Rheology of Southern California from Mineral to Regional Scale

Laurent G. Montesi, Kristel Izquierdo, Kali L. Allison, William E. Holt, Alireza Bahadori, Greg Hirth, William Shinevar, & Michael E. Oskin

Published August 15, 2019, SCEC Contribution #9735, 2019 SCEC Annual Meeting Talk on Tue 1400 (PDF)

The stresses that load faults to rupture and trigger earthquakes have their origin in the global dynamics of our planet. Mantle convection and plate motion results in a far-field loading at regional scale, locally modified by topography, magmatic intrusion, static and dynamic stress transfer, groundwater, passing seismic waves, and probably a myriad of other phenomena. Understanding how these stress sources influence each fault requires a good knowledge of the rheology of the rocks both inside and between fault zones. We focus here on understanding how geological complexity and heterogeneity influences rock rheology, particularly in the ductile regime.

The wealth of datasets available over Southern California, many of which collected under the auspices of SCEC, makes it possible to develop a first-order view of various tectonic domains and the generic rock types present at depth. Mixing relations that utilize estimates of mineral assemblages in each rock type and experimentally calibrated flow laws provide rough estimates of rheology, strength, and effective viscosity and how it varies with depth and throughout the region. These estimates may be used in regional-scale numerical models to evaluate stress transfer, for example, but they must first be tested and evaluated against independent observations. For example, we discuss how integrated strength or equivalently depth-averaged viscosity varies throughout Southern California. Comparison with semi-independently derived estimates of these quantities based on geodynamic analysis shows a general qualitative agreement, but with important differences. For example, rheological estimates generally imply a stronger lithosphere than geodynamics estimates and a greater spatial variability in effectivity viscosity. This implies that strength reduction is a common phenomenon throughout the region. In the downdip continuation of seismogenic faults, the origin of strength reduction may be grain size reduction and fabric development. In the blocks that separate the faults, it is more likely that stress level is insufficient to reach the maximum strength envelope assumed in this study. In that case, rheology cannot be estimated independently from stress. Our work highlights the importance of including geological complexity when considering lithospheric strength and also the limitation of traditional strength envelope approach.

Key Words
rheology, stress

Citation
Montesi, L. G., Izquierdo, K., Allison, K. L., Holt, W. E., Bahadori, A., Hirth, G., Shinevar, W., & Oskin, M. E. (2019, 08). Rheology of Southern California from Mineral to Regional Scale. Oral Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)