## Trimming the UCERF3-TD Hazard Tree with a New Probabilistic Model-Reduction Technique

Keith A. Porter, Edward H. Field, & Kevin R. MilnerSubmitted August 15, 2016, SCEC Contribution #7017, 2016 SCEC Annual Meeting Poster #300

The Uniform California Earthquake Rupture Forecast Version 3, Time-Dependent (UCERF3-TD) logic tree has 5,760 branches, which can be computationally problematic for risk analysis of large portfolios, especially when multiplied by five NGAWest-2 ground-motion prediction equations and two site-effect models. An insurer or catastrophe risk modeler concerned with rare, catastrophic losses to a portfolio of assets would have to evaluate the portfolio 57,600 times to create a loss exceedance curve that explores the entire possibility space. Which branches matter most, and which can be ignored? We employed two model-reduction techniques to find a subset of UCERF3-TD parameters that must vary and fixed baseline values for the remainder such that the reduced model produces approximately the same distribution of loss that the full model does. The two techniques are (1) the same tornado-diagram approach we employed previously for UCERF2, and (2) an apparently novel probabilistic sensitivity approach that is better suited to functions of nominal random variables and that produced a smaller reduced model with only 60 leaves, as opposed to 57,600. Results can be used to reduce computational effort in loss analyses by orders of magnitude.

**Key Words**

logic tree; UCERF; model reduction

**Citation**

Porter, K. A., Field, E. H., & Milner, K. R. (2016, 08). Trimming the UCERF3-TD Hazard Tree with a New Probabilistic Model-Reduction Technique. Poster Presentation at 2016 SCEC Annual Meeting.

**Related Projects & Working Groups**

Working Group on California Earthquake Probabilities (WGCEP)