Orientation of faults, fault roots, rock fabric, stress, and deformation in Southern California: Geographical comparisons and field and numerical experiments

Thorsten W. Becker, Vera Schulte-Pelkum, Whitney M. Behr, Robert Porritt, & Meghan S. Miller

Published August 15, 2018, SCEC Contribution #8699, 2018 SCEC Annual Meeting Poster #157

How do faults root into the ductile deep crust? Do pre-existing faults and shear zones affect present-day faulting and deformation? Here, we discuss constraints from existing datasets across Southern California and introduce a new seismic experiment in the Mojave designed to advance our understanding of lithospheric shear zone structure.

The spatial alignment between geological and geophysical indicators of deformation can provide interesting clues. Here, we comprehensively compare mapped fault traces, stress and strain patterns derived from different methods, estimates for seismic anisotropy from a range of depths from the upper crust to the mantle, and constraints from geological history. The results suggest reorientation of stress near major fault traces as well as faulting at non-optimal angles to stress. Anisotropic rock fabric appears pervasive across the region, and may explain some of the mismatch between deformation patterns and anisotropic “fast axes” within the crust. This has potential implications for mechanical anisotropy and stress transmission on faults from tectonic loading.

A closer look at the downward continuation of crustal faults into lithospheric shear zones is possible with dense temporary seismic deployments. Past deployments often cross faults where near-surface structure changes across the fault (usually crossing into a sedimentary basin on one side), making it difficult to disentangle near-surface effects on seismic waveforms from deeper structure. We report on a new 19-station, partially SCEC-funded deployment crossing several fault strands in the Mojave that is designed to place all stations on bedrock to reduce bias from near-surface effects. Separately funded work on crustal and mantle xenoliths erupted nearby provides ground truth constraints on seismically inferred rock fabric. Stations were installed in Spring 2018.

Key Words
stress; strain; seismic anisotropy; shear zones

Citation
Becker, T. W., Schulte-Pelkum, V., Behr, W. M., Porritt, R., & Miller, M. S. (2018, 08). Orientation of faults, fault roots, rock fabric, stress, and deformation in Southern California: Geographical comparisons and field and numerical experiments. Poster Presentation at 2018 SCEC Annual Meeting.

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