How Sensitive are Inferred Stresses and Stressing Rates to Rheology? Clues from Southern California Deformation Models

Elizabeth H. Hearn

Submitted September 20, 2016, SCEC Contribution #6447, 2016 SCEC Annual Meeting Talk on 9/12 14:00 (PDF)

Variations in fault zone strength and lower lithosphere viscoelastic rheology have been shown to influence deformation patterns in fault systems over a range of time scales (e.g. Bird and Kong, 1994; Fay and Humphreys, 2005). Because of our mandate to characterize stresses in the southern California lithosphere, SCEC has proposed a resource (the Community Rheology Model) that will facilitate representing Southern California’s rheology in deformation models. I will present some recent modeling results demonstrating how assumed fault zone and rock rheologies influence estimates of stresses, stressing rates and fault slip rates.

My kinematic and dynamic finite-element models incorporate the UCERF3 block model-bounding fault geometry (Field et al., 2014), with refinements suggested by geologists involved in the UCERF3 effort. The kinematic models are fit to either the SCEC CMM4 velocity field or a strain energy minimization criterion by varying fault slip rates, within the UCERF3 ranges. The (preliminary) dynamic models are tuned to fit geological fault slip rates and SHmax orientations by varying fault zone and rock rheologies. Deformation is driven from the sides in both sets of models.

The kinematic models suggest that 22-35% of the strain accumulation across Southern California is not associated with interseismic locking of the modeled faults, consistent with other studies (e.g. Bird, 2009, Johnson, 2013; Zheng and Shen, 2016). This is true whether or not the GPS velocity field is corrected for possible seismic cycle perturbations due to large SAF earthquakes. Plasticity exerts a profound influence on slip rates, deformation patterns and stresses obtained from dynamic deformation models. Variation in fault zone strength (quantified as shear traction) also affects stress magnitudes and orientations, but not surface velocities or slip rates in models with just the San Andreas Fault zone represented. Simulations are underway to assess how variations in fault zone strength and lithosphere rheology affect deformation in dynamic models incorporating the full fault network.

Key Words
lithosphere deformation, rheology, stress, fault slip rates, CSM, CRM

Citation
Hearn, E. H. (2016, 09). How Sensitive are Inferred Stresses and Stressing Rates to Rheology? Clues from Southern California Deformation Models. Oral Presentation at 2016 SCEC Annual Meeting.


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