Using Postseismic Relaxation Test Southern California Viscosity Models

Kaj M. Johnson, Jacob H. Dorsett, Simone Puel, & Thorsten W. Becker

Published August 15, 2020, SCEC Contribution #10674, 2020 SCEC Annual Meeting Poster #114

We continue to develop 3-D heterogeneous viscoelastic static Green's functions (GFs) for deformation in southern California for the SCEC Community Rheology Model. We are testing viscoelastic mantle models using GPS-derived postseismic displacements following the 1999 Hector Mine, 2010 El Mayor – Cucapah, and 2019 Ridgecrest earthquakes.

To build the 3D viscoelastic model, we first construct a 1D reference viscosity model from previously published viscosity models from postseismic deformation studies. We infer temperature anomalies from simple empirical relationships between temperature and seismic velocity anomalies starting from a combination of regional and global tomographic models. For the lower crust, we estimate the viscosity as linear combination of multiple empirical components (dislocation creep formula) that depend on temperature, seismic velocity, and strain rate.

Results from modeling Ridgecrest postseismic displacements suggest a transient viscosity is required to fit the data (Burgers and power-law rheology). All models which match the data require a low effective postseismic viscosity on the order of 1-5x1017 Pa s at 60-100 km depth. From suites of postseismic viscosity models, we back-calculate a permissible range of steady-state (pre-earthquake) effective viscosity depth profiles showing a viscosity range of 1-8x1018 Pa s at 60-100 km depth, which tends towards lower estimates inferred in previous studies, and lower than the 1D reference model, but consistent with relatively low mantle viscosity inferred in this region from tomography.

The 3D viscoelastic model shows lateral variations in upper mantle viscosity spanning nearly 4 orders of magnitude in southern California. Comparisons of preliminary models of deformation following the 1999 Hector Mine earthquake with 1D and 3D viscosity models show that the lateral viscosity variations contribute surface displacement deviations from the 1D model up to about 10 cm at wavelengths of 100s of km. These deviations are of similar magnitude to measured displacements ~200 km from the earthquake source. This demonstrates that models of postseismic displacements 10-20 years after the 1999 Hector Mine earthquake will be important constraints on building the 3D viscosity model for southern California.

Key Words
rheology, mantle, postseismic

Citation
Johnson, K. M., Dorsett, J. H., Puel, S., & Becker, T. W. (2020, 08). Using Postseismic Relaxation Test Southern California Viscosity Models. Poster Presentation at 2020 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)