How certain are regional strength envelopes in Southern California?

Kristel D. Izquierdo, Laurent G. Montesi, William E. Holt, Alireza Bahadori, & William Shinevar

Published August 15, 2019, SCEC Contribution #9702, 2019 SCEC Annual Meeting Poster #316

Comparing different estimates of viscosity in the lithosphere can lead to a more robust understanding of the geological origin of rheological variations in the study area. Similar patterns of viscosity increase confidence in the validity of the rheological model or methods used, while differences might suggest new observations are needed to reconcile them. In this study, we compare depth averaged viscosity estimates from a geodynamic model with estimates from rheological models and SCEC-related datasets in a grid covering Southern California. The geodynamic model is constrained by gravitational potential energy values (associated with topography, crustal structure, and crustal and upper mantle densities) and boundary condition velocities and surface kinematics (from GPS data). We create eight alternative rheological models that assume either wet or dry rheologies, a uniform grain size (1mm) or a grain size tied to a piezometer, and a maximum stress at depth limited to 300 or 30 MPa. We generate a strength envelope at each point of the grid based on various SCEC-related datasets and average the strength envelop 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 geodynamic and rheologic strength estimates best agree when lower crustal rheology is weakest. This can be achieved through the use of wet flow laws, piezometric grain size reduction, and capping upper crustal strength to 30 MPa. Both methods show a strong Sierra Nevada Block and a weak Salton Trough area while the Colorado Plateau appears strong in the geodynamic model but weak in the rheological model. Rheological variations due to mineralogy are likely to occur and the strength envelope is unlikely to be at failure at every depth.

Key Words
Rheology, viscosity, geodynamic models

Citation
Izquierdo, K. D., Montesi, L. G., Holt, W. E., Bahadori, A., & Shinevar, W. (2019, 08). How certain are regional strength envelopes in Southern California?. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)