SCEC Award Number 17174 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Are Observations Of Stress Field Orientation From Borehole Breakouts And Earthquake Focal Mechanisms Locally Consistent? Implications For 3-D Stress Field Heterogeneity And The Development Of CXM
Name Organization
Karen Luttrell Louisiana State University Jeanne Hardebeck United States Geological Survey
Other Participants LSU undergraduate researcher
SCEC Priorities 1d, 1c SCEC Groups SAFS, SDOT, CXM
Report Due Date 06/15/2018 Date Report Submitted 07/17/2018
Project Abstract
We analyze observations of stress field orientation provided by earthquake focal mechanisms and borehole breakouts to quantify relative consistency between these two sets of observations and characterize the implications for crustal stress heterogeneity. We identify 6 geographic subsets of high quality earthquake focal mechanisms that are near borehole observations, each with two possible depth criteria and three possible focal mechanism quality criteria. We then invert each subset for local stress field orientation using established methods and compare SHmax from these to SHmax observations from boreholes to assess the compatibility of the stress fields indicated by the two methods. In particular, we test the hypotheses that discrepancies are caused either by (1) the smoothing of the focal mechanism inversion process, or (2) the difference in crustal depth sampled by each method. We find that the actual improvement in fit between SHmax indicated by boreholes and local inversions of focal mechanisms over regional smoothed inversions of focal mechanisms is moderate at best. This suggests that while over-smoothing may be a factor, it is not principal cause of the discrepancy. Furthermore, we find that in most sub-regions, inverting only shallow focal mechanisms makes very little difference relative to either the regional smoothed focal mechanism inversion or the local inversion of focal mechanisms of all depths. These two factors are the ones most often cited by stress researchers when discussing the merits of one stress state indicator over another, but our analysis demonstrates they are insufficient mechanisms for explaining observed differences.
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 project has enabled one LSU undergraduate student to conduct research and gain valuable experience in data mining, computer programming, figure preparation, and writing skills. This research was presented at the 2017 SCEC Annual Meeting.
Exemplary Figure Figure 3: Comparison of SHmax derived from borehole (red) and regional inversion of focal mechanisms (black) with local inversion of focal mechanisms (blue) and local inversion of shallow focal mechanisms with depth < 5 km (brown).