Understanding Crustal Stress Heterogeneity in Southern California using Focal Mechanism Inversions and Shear Wave Splitting Fast Velocities

Leora J. Wilson, Karen M. Luttrell, & Jeanne L. Hardebeck

Published December 17, 2021, SCEC Contribution #11850

The Los Angeles region of southern California is structurally complex with demonstrated heterogeneity in crustal stress orientation. Maximum horizontal compression azimuth (SHmax) can be calculated from focal mechanism inversion, inferred from shear wave splitting fast directions (presumably corresponding to stress-induced crustal fabric), or sometimes directly measured from borehole breakouts. These different techniques are sensitive to stress over different depth ranges and crustal volumes, making it unclear how these observations should be jointly interpreted and incorporated into ongoing stress modeling efforts. In this study, we use published catalogs of earthquake focal mechanisms and individual earthquake-to-station raypath shear wave splitting fast direction measurements from across southern California. We perform new focal mechanism stress inversions over an assortment of crustal volumes and compare the indicated SHmax direction with the shear wave fast directions indicated by ray paths through the same crustal volumes. In particular, we investigate the degree to which these two indicators of crustal stress orientation are consistent, and over what spatial scales they are consistent, both vertically and laterally. These comparisons can help us both illuminate the present crustal stress heterogeneity and potentially understand the linkages between the development of crustal fabric and the modern stress state. Preliminary results suggest clear differences between stress orientations indicated by shear wave splitting and focal mechanisms, with subregions of disagreement largely correlating with major structural and fault features of the area. These discrepancies may be reconciled by more detailed analysis of particular crustal volumes, but further research is needed. If the discrepancies persist, this may record changes in southern California crustal stress during basin development, or could indicate low stress magnitudes such that crustal fabric is sensitive to factors other than stress orientation. These results have implications for the broader understanding of the interplay between the evolution of crustal stress and sediment basin structures.

Wilson, L. J., Luttrell, K. M., & Hardebeck, J. L. (2021, 12). Understanding Crustal Stress Heterogeneity in Southern California using Focal Mechanism Inversions and Shear Wave Splitting Fast Velocities. Poster Presentation at AGU Fall Meeting.