A 3-D crustal compositional model of S. California and its implications on the instantaneous stress state of the lithosphere

Weisen Shen, Siyuan Sui, Alireza Bahadori, William E. Holt, Jeonghyeop Kim, & Lajhon Campbell

Published August 16, 2021, SCEC Contribution #11633, 2021 SCEC Annual Meeting Poster #015

Constraining the stress and stressing rate of the California area with Geodynamic modeling requires knowledge of the seismic, thermal, and rheological properties of the lithosphere. In this presentation, we present a 3-D simulation of the dynamics of the crust and uppermost mantle based on the rheological model defined by a high-resolution chemical compositional model. This compositional model, constrained by seismic properties, provides rheological layerization of the crust, as well as the crustal heat generation which is sensitive to the crustal thermal structure. Particularly, four efforts of the work are summarized: 1) using more than 300 seismic stations in the Southern California area, we map the variations in crustal Vp/Vs ratios. 2) These Vp/Vs ratios are then combined with the crustal shear velocity to quantify the SiO2 content of the crystalline crust and a 3-D compositional model of the crust of S. California is built. 3) Based on the compositional model, we map the variations in thicknesses of the weak, felsic upper crust, intermediate middle crust, and a strong, mafic lower crust, and assign rheological parameters to each of these crustal layers accordingly. 4) Finally, we perform a set of geodynamic modeling to the lithosphere using the UnderWorld Geodynamic modeling tool (UWG) based on the new crustal rheological model. In the last stage, we apply plate-motion boundary conditions to the left, right, front, and back of the model. Material deformation is expressed based on non-Newtonian visco-plastic rheologies with viscosity dependence on temperature, stress, and strain rate. We pay special attention to the resulting strain rate and deviatoric stress fields, which are critical contributions to the seismic hazard analysis in this area. These efforts help us identify the geological features that contribute to the spatial variations of the seismicity and GPS-observed deformation in the S. California area. For example, we identify that the mafic lower crust beneath the Peninsular Ranges is generally thicker than the areal average, leading to a higher crustal strength and lower strain rate. In the geodynamic modeling, the flow is computed through diffusion and dislocation creep for the viscous component and Drucker-Prager yield criterion for the plastic component. The flow law parameters for the layered rheologies are guided by the seismically defined composition model.

Key Words
Compositional model, Crustal strength, Geodynamic modeling

Citation
Shen, W., Sui, S., Bahadori, A., Holt, W. E., Kim, J., & Campbell, L. (2021, 08). A 3-D crustal compositional model of S. California and its implications on the instantaneous stress state of the lithosphere. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)