SCEC Award Number 22129 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Reconciling Stress Heterogeneity Within and Between Fault-Bounded Crustal Blocks From Focal Mechanisms, Shear Wave Splitting, and Borehole Breakouts
Investigator(s)
Name Organization
Karen Luttrell Louisiana State University Jeanne Hardebeck United States Geological Survey
Other Participants Udergraduate Student Researcher (LSU)
SCEC Priorities 1d, 1c SCEC Groups SDOT, CXM
Report Due Date 03/15/2023 Date Report Submitted 03/29/2024
Project Abstract
 The goal of this research has been to assess heterogeneity in crustal stress orientation within and between fault-bounded crustal blocks, particularly improving stress orientation estimates by identifying and removing the influence of fault structures on seismic anisotropy measurements. We analyzed shear wave splitting fast direction estimates, and calculated mean fast directions within a grid of crustal blocks across southern California, and compared those from fault-crossing raypaths with those from raypaths that did not cross a fault. We also considered a detailed investigation of fast direction estimates from individual raypaths near the Newport Inglewood fault, and compared these to SHmax estimates from nearby boreholes and focal mechanisms. We find SWS fast directions to be mostly bimodally distributed, and that dividing raypaths by whether or not they cross a fault is moderately effective at isolating stress- and structurally-induced anisotropy across the region. However there is clear room for improvement in the analysis procedures before we could confidently adopt these results as indicative of SHmax direction. In our detailed investigation of an area bounded by both strike slip and blind thrust faults, we find the division of raypaths by whether or not they cross a fault segment to be insufficient to explain the bimodal distribution of fast orientation directions. Further comparison with SHmax estimates from nearby boreholes and focal mechanisms is consistent with previous findings of lateral heterogeneity along this basin margin, but further testing is needed to fully understand this relationship.
Intellectual Merit These findings directly support the objectives of the community models (CXM) and Stress and Defamation Over Time SDOT interdisciplinary working groups to answer the basic earthquake science question of “How are faults loaded across temporal and spatial scales?” by constraining how absolute stress and stressing rate vary laterally and with depth on faults, and by evaluating the time dependence of stress transfer on faults.
Broader Impacts This project has enabled two LSU undergraduate students from underrepresented backgrounds to conduct research and gain valuable experience in data analysis, computer programming, critical thinking skills, and scientific communication. The research was presented at the 2023 SCEC annual meeting.
Exemplary Figure Figure 2: Mean fast direction (bar color and orientation) for raypaths within a grid of 10 km crustal blocks, calculated using a) all raypaths within each block, b) only raypaths that intersect a CFM 3D fault patch, and c) only raypaths that do not intersect a CFM 3D fault patch. Gray lines indicate surface trace of CFM v6.1 faults [Marshall et al., 2023]. Polar histograms of mean fast directions calculated across southern California, using d) all raypaths, e) only raypaths that cross faults, and f) only raypaths that do not cross faults. Location of Figure 3 shown in a).