Inferring Southern California Crustal Viscosity Structure from the SCEC Community Velocity Model

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

Submitted July 25, 2016, SCEC Contribution #6363, 2016 SCEC Annual Meeting Poster #339

In this study we constrain the viscosity of the lower crust through a joint analysis of the role of rock composition on seismic P-wave (Vp) , S-wave (Vs) velocities and rheology. 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. Our analysis follows a three-step approach. First, we use the Gibbs free energy minimization routine Perple_X to calculate equilibrium mineral assemblages for a global compilation of crustal rocks at various pressures and temperatures. Second, we use the Huet et al. (JGR, 2014) mixing model and single-phase flow laws for major crust-forming minerals to calculate bulk viscosity for the predicted equilibrium mineral assemblages. Third, we use Perple_X to calculate seismic velocities for the mineral assemblages. Our results are strongly 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 strong correlation between crustal viscosity and the seismic Vp/Vs ratio. This implies that even in regions where seismic velocity does not give a strong constraint on composition, it still provides robust estimates of crustal viscosity. We apply our method to seismic data from Southern California to calculate a viscosity for the regional lower crust.

Key Words
Viscosity Velocity

Citation
Shinevar, W., Behn, M., Hirth, G. H., & Jagoutz, O. (2016, 07). Inferring Southern California Crustal Viscosity Structure from the SCEC Community Velocity Model. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Community Modeling Environment (CME)