Instantaneous stress state of the lithosphere of Southern California: A synthesis of geophysical and compositional products of SCEC

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

Published September 11, 2022, SCEC Contribution #12488, 2022 SCEC Annual Meeting Poster #191 (PDF)

The southern California region is defined by complex seismotectonics in which there is an equally complex stress state that is yet to be fully understood. Our goal is to build a geodynamic framework that quantifies the instantaneous stress state of the lithosphere of southern California. 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 (Chapman, 1986). As such, we develop a 3-D coupled thermo-mechanical model of the lithosphere (crust and uppermost mantle) in southern California using UWGeodynamics code (Moresi et al., 2002, 2003, 2007; Beucher et al., 2019). We use a free-slip boundary condition for defined air and sticky-air layers on top of the surface topography to simulate the free surface and apply the Pacific-North America velocity boundary condition below. We have adapted the UWGeodynamics code to account for the dynamic influences of variable topography of the surface, Moho, and internal compositional layers (Bahadori et al., 2022).

We define the geometries (of variable thickness) of the compositional layered crust according to the estimated SiO2 wt%, derived from the latest crustal Vs and Vp/Vs structures (Sui et al., 2022). Rheologies for the geodynamic model are quartz dislocation (Burov, 2011) for the upper crust, wet diorite (Carter and Tsenn, 1987) for the middle crust, dry Maryland diabase for lower crust, strong dry Maryland diabase for oceanic crust, and wet olivine for a hydrated mantle lithosphere (Burov, 2011). We also incorporate the thermal state of the model, as constrained from the seismic data in the mantle (Shen and Ritzwoller, 2016) and from published data products for the crust (Shinevar et al., 2018). We also investigate the role of mantle flow tractions applied to the base of the lithosphere, obtained from separately computed mantle flow calculations using the global density distribution model TX2008 (Moucha et al., 2009; Simmons et al., 2009). Earliest results indicate that thickened zones of felsic composition have a positively correlated influence on strain rates within the crust. This suggests that a component of the geometry of the shear zones within the plate boundary zone in Southern California owe their existence to the 3-D rheological structure. The location of these shear zones in the crust also shows a strong correlation with the geographic location of fault zones at the surface.

Key Words
Dynamics, Rheology, Stress, 3-D

Citation
Campbell, L., Sui, S., Bahadori, A., Shen, W., & Holt, W. E. (2022, 09). Instantaneous stress state of the lithosphere of Southern California: A synthesis of geophysical and compositional products of SCEC. Poster Presentation at 2022 SCEC Annual Meeting.

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