SCEC Award Number 19070 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Quantifying Uncertainty In Models Of Absolute Crustal Stress In Cajon Pass
Investigator(s)
Name Organization
Karen Luttrell Louisiana State University Bridget Smith-Konter University of Hawaii at Manoa
Other Participants Elliott Helgans (LSU graduate student)
SCEC Priorities 1c, 1d, 1e SCEC Groups SDOT, CXM, SAFS
Report Due Date 04/30/2020 Date Report Submitted 04/23/2020
Project Abstract
 We model the in situ stress field in Cajon Pass as the superposition of stress from three tectonic processes: the accumulation of stress on locked faults over variable loading times, the load of topography, and the far field geodynamic driving stress. We resolve our modeled stress fields onto 5 segments of the SCEC Community Fault Model in the Cajon Pass area, calculate the in-plane maximum shear stress to determine the predicted rake direction, and compare to the observed optimal rake direction indicated by earthquake focal mechanisms. By comparing mean rake angle difference along each segment, we find that model fit is most sensitive to the orientation of geodynamic driving stress and least sensitive to fault loading time, with intermediate sensitivity to the magnitude of geodynamic driving stress. Mean rake misfit values for the best fitting models range from 6º to 16º difference. The optimal driving stress orientation varies from west of north (-15º ± 10º on the Mission Creek segment) to east of north (10º ± 10º on the Claremont segment). We identify two end-member best-fitting models: one with varying driving stress along each fault segment, and one with uniform driving stress across the entire region. We calculate the magnitude of resolved shear stress along the CFM fault segments for each and determine the model incorporating heterogeneous driving stress exhibits considerable more segment-scale variations in shear stress. These observations suggest that stress field heterogeneity could be sufficient to explain some of the proposed “earthquake gate” behavior in Cajon Pass.
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 graduate student to conduct research and gain valuable experience in computational modeling, critical thinking skills, and scientific communication. This research was presented at the 2019 SCEC Annual Meeting, the 2018 SSA Annual Meeting, at the 2019 SCEC Community Stress Model Workshop, and has resulted in the successful defense and publication of a Louisiana State University MS thesis.
Exemplary Figure Figure 3: Summary parameter sensitivity and implied resolved shear stress: Mean rake misfit values for individual Cajon Pass fault segments for a range of (a) driving stress orientations (Gθ) and (b) fault segment loading times (tload) for a uniform driving stress magnitude of 30 MPa. Resolved shear stress on CFM fault segments for (c) best fitting composite stress model with uniform driving stress (dashed gray line in (a)) and (d) best fitting composite stress model with variable driving stress (colored bars in (a)).