SCEC Award Number 16106 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Rheological Mixing Laws for Application to the Community Rheology Model
Name Organization
Greg Hirth Brown University Mark Behn Woods Hole Oceanographic Institution
Other Participants Billy Shinevar, MIT/WHOI graduate Student
Brown undergraduate student
SCEC Priorities 1b, 2d, 3a SCEC Groups CME, SDOT
Report Due Date 03/15/2017 Date Report Submitted 05/01/2017
Project Abstract
We initiated development of a catalogue of rheological flow laws and construct quantitative tools to calculate effective viscosities of crustal rocks appropriate for application in the nascent Community Rheological Model (CRM). Over the last few years, there has been a growing appreciation for how the rheology of the lithosphere is important for several SCEC goals linked to the Community Stress Model (CSM), Community Geodetic Model (CGM), and the SDOT group’s goals of constraining the loading of faults at the time scales much greater than an earthquake cycle. We investigated how pertinent mineral and rock flow laws, as well as rheological mixing laws can be used to calculate effective viscosities for crustal rocks. In anticipation of preparatory efforts to construct a Community Thermal Model (CTM), as well as efforts to constrain pertinent rock types based on relationships between seismic velocity (Vp, Vs, Vp/Vs) and rock composition, we explored the efficacy of calculating rock viscosity based on composition and highlight where important assumptions about rheology need to be considered.
Intellectual Merit In anticipation of on-going efforts to construct a Community Thermal Model (CTM), as well as efforts to constrain pertinent rock types based on relationships between seismic velocity (Vp, Vs, Vp/Vs) and rock composition, we describe the efficacy of calculating viscosity based on rock composition. The products of our analyses will be tractable macros and catalogues that can be easily incorporated into a CRM as the project matures – in collaboration with the other SCEC scientists leading efforts to develop the CTM, seismic imaging and geodynamic modelling pertinent to the effort. These efforts will also provide a “rheological backbone” onto which additional processes can be studied, such as grain size evolution, transient creep and the evolution of macroscopic shear zone structure – all of which are important for understanding processes responsible for lithospheric-scale strain localization, the interpretation of post-seismic creep, and understanding earthquake rupture dynamics near the brittle-plastic transition.
Broader Impacts One of the most important reasons to understand crustal rheology is for the accurate assessment of earthquake hazards produced by time-dependent loading of seismogenic faults – as quantified by geodetic studies. Our research also involves routine broader impacts associated with educating undergraduates and graduate students. We have successfully attracted numerous undergrads to work on experimental and modeling projects for honors theses at Brown and through the WHOI Summer Student Fellowship Program. We will strive to recruit undergrads from traditionally under-represented groups, as engagement in undergraduate research is perhaps the most promising way to increase the number of under-represented minorities in the “pipeline” to graduate school and academic positions in Earth sciences. Finally, we support the development of shared, open-source modeling software – one of the primary goals of our SCEC work on the CRM. We have previously released codes for simulating long-term lithospheric deformation and olivine fabric development.
Exemplary Figure Figure is called EXEMPLARY FIGURE at end of the report:

We use seismic velocity (P and S wave) from the SCEC CVM to constrain lower 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 and seismic velocities for a global compilation of lower crustal rocks at various pressures and temperatures. Second, we use a mixing model and single-phase flow laws for major crust-forming minerals to calculate bulk viscosity for the predicted equilibrium mineral assemblages. Finally, we linearly fit the viscosity calculations to the seismic velocity calculations. This method provides a strong (R2>0.9) fit in the alpha-quartz regime. Our figure shows a viscosity map for 25km depth assuming T=700°C and έ=10-14 s-1 as well as the viscosity-velocity data shown in the upper right hand corner. High viscosity in the Salton Trough is measuring mantle, rather than crustal behavior, owing to shallow Moho. The colored lines are the estimated fit. Our results are in agreement with regional geodetic estimates.