SCEC Award Number 17241 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Modeling the effects of material property contrasts on the development of asymmetric deformation and uplift fields in restraining bends
Investigator(s)
Name Organization
George Hilley Stanford University
Other Participants Curtis W. Baden
SCEC Priorities 2a, 2e, 1d SCEC Groups SDOT, SAFS
Report Due Date 06/15/2018 Date Report Submitted 11/15/2018
Project Abstract
 Restraining bends generate zones of regional plate convergence, uplift, and seismic activity along the Pacific-North American plate boundary. Interestingly, topography and deformation occur asymmetrically about these features along the San Andreas Fault, which is not predicted by idealized elastic and plastic numerical models of deformation around these features. We tested the hypothesis that contrasts in crustal strength drive plastic deformation and uplift surrounding these bends, which ultimately generates the observed asymmetric topography, using the finite element software Abaqus. We varied elasto-plastic parameters of a Drucker Prager and Mohr Coulomb constitutive model, and the basal and horizontal boundary conditions, to determine each’s effect on modeled long-term deformations.

We tailored the modeling after the Santa Cruz Mountains restraining bend, where previous work quantifies deformation using geodetic, geomorphic, thermochronologic, and geologic methods. We show that, under certain constitutive rules and boundary conditions, crust may be asymmetrically advected and uplifted along the modeled restraining bend, consistent with thermochronologic, topographic, and geomorphic observations within the Santa Cruz Mountains, but inconsistent with purely elastic model predictions. We find that, in addition to producing asymmetric distributions of topography and deformation, irrecoverable off-fault failure may be essential in reconciling fault zone behavior observed over geodetic and geologic timescales.
Intellectual Merit We developed a mechanistic understanding of how strike-slip plate boundary processes build structures that we geologically observe in restraining bend structures. The use of an elasto-plastic constitutive rule in our models localized deformation within the restraining bend, and allowed lateral advection of crust into the structure, consequently producing exhumation histories that were quantitatively consistent with our low-temperature thermochronologic ages. These models reproduce the geologically resolvable deformation present in the SCM, which accrued over numerous earthquake cycles through the plastic yielding of Earth’s crust. Thus, to first order, the geologically observed deformation within the Santa Cruz restraining bend can be modeled by extending previously used elastic models to include the effects of plastic yielding.
Broader Impacts San Andreas fault hosts two notable restraining bends along its length, which together pose a seismic hazard to 25 million California residents: the Big Bend, centered within the Los Angeles area, and the Santa Cruz Bend, which lies adjacent to the San Francisco Bay Area. Each of these restraining bends hosts large magnitude earthquakes, which threaten the lives and infrastructure each of these densely populated areas. This project shapes our understanding of the localization of inelastic, off-fault deformation in the vicinity of these restraining bend structures, which assists in further quantifying the seismic hazards they pose.

This SCEC award funded a graduate student, and fostered collaborations between the graduate student and more senior SCEC-affiliated scientists within the field. The graduate student participated in the SCEC Annual Meeting and presented work associated with this award, which in turn lead to constructive and collaborative discussions with the SCEC community.
Exemplary Figure Figure 4 - Plausible thermochronologic ages inferred from the presented modeled uplift fields agree with measured ages. A) Modeled uplift fields calculated for elastoplastic materials with homogeneous internal angles of friction ( i: phi = 30°; ii: phi = 40°) across the model domain exhibit asymmetry similar to that observed in the Santa Cruz Mountains topography. B) We plot the thermochronologic measurements presented above as a function of distance from the bend initiation. The yellow, green, and blue lines represent the free, intercept-constrained, and geodetically constrained regression fits to the data, respectively. We plot model-inferred ages alongside these data, and find that, within error, our observations are consistent with the presented mechanical models. The systematically low modeled ages (relative to the measured data) at the farthest distances from the modeled bend indicate that a tighter initial bend geometry may more closely reproduce our observations.
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