SCEC Award Number 18083 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Testing the effect of anisotropic rheology on lithospheric deformation in Southern California
Investigator(s)
Name Organization
Thorsten Becker University of Texas at Austin Whitney Behr University of Texas at Austin Vera Schulte-Pelkum University of Colorado Boulder
Other Participants Wanying Yang
SCEC Priorities 1c, 1e, 3b SCEC Groups SDOT, CXM, FARM
Report Due Date 03/15/2019 Date Report Submitted 03/16/2019
Project Abstract
 The goal of this project is to test the influence of anisotropic viscosity on the deformation of the lithosphere in general and of Southern California in particular. An important question for fault loading stresses and SCEC's Community Rheology Model (CRM) is whether viscous ani\-so\-tropy is required to represent the mechanical behavior of the lithosphere, or if isotropic rheology is sufficient. During the initial stages of this project,
we compiled evidence for the existence of mechanical heterogeneity in the region. We then started to investigate how and on what scale such heterogeneity needs to be incorporated into mechanical models in terms of their effect on force transmission. An improved representation of lithospheric deformation may help to resolve discrepancies in observed deformation patterns as inferred from geodesy, focal mechanisms, seismic anisotropy, and the geological record. To this end, we conducted initial modeling exercises to investigate the effect of anisotropic viscosity on the alignment of stress and strain under simple shear. We find that anisotropic viscosity changes the deformation response significantly, as expected, with strong sensitivity to the orientation of the anisotropic alignment. This opens up a possible avenue of comparison with field observations.
Intellectual Merit An improved representation of lithospheric deformation may help to resolve discrepancies in observed deformation patterns as inferred from geodesy, focal mechanisms, seismic anisotropy, and the geological record. Improving our understanding of such discrepancies may help resolve how faults are loaded, and how plate boundaries evolve. In particular, it is unclear if the development of SCEC5's proposed CRM requires anisotropic rheology, and how the CRM might include ductile shear zones and their geometries.
Broader Impacts The PIs collaborated with an external graduate student, Haibing Yang (University of Melbourne/ANU). The project formed new interdisciplinary links between geology, seismology, and geodynamics, and supported international collaboration. It funded modifications and new use of the CitcomCU and Underworld codes to incorporate and test anisotropic viscosity. Both codes are on GitHub and openly shared.
Exemplary Figure Effect of mechanical anisotropy on stress and strain-rate alignment.
Interior cross-section segments of a 3-D box sheared in $y$ direction ($v_y$, box dimensions: $x\times y \times z=4\times 6\times 1$) at box center (left panels, at $x=2$) and mid section depth-profiles (right panels, at $x=2$, $y=3$) for four different anisotropic director alignments (a-d) in terms of dip from the vertical, $\delta(n)$ ($\delta (n) = 90^\circ$ case is equivalent to a). The orthothropically anisotropic layer applies for $y\in[0.5;0.9]$ and has a weak to strong shear viscosity ratio of $0.01$.
Cross-sections show pressure in background, velocity (white vectors), and major compressive stress amplitude and orientation as orange sticks. Profiles show $v_y$ (blue lines) and maximum shear stress, $\left(\sigma_1-\sigma_3\right)/2$ (red, log-scale). All quantities are non-dimensionalized.