SCEC Award Number 25327 View PDF
Proposal Category Collaborative Research Project (Multiple Investigators / Institutions)
Proposal Title Initial exploration of correlated uncertainties within deformation models used for probabilistic seismic hazard assessment: towards development of a geologically informed model of fault behavior and fault interactions within southern California
Investigator(s)
Name Organization
James Dolan University of Southern California Alex Hatem United States Geological Survey Kevin Milner United States Geological Survey
SCEC Milestones D1-1, D2-2, D3-1, A1-3, A3-4 SCEC Groups SDOT, Geology, PBS
Report Due Date 03/15/2026 Date Report Submitted 04/03/2026
Project Abstract
 Emerging geologic datasets indicate that mechanically complementary faults within plate-boundary systems not only exhibit pronounced accelerations and decelerations of fault slip spanning multiple earthquake cycles, but that they also alternate periods of faster-than-average and slower-than-average slip rates while maintaining an overall constant system-level rate. Most modern seismic hazard models do not consider these behaviors and instead rely heavily on single values of slip rate that are averaged over a wide range of time and displacement scales. Use of single average slip rates may under- or over-estimate near-future hazard. Ideally, fault slip rates would be expressed as probability density functions (PDFs) that could be sampled. However, a challenge arises when mapping potentially correlated or uncorrelated epistemic uncertainties in slip rates across a fault system. We present preliminary development of a covariance matrix using geologic observations of intra-system fault behavior, beginning with a simple model of four parallel strike-slip faults with the same average slip rate and variance to establish a basic framework methodology to sample slip rates from multiple PDFs representing each fault. This framework samples the slip rate distribution of each fault under an imposed covariance matrix, producing numerous valid interpretations of slip rates on multiple faults from the entire uncertainty space of a single deformation model. We also prescribe a faster-than-average slip rate on one fault and conditionally sample commensurately slower slip rates on the remaining faults. Ultimately, this project will continue to improve seismic hazard assessment by considering a broader uncertainty space within mechanically integrated fault systems.
Intellectual Merit We have begun development of a preliminary deformation model correction. A deformation model prescribes an on-plane slip rate per unit fault considered in an earthquake rupture forecast. The ability to sample slip rates of multiple faults from their respective PDFs under an imposed covariance matrix would improve consideration of slip rate uncertainty and advance the use of epistemic uncertainties within modern seismic hazard models. This is a novel area of study, and our proposed work fills an immediate need for a covariance model module that could become influential in future iterations of the U.S. National Seismic Hazard Model.
Broader Impacts The primary impact of this work revolves around developing seismic hazard strategies that facilitate more accurate forecasting of future earthquake occurrence, which is paramount to building a more resilient society. Highly variable, covarying fault slip rates represent a potential source of over- or under-estimating near-future hazard. The research discussed herein represents USC PhD student Caje Kindred Weigandt’s initial foray into seismic hazard analysis, expanding his expertise and future research efforts. This work constitutes a chapter of his dissertation and, ultimately, a peer-reviewed journal article. He presented this work at the 2025 SCEC Annual meeting.
Project Participants Principal investigator: James Dolan (USC), Alexandra Hatem (USGS), Kevin Milner (USGS)
Participating Graduate Student: Caje Kindred Weigandt (USC)
Conceptualization of the sampling framework was born out of conversations among Caje Kindred Weigandt and the PIs, and Caje has taken the lead role in development and implementation
Exemplary Figure Figure 3. Total (dark) and accepted (white) histograms from conditional sampling of three parallel strike slip faults with slip rates of mean rate 10 mm/yr and standard deviation of 5 mm/yr based on assumption that Fault 1 is slipping at 30 mm/yr. Accepted samples are those for which (1) total slip rate is constrained between 35-45 mm/yr and (2) no fault slips slower than 0.001 mm/yr. Colored lines indicate results from individual accepted sample scenarios and illustrate capability of our conditional sampling framework to produce variable results from the uncertainty of a single deformation model under an imposed covariance matrix.
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