SCEC Award Number 18156 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Incorporating the effects of hydrous phases and strain localization into seismic-velocity-based models for the Community Rheology Model
Investigator(s)
Name Organization
Greg Hirth Brown University Mark Behn Woods Hole Oceanographic Institution
Other Participants Billy Shinevar (MIT/WHOI)
Leif Tokle (Brown)
SCEC Priorities 1b, 2d, 3a SCEC Groups CXM, SDOT
Report Due Date 03/15/2019 Date Report Submitted 05/06/2019
Project Abstract
 We extended our analysis of correlations between seismic velocity (P and S wave) and effective viscosity to include the influence of weak hydrous phases (e.g., micas) and the role of fabric on strain localization. As in earlier analyses on anhydrous rocks (Shinevar et al., 2015), we follow a three-step approach. First, we calculate equilibrium mineral assemblages and seismic velocities for a global compilation of lower crustal rocks at various pressures and temperatures relevant for the mid- to lower crust in Southern California (using Perple X; Connolly, 2009). Second, we use rheological mixing models and single-phase flow laws for major crust-forming minerals to calculate aggregate viscosity for the predicted equilibrium mineral assemblages following Huet et al., (2014). Third, we fit the viscosity calculations to the seismic velocity calculations for the same lithology. Using input from the SCEC Community Velocity Model (CVM), our methodology provides an independent constraint on crustal viscosity in Southern California (Figure 1a). A focus of these calculations was including the potential rheological effects of modest amounts of hydrous mineral phases, which are known to be present in rocks from Southern California (e.g., Figure 1b) and processes that lead to strain localization. Inclusion of hydrous phases is particularly pertinent given that the Geologic Framework for the CRM in the Mojave region highlights the importance of biotite (e.g., Oskin et al., SCEC CRM Workshop talk, 2017). Further, our modelling strategy provides a methodology for exploring the potential the rheological effects of fabric formation and grain size evolution on strain localization.
Intellectual Merit The on-going development of the CRM in SCEC5 provides a platform to assess the role of lithosphere rheology on fault loading at time-scales relevant for post-seismic creep to time-scales much greater than the earthquake cycle. The CRM leverages (and integrates) the community efforts involved with SCEC goals linked to the Community Stress Model (CSM), Community Geodetic Model (CGM), and SDOT. In concert with on-going efforts to refine the Community Thermal Model (CTM), as well as the Geologic Framework for the CRM based on structural data and relationships between seismic velocity (Vp, Vs, Vp/Vs) and rock composition, we describe the efficacy of calculating effective viscosity based on rock composition. These studies will facilitate analysis of processes responsible for lithospheric-scale strain localization, the interpretation of post-seismic creep, and earthquake rupture dynamics near the brittle-plastic transition.
Broader Impacts In this project, we have engaged graduate students at Brown and WHOI/MIT in research projects aimed at increasing knowledge of seismic hazards - integrating a broad range of expertise and approaches.
Exemplary Figure Figure 2 from the report (the figure is from Shinevar et al., EPSL, 2018)

Figure 2: (a) Regional seismic-aseismic transition. (b) Predicted brittle-ductile transition. (c) Viscosity versus temperature (depth) for a 17oC/km geotherm for dislocation creep of quartzite, plagioclase, and calculated aggregate viscosities for granite and gabbro. Black arrows illustrate potential rheologic effect of fabric formation during weakening of granite and gabbro to produce interconnected quartz and plagioclase, respectively. (d) Diagram showing: (i) stress versus depth for the same flow laws in (c); (ii) mean locking depths; and (iii) locking depth temperature for regions east (red) and west (blue) of the SAF.