SCEC Award Number 15187 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Improved Analysis of Crustal Stress in Southern California, with constraints from Seismology, Geodesy, Topography, and Gravity
Investigator(s)
Name Organization
Karen Luttrell Louisiana State University Bridget Smith-Konter University of Hawaii at Manoa David Sandwell University of California, San Diego
Other Participants one undergraduate LSU student
SCEC Priorities 2d, 1b SCEC Groups SDOT, Geodesy, CEO
Report Due Date 03/15/2016 Date Report Submitted 03/14/2016
Project Abstract
 We combine models of individual processes contributing to the in situ stress field in southern California into a synthesis total model in order to investigate the characteristics of both the in situ stress field as a whole and the relative importance of its constituent components. We estimate the magnitude of the non-lithostatic in situ stress field by balancing the revised topography stress model against the in situ orientation indicated by earthquake focal mechanisms. Our results, based on the most rugged topography along the San Andreas Fault System, indicate that differential stress at seismogenic depth must exceed 62 MPa, suggesting that pore pressure must be less than hydrostatic and that coefficient of friction cannot be very low. We then use our synthesis model of near-fault in situ stress to assess the extent to which heterogeneity in the orientation of the in situ stress field is governed by the fault segment scale processes of the earthquake cycle. We find that observed rotations in the in situ stress field cannot be fully explained by the variations in the geometry or loading stage of the major locked fault segments, but rather that the driving stress field itself must be heterogeneous at the fault segment scale. Finally, we compare in situ stress orientations indicated by earthquake focal mechanisms compared with those observed in boreholes, and determine that orientations estimate by these two methods generally differ greatly, possibly indicating the shallow stress field is fundamentally more heterogeneous than that at seismogenic depth.
Intellectual Merit These findings directly support an important objective of SCEC and the CSM, to synthesize the insights offered by diverse contributed models in order to gain a more holistic understanding of the 4D stress field and be better situated to present a community-endorsed stress model to the broader SCEC community. Our investigations have developed novel estimates of stress magnitude and established novel techniques to infer the near-fault character of the tectonic driving stress and explore the relative importance of locked faults on the in situ stress state.
Broader Impacts This project has enabled two LSU undergraduate students to conduct research on southern California stress state and gain valuable experience in data mining, computer programming, figure preparation, and writing skills. This research was presented at the 2015 SCEC Annual Meeting.
Exemplary Figure Figure 2: a) Brittle yield strength in the upper crust for coefficient of friction (µ ) 0.6, with varying ratios of pore pressure to hydrostatic pressure (λ) as indicated. Dashed gray lines represent ductile yield strength for given strain rate [e.g., Hirth et al., 2001]. Yellow and blue circles indicate differential stress estimates from scientific drilling and petrologic analysis estimates, respectively [Behr and Platt, 2011; Zoback and Healy, 1992]. Red region indicates differential stress estimate from this study. b) Contours of brittle yield strength at seismogenic depth (5 km) as a function of λ and µ . Red region indicates λ and µ values consistent with the results of this study. After Luttrell and Smith-Konter [2016].
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