SCEC Award Number 21129 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Crustal Stress Heterogeneity and the Role of Sediment Basins Indicated by 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 Undergraduate Student Researcher
SCEC Priorities 1d, 1c SCEC Groups CXM, SDOT
Report Due Date 03/15/2022 Date Report Submitted 05/04/2022
Project Abstract
 The goal of this research has been to assess heterogeneity in crustal stress orientation and test the hypothesis that heterogeneity is more pronounced within and around sedimentary basins relative to either surrounding areas or underlying basement rock. We compiled existing indications of borehole (BH) SHmax and shear wave splitting (SWS) fast directions within southern California, and compared them to new estimates of SHmax from local inversions of earthquake focal mechanisms (FM). Initially, we compared published station-averaged SWS fast directions with FM SHmax estimates derived from inverting all nearby earthquakes, but found the estimates had very different degrees of heterogeneity across the region, with little correlation between the two. We subsequently performed additional local FM inversions and new averages of fast directions from earthquake-station pairs selected to ensure the two methods were sampling the same crustal depths. We found that there are strong discrepancies between all three types of measurements that persist even when we performed new inversions and averages to ensure that the techniques are sampling the same crustal volume. This suggests that the observed crustal heterogeneity may have more to do with lateral variations than depth variations in this region. To test this further hypothesis, we performed preliminary investigations considering crustal volumes and earthquake-station raypaths that cross active fault structures, and noted the tendency of these fast direction measurements to be bimodal. We interpret this as a possible superposition of stress-induced and structure-induced anisotropy that can be tested in future work.
Intellectual Merit These findings directly support the objectives of the Community Models (CXM) and Stress and Deformation 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 one LSU undergraduate student to conduct research and gain valuable experience in data analysis, computer programming, critical thinking skills, and scientific communication. This research was presented at the 2021 SCEC Annual Meeting and the 2021 Fall AGU Meeting.
Exemplary Figure Figure 2: SWS mean fast directions at 127 seismic stations from a) all earthquake depths [after Li and Peng, 2017], b) deep earthquakes (> 5 km), and c) shallow earthquakes (< 5 km). FM SHmax directions from d) earthquakes of all depths, and e) shallow earthquakes. f) BH SHmax directions at 57 boreholes [after Luttrell and Hardebeck, 2021]. Empty circles indicate locations of seismic stations with too few FMs within the specified crustal volume to invert for a stress state.