Performance-Based Fault Displacement Estimation With Uniform California Earthquake Rupture Forecast 3 Probabilities

Glenn P. Biasi

Submitted August 24, 2016, SCEC Contribution #6948, 2016 SCEC Annual Meeting Poster #297

We are implementing a performance-based engineering approach for probabilistic fault displacement hazard analysis (PFDHA) that builds on the third Uniform California Earthquake Rupture Forecast (UCERF3). UCERF3 developed probabilities of surface rupture for a state-wide system of active faults. The USGS has adopted UCERF3 results for California, so its implementation in PFDHA will be part of the next advance in state-of-practice in California. This work specifically addresses probability distributions for rupture displacement given fault location.

Displacement estimates are developed in a staged approach that preserves both best estimates and uncertainties. In the initial stage, all ruptures crossing the site are gathered with their magnitudes, lengths, rupture probabilities, and site location within the rupture. Implied displacements given rupture can be estimated using length vs. average displacement, and magnitude vs. area scaling relations. Results are combined with UCERF3 rupture rates to form a displacement hazard curve. This estimate is only a first approximation displacement hazard because displacement variability within ruptures is not included.

To estimate rupture displacement variability we apply a database of displacement measurements from historical ruptures. We find that variability is not constant across ruptures, but depends on where the fault crossing site is within the rupture, and may be magnitude dependent. We are also evaluating whether variability can be reduced using a statistical autoregressive approach. These steps give a per-rupture displacement and uncertainty for a crossing site. Two paths are considered to combine displacement probabilities over multiple ruptures. The brute-force method repeats per-rupture estimates for all UCERF3 ruptures that pass the site, and combines results using UCERF3 rupture probabilities. For some fault crossings such as the San Andreas at the Los Angeles Aqueduct, this strategy would involve tens of thousands of ruptures. In a second path, we first extract the UCERF3 magnitude-frequency distribution (MFD) from the site-crossing ruptures. Given the MFD, we vary the site location within the ruptures. Three locations suffice for 6<M<7 ruptures, and of order 5 for larger events. We apply probabilities in the site MFD to develop final displacement estimates and uncertainties.

Key Words
UCERF3, fault displacement

Citation
Biasi, G. P. (2016, 08). Performance-Based Fault Displacement Estimation With Uniform California Earthquake Rupture Forecast 3 Probabilities. Poster Presentation at 2016 SCEC Annual Meeting.


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