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

Caje A. Kindred Weigandt, Kevin R. Milner, Alex Hatem, & James F. Dolan

Submitted September 7, 2025, SCEC Contribution #14579, 2025 SCEC Annual Meeting Poster #TBD

Emerging geologic datasets indicate that mechanically complementary faults within plate-boundary fault systems not only exhibit pronounced decelerations and accelerations of fault slip that span 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 moment release rate. However 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. Thus, use of single average slip rates may under- or over-estimate hazard. Ideally, fault slip rates would be expressed as probability density functions 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 that exhibit the same average slip rate and variance to establish a basic framework methodology to sample slip rates from multiple probability distribution functions representing each fault. This framework samples the slip rate distribution of each fault under an imposed covariance matrix, successfully producing numerous valid interpretations of slip rates on multiple faults from the entire uncertainty space of a single deformation model. We are exploring the case of a prescribed faster-than-average slip rate on one fault and conditionally sampling commensurately slower slip rates on the remaining faults. Continuation of this study will include exploration of the uncertainty space by using distance between faults, various probability distribution function shapes (e.g., skewed triangular, trapezoidal, bimodal) for individual faults, and construction of a systematic method for defining the magnitudes of correlation and anticorrelation among the faults within the covariance matrix. This will involve leveraging incremental field-based slip rate records and the relative complexity of regional fault systems. Ultimately, this project will continue to improve seismic hazard assessment by considering a broader uncertainty space in seismic hazard modeling within mechanically integrated fault systems.

Key Words
Probabilistic seismic hazard analysis, deformation model, covariance

Citation
Kindred Weigandt, C. A., Milner, K. R., Hatem, A., & Dolan, J. F. (2025, 09). 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. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)