Using relative structural complexity of fault segment barriers to model prehistoric earthquake rupture histories

Christopher B. DuRoss, Ryan D. Gold, Rich W. Briggs, & Scott E. Bennett

Published August 15, 2018, SCEC Contribution #8644, 2018 SCEC Annual Meeting Poster #224

Geometrically complex structures along faults, such as fault steps and bends, commonly limit the lateral extent of surface-rupturing earthquakes. However, reconstructions of prehistoric fault ruptures often treat these structures as binary limits to rupture. We hypothesize that for normal faults, the degree of structural complexity can inform the likelihood of throughgoing ruptures at proposed rupture barriers. That is, complex barriers are more likely to impede ruptures than simple barriers. Using a 100-km-long section of the 350-km-long Wasatch fault zone (WFZ) in Utah, we combine measures of fault structural complexity with earthquake-timing probability density function (PDF) data from fault trench studies to evaluate alternative earthquake correlations along strike. For six geometric structures along the Salt Lake City and Provo segments of the WFZ, we calculate values of relative structural complexity based on kilometer-scale fault geometry (e.g., fault steps, bends, and changes in orientation relative to a regional slip vector). We then compare the overlap of Holocene earthquake PDFs across these structures to both the relative structural complexity values and the timing uncertainty of the constituent PDFs to identify the most probable along-strike earthquake correlations. Our method yields a best-fit prehistoric rupture model that leverages all available structural and paleoseismic data. For the WFZ, our results suggest a complex history of rupture on the Salt Lake City and Provo segments, including rupture at and likely across the proposed Traverse Mountains structural barrier between them. These ruptures yield a cumulative fault displacement curve that closely resembles that from published point observations of Holocene fault-scarp offset. Our approach can be used to limit the range of possible rupture models and help define the epistemic uncertainty in rupture length for seismic-hazard analyses.

Key Words
Basin and Range Province, normal faulting, paleoseismology, Wasatch fault zone, segmentation, rupture length, rupture models

Citation
DuRoss, C. B., Gold, R. D., Briggs, R. W., & Bennett, S. E. (2018, 08). Using relative structural complexity of fault segment barriers to model prehistoric earthquake rupture histories. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology