The Dependence of Surface Deformation on Rheology Throughout the Seismic Cycle

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A fundamental question in lithospheric dynamics is if the lower crust is stronger or weaker than the upper-most mantle. This question is not only of importance to understanding the physical state of the lithosphere, but is also essential to being able to describe the earthquake cycle on major faults (i.e., the repeated cycle of stress build-up on faults during a period of hundreds to thousands of years, and then the eventual release of that stress during an earthquake).Observations of the earthquake cycle are largely composed of measurements of the deformation of Earth?s surface at times during the earthquake cycle. In order to translate those observations of surface deformation to stresses in the seismogenic crust, we require mechanical models that accurately describe the mechanical properties (i.e., rheologies) of the lithosphere. Whether the lower crust is stronger or weaker than the upper-most mantle has strong implications for the evolution of stresses on faults during the earthquake cycle. As the earthquake cycle is significantly longer than the era of modern measurements of Earth deformation, we do not have a complete record of observations throughout an earthquake cycle. Without complete observations, it is not always possible to uniquely validate mechanical models directly from observations. Furthermore, models of the earthquake cycle with significant rheologic complexity are often computationally expensive, and thus not suited to explore the full range of permissible model parameters that are consistent with observations.Mechanical models of the earthquake cycle most often only contain Maxwell viscoelasticity, and do not consider either a robust depth- dependence of material properties or localized shear zones within the lower crust or mantle. However, the rheology of the lithosphere is likely more complicated than Maxwell viscoelasticity and material properties of the lithosphere are expected to be depth-dependent. Rheologic complexities that likely have significant impact on deformation of the lithosphere throughout the earthquake cycle include depth-dependent viscosity, power-law creep, Burgers viscoelasticity, and localized creep at depth. In this project we will: (1) Quantify the sensitivity of surface deformation throughout the earthquake cycle to rheologies at various depths.; (2) Determine if Burgers viscoelasticity and power-law creep have the same affect in models of the earthquake cycle.; (3) Systematically test the similarity of interseismic surface deformation due to localized or distributed creep at depth.; (4) Establish the time-dependent correspondence between surface deformation in idealized models and deformation in models with depth dependent viscosity, power-law creep, transient viscoelasticity, or localized creep at depth. The last point will contribute to an understanding of the non-uniquenesses inherent in trying to validate models of the earthquake cycle from observations of surface deformation. Additionally, by knowing the correspondences between idealized models and classes of more complicated models, one will be able to constrain computationally efficient idealized models to geodetic data, and then determine the range of the more complicated models directly from the idealized model inferences.