Inferring Crustal Viscosity from Seismic Wavespeeds: Applications to the Rheologic Structure of Southern California

William Shinevar, Mark Behn, Greg Hirth, & Oliver Jagoutz

Published August 12, 2017, SCEC Contribution #7517, 2017 SCEC Annual Meeting Poster #230 (PDF)

We constrain the viscosity of the lower crust through a joint inversion of seismic P-wave (Vp) and S-wave (Vs) velocities. Previous research has demonstrated robust relationships between seismic velocity and crustal composition, as well as between the composition and viscosity of the lower crust. Here we extend these analyses to determine whether seismic velocity can be used as a robust indicator of crustal viscosity. First, we use the Gibbs free energy minimization routine Perple_X to calculate equilibrium mineral assemblages and seismic velocities for a global compilation of crustal rocks at various pressures and temperatures. Second, we use a rheological mixing model that combines single-phase flow laws for major crust-forming minerals to calculate bulk viscosity from the predicted mineral assemblages. Our results are influenced by the choice of the garnet flow law, which is poorly constrained at crustal conditions. However, assuming a strong garnet flow law (with a strength similar to that of pyroxene) we find a robust correlation between crustal viscosity and Vp together with Vs. Applying our method to seismic data from southern California to calculate lower crustal viscosity for the Community Rheology Model (CRM), we find that the lower crustal viscosity regionally varies over five orders of magnitude and is most closely approximated by the wet plagioclase creep flow. In addition, we provide a best-fit flow law for our predicted viscosities in Southern California. Our results agree with regional geology and geodetic estimates for the lower crustal viscosity.

Key Words
Community Rheology Model, (CRM), Viscosity, Composition, Temperature, Rheology, Lower Crust

Citation
Shinevar, W., Behn, M., Hirth, G., & Jagoutz, O. (2017, 08). Inferring Crustal Viscosity from Seismic Wavespeeds: Applications to the Rheologic Structure of Southern California. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)