SCEC Award Number 18148 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Reconciling Stress Field Orientation From Borehole Breakouts And Local Earthquake Focal Mechanism Inversions: Implications For The Length Scale Of Stress Heterogeneity
Name Organization
Karen Luttrell Louisiana State University Jeanne Hardebeck United States Geological Survey
Other Participants
SCEC Priorities 1d, 1c, 1e SCEC Groups CXM, SDOT
Report Due Date 03/15/2019 Date Report Submitted 05/02/2019
Project Abstract
We compiled a thorough set of published observations of borehole-derived stress orientation across the Los Angeles region for detailed study. We then performed 1800 new stress state inversions of local focal mechanisms centered around boreholes, covering a volume of crust defined by varying maximum distance and maximum depth criteria. Comparing these two stress indicators over a variety of length scales allows us to describe the length scale of stress heterogeneity across this region, and to test the hypothesis that previously observed discrepancies are caused either by differences in crustal depth sampled or by over-smoothing during focal mechanism inversion. We expect that more boreholes would have adequately fit SHmax directions when compared with SHmax directions inverted from focal mechanisms within a more localized volume (shorter maximum distance and shallower maximum depth). We find instead that, while the local results match the borehole observations better in some areas (e.g., central Ventura), in most areas the improvement in fit with decreasing crustal volume sampled is negligible, even when sampling within a few kilometers. This implies that neither differences in depth nor differences in lengthscale are able to satisfactorily account for the discrepancy between focal mechanism SHmax and borehole SHmax. Overall, these analyses suggest that legitimate stress heterogeneity exists at small lengthscales, in some cases down to a few kilometers or less. These results have immediate implications for the SCEC CSM, for both developers and users.
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 quantifying the spatial scale of stress heterogeneity and synthesizing the available observations used to constrain absolute stress and stressing-rate.
Broader Impacts This work is part of a broader project that has enabled three LSU undergraduate students to conduct research and gain valuable experience in data mining, computer programming, figure preparation, and writing skills. This research was presented at the 2018 SCEC Annual Meeting, at the 2019 SCEC Community Stress Model Workshop, and at the 2019 SSA annual meeting.
Exemplary Figure Figure 1: a) SHmax azimuth (black bars and central circle color) derived from borehole breakouts across the Los Angeles region [Mount and Suppe, 1992; Kerkela and Stock, 1996; Wilde and Stock, 1997]. Subregions discussed in this study identified by black boxes. b) SHmax azimuth (black bars and central circle color) derived from southern-California wide inversion of earthquake focal mechanisms (gray dots), shown at locations of borehole observations shown in (a) for comparison [Yang et al., 2012; Yang and Hauksson, 2013]. SHmax azimuth (black bars and central circle color) derived from local inversion of earthquake focal mechanisms within c) 20 km or d) 5 km of each borehole observation shown in (a).