SCEC Award Number 17202 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Integrating Seismic Velocity Data and Experimental Flow Laws into the Community Rheology Model
Investigator(s)
Name Organization
Greg Hirth Brown University Mark Behn Woods Hole Oceanographic Institution
Other Participants
SCEC Priorities 1b, 2d, 3a SCEC Groups CXM, SDOT, CME
Report Due Date 06/15/2018 Date Report Submitted 11/13/2018
Project Abstract
 We extend our analysis of correlations between seismic velocity (P and S wave) and effective viscosity with applications for the SCEC Community Rheology Model (CRM). Our analyses 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 viscosities for the predicted equilibrium mineral assemblages. Third, we fit the viscosity calculations to seismic velocity calculations for the same lithology. As shown below (using input from the SCEC Community Velocity Model (CVM)), this method provides an independent constraint on crustal viscosity in Southern California. We find a robust correlation between crustal viscosity and together with in the α–quartz regime. Using seismic data from Southern California, our method predicts that lower crustal viscosity varies regionally by four orders of magnitude, and lower crustal stress varies by three orders of magnitude. At least half of the total variability in stress can be attributed to composition, implying that regional lithology has a significant effect on lower crustal geodynamics.
Intellectual Merit The continued 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 Use figure 2 from the report