SCEC Award Number 16177 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title 4D Stress Evolution Models of the San Andreas Fault System Using Improved Geodetic and Paleoseismic Constraints
Investigator(s)
Name Organization
Bridget Smith-Konter University of Hawaii at Manoa Karen Luttrell Louisiana State University David Sandwell University of California, San Diego
Other Participants A UH graduate student researcher
SCEC Priorities 2d, 1b SCEC Groups SDOT, Geodesy
Report Due Date 03/15/2017 Date Report Submitted 06/01/2017
Project Abstract
 Major earthquakes of the San Andreas Fault System (SAFS) are thought to occur when accumulated fault stress in the upper locked portion of the crust exceeds some threshold value. 4D simulations of stress evolution provide rare insight into earthquake cycle crustal stress variations at seismogenic depths where earthquake ruptures nucleate, however modeled stress accumulation through time is largely prescribed by the assumed slip history; these results highlight the need for continual improvements to and utilization of contemporary paleoseismic chronologies. Likewise, improved resolution and accuracy of the near-fault velocity field using new, integrated GPS and InSAR data is critical for improving strain and stress rate models of the SAFS. Utilizing new SCEC community paleoseismic and geodetic data, our refined 4D stress models are used to explore the temporally and spatially varying stress rates and threshold conditions, and the sensitivity of these results on fault locking depth, for major segments of the SAFS. Our new estimates of stress accumulation rate in Southern California are most significant along the Imperial (2.8 MPa/100yr) and Coachella (1.2 MPa/100yr) faults, consistent with an 11-17% change in stress rate due to locking depth and geologic slip rate parameter updates over the last 5 years. Revised estimates of present-day earthquake cycle stress accumulation in Southern California are most significant along the Imperial (2.25 MPa), Coachella (2.9 MPa), and Carrizo (3.2 MPa) segments, consistent with a 15-29% decrease in stress due to locking depth, geologic slip rate parameter updates, and postseismic relaxation from the El Mayor-Cucapah earthquake.
Intellectual Merit A fundamental objective of SCEC4, and in particular, of the Stress and Deformation Over Time (SDOT) focus group, has been to develop a Community Stress Model (CSM), a large-scale effort to deliver a set of spatio-temporal (4D) representations of the stress tensor in Southern California. Complex earthquake cycle modeling of the full range of temporal and spatial scales using nonlinear rheology with realistic spatial variations requires computationally intensive 4-D finite element models that are currently being developed within the SDOT group. Simpler semi-analytic models, using computationally fast algorithms, however, can be used to assess the importance of basic stress model components and the first-order consistency of paleoseismic data and time-dependent earthquake cycle stress calculations. Using the later, this project has focused on the development of 4D simulations of stress evolution that provide rare insight into earthquake cycle crustal stress variations at seismogenic depths where earthquake ruptures nucleate. As these models develop, several critical science questions have emerged that are helping to advance the body of knowledge of earthquake cycle crustal dynamics: Is stress accumulation consistent over multiple earthquake cycles? Can improved earthquake cycle stress models be reconciled with paleoseismic data to determine a characteristic stress threshold for major fault segments of the SAFS? How do long-term (1000-yr) variations in stress accumulation influence the spatial and temporal pattern of earthquake occurrence? Utilizing new SCEC community paleoseismic and geodetic data, our refined 4D stress models suggest a high degree of sensitivity to fault locking depth along the San Andreas Fault System, a quantity best estimated by spatially dense geodetic observations. The findings of this work promote further investigations into the relationship between fault stress accumulation rate and earthquake occurrence, guided by the SCEC Community Geodetic Model.
Broader Impacts A component of this SCEC4 funded project emphasized Earth Science education and communication of pertinent and accessible earthquake information to the general public. Directly aimed at disseminating geoscience educational material to our local community, we worked closely with the Earth Science on Volcanic Islands summer REU program, hosted by UH’s Department of Geology and Geophysics, to develop visualization content for a visiting student from the University of South Carolina (J. Burnstein). We have been actively sharing relevant interactive visualization products with teachers and colleagues in Honolulu. Manao Elementary School and Waialae Public Charter School have benefited greatly from interactive educational products provided by our team, in conjunction with the research activities supported by this award. We have also utilized the UH Hawaiian Institute of Geophysics visualization center many times over the year to display San Andreas visualizations for classroom and public education activities. Two undergraduate UH students contributed to the body of this work (A. Lee and L. Burkhard), and coursework lectures and visualization exposure of these datasets was provided to 18 UH undergraduate students enrolled in GG451 Earthquakes and Crustal Deformation.
Results from these tasks were presented at the 2016 SCEC Annual Meeting in 3 poster presentations and 1 invited plenary presentation. Results were also presented at the 2016 AGU, GSA, and SSA meetings. One manuscript submitted to GJI (Luttrell and Smith-Konter, 2017) is currently undergoing minor revision, and two additional mature manuscripts are in preparation to be submitted to BSSA and JGR. We also welcome future collaborative activities with the SCEC Community Stress Model, of which we will eagerly share and distribute these results.
Exemplary Figure Figure 1. Sliced view of present-day (2017) Coulomb stress accumulation model of the San Andreas Fault System based on interseismic stress accumulation rates and stress changes from 112 historical and prehistorical earthquake ruptures. Stress variations with depth are due to transitions in along-strike locking depth.