SCEC Award Number 18150 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Interrogating Synthesis Models Of Crustal Stress In Cajon Pass With Implications For Stress Field Heterogeneity
Investigator(s)
Name Organization
Karen Luttrell Louisiana State University Bridget Smith-Konter University of Hawaii at Manoa
Other Participants Elliott Helgans, LSU MS student
SCEC Priorities 1c, 1d, 1e SCEC Groups SDOT, CXM, SAFS
Report Due Date 03/15/2019 Date Report Submitted 04/05/2019
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 compare the orientations of the resulting 3-D stress field with available observations of stress state in the Cajon Pass region. We find that along each individual fault segment, the best fitting models exhibit a tradeoff between fault segment loading time and geodynamic driving stress orientation, and that a single geodynamic driving stress with compression oriented at 16ºEofN does the best job reproducing the observed orientations, but only for very long loading times, in some cases an order of magnitude more than paleoseismic estimates of last rupture. It is more likely, instead, that driving stresses vary in orientation or magnitude across the region, or that a distinct source of stress not considered is contributing to the observed orientations. We also find that the ability of the best-fitting synthesis models to reproduce observed stress orientations varies considerably along fault segments. Overall, our results indicate that major features of the in situ stress field orientation indicated by earthquake focal mechanism can only be reproduced with a heterogeneous plate driving stress: variations from topography and fault loading are insufficient to account for the observed stress rotations.
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 graduate student to conduct research and gain valuable experience in computational modeling, critical thinking skills, and scientific communication. This research was presented at the 2018 SCEC Annual Meeting, the 2018 Fall AGU Meeting, in an invited university seminar, and at the 2019 SCEC Community Stress Model Workshop.
Exemplary Figure Figure 3: Comparison of best-fitting forward model using a uniform geodynamic driving stress with observed stress field orientation: (a) SHmax orientation of the best-fitting forward model from this study using a uniform geodynamic driving stress with parameters indicated by yellow star in Figure 2. (b) SHmax orientation of the stress field observed from earthquake focal mechanisms [Yang and Hauksson, 2013]. (c) map of scalar misfit between best-fitting forward model and observed stress orientations (0 indicates perfect 3-D alignment, 2 indicates perfect 3-D misalignment, and 1 indicates arbitrary misalignment).