SCEC Award Number 18187 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Collaborative research: Lithosphere-scale viscosity variations in South California
Investigator(s)
Name Organization
Laurent Montesi University of Maryland William Holt Stony Brook University
Other Participants Kristel Izquierdo, University of Maryland;
Alireza Bahadori, Stony Brook University
SCEC Priorities 1c, 1b, 3b SCEC Groups CXM, SDOT, Geodesy
Report Due Date 03/15/2019 Date Report Submitted 08/11/2019
Project Abstract
 There are several ways of constraining the state of stress and loading conditions of faults in Southern California. In this project, we compare estimates of viscosity based on geodynamic modeling and rheological considerations. Stress at each point on a grid covering Southern California is provided by a geodynamic model constrained by gravitational potential energy values, boundary condition velocities, and surface kinematics. That stress and the original strain rate field lead to a geodynamic estimate of the lithosphere viscosity. Then, we generate a strength envelope at each point and average the strength envelope with depth for various strain rates to produce an effective rheology of the lithosphere. We determine the strain rate associated with the geodynamically-inferred stress and use it to produce a viscosity estimate from the rheological model. The two viscosity estimates are similar only if the weakest crust and mantle rheologies are used (wet rheologies, and grain size tied to a piezometric relation) or the strength is not allowed to exceed a few tens of MPa. Even so, the magnitude of viscosity variations from the rheological model are much larger than in the geodynamical model. Although certain variations are consistent between the models (strong Sierra Nevada block, weak Salton trough area) the Colorado Plateau appears strong in the geodynamic model but weak in the rheological model. This study shows that there are likely important rheological variations due to mineralogy but also that the strength envelope is unlikely to be at failure at every depth.
Intellectual Merit This project provides a first evaluation of a rheology model encompassing Southern California. The first test of this model considers the integrated strength of the lithosphere, or equivalently its depth-averaged viscosity, and highlights both agreement and discrepancies with geodynamic models. This project reveals the limits of building a rheological model from laboratory flow laws and just ways of modifying the rheological understanding of the Southern California lithosphere: We suggest that reduced grain size is important near fault and that failure stress is not reaches in the blocks between the faults. Differences in temperature can explain some of the broadest patterns observed in the region, although the amplitude of variations is probably overestimated at this point.
Broader Impacts The project involved graduate students at both Stony Brook University and University of Maryland. The UMD student is female and native of Mexico. She has worked until now on planetary science applications and this constitutes her first earth-focused project. The student will participate to the 2019 SCEC meeting.
Exemplary Figure Figure 4: Comparison between the viscosity field expected from geodynamics (left) and rheological modeling (right). Even though there are similarities in the patterns, the amplitude of variations are very different between these calculations (note the different scale bar).