to investigate how deformation rate and mechanical properties of the shallow crust affect fault behavior. Despite four orders-of-magnitude difference in the rupture velocities, MLS-derived shear strain fields are remarkably similar at the two sites and suggest deceleration of the co-seismic rupture near Earth's surface. Constrained by the MLS and seismic data, finite element models indicate shallow faulting is more sensitive to lithologic layering and plastic yielding than to the presence of fault compliant zones (i.e., regions surrounding faults with reduced stiffness). Although both elastic and elastoplastic models can reproduce the observed surface displacement fields within the uncertainty of MLS data, elastoplastic models likely provide the most reliable representations of subsurface fault behavior, as they produce geologically reasonable stress states and are consistent with field, geodetic, and seismological observations. strain within compliant zones 30,31 . Such deformation may arise from mechanisms of plastic yielding, for which evidence exists in paleoseismic trenches 32,33 . Although gravitational effects in layered media, including the acceleration of viscoelastic relaxation at long wavelengths and attenuation of the overall vertical displacement field, become important at length scales greater than several elastic plate thicknesses and/or over time periods much greater than the relaxation time, the effect on strains is negligible in the near-field and within a single earthquake cycle [34][35][36][37][38] .Relatively unstudied factors that may affect near-field deformation include layered elastic properties 39,40 and deformation rate (e.g., co-seismic versus post-seismic slip) 41,42 . Without greater confidence in how these proposed factors affect faulting near Earth's surface, we cannot reliably use the results of near-field geodetic analyses to infer shallow deformation, nor can we formulate models to predict fault slip and deformation in future events.
Scientific RepoRtS |(2020) 10:5031 | https://doi.www.nature.com/scientificreports www.nature.com/scientificreports/ zone surrounding the fault. The corresponding surface deformation fields, however, do not obviously respond to these distinct elastic structures. In addition, the compliant zone width at Saintsbury (Fig. 2f) is nearly twice that of the observed shear zone (Fig. 2d), further indicating that the compliant zone does not strictly control the strain distribution. Our estimate of compliant zone width represents a minimum constraint 28 , since surveys covering the greater Napa ...