Testing Magnitude Distributions Near Faults in Modern PSHA Models

Morgan T. Page, & Kevin R. Milner

Submitted September 7, 2025, SCEC Contribution #14459, 2025 SCEC Annual Meeting Poster #TBD

Some large, mature faults, such as the San Andreas fault in California, have low rates of recent seismicity compared to the geologically inferred rate of large earthquakes. This discrepancy led to the characteristic magnitude distribution hypothesis, which predicts a higher rate of large earthquakes than a Gutenberg-Richter extrapolation of the small-earthquake rate would give (Schwartz and Coppersmith, 1984). Characteristic magnitude distributions on faults are common in probabilistic seismic hazard analysis (PSHA). We test this hypothesis by comparing the fault-based magnitude-frequency distributions from the 2023 update to the National Seismic Hazard Model (NSHM23) in the Western U.S. with observed seismicity over the past 93 years. We find that observations fall outside of the confidence limits predicted by the model in “characteristic” regions, as defined by the model’s magnitude distributions. An analysis of the earlier California model (UCERF3) also reveals discrepancies between the modeled and observed magnitude distributions. These results challenge the widespread use of characteristic magnitude distributions in fault-based seismic hazard models and suggest, alternatively, large temporal changes in seismicity rates near faults.

Key Words
seismic hazard, magnitude distributions, NSHM23

Citation
Page, M. T., & Milner, K. R. (2025, 09). Testing Magnitude Distributions Near Faults in Modern PSHA Models. Poster Presentation at 2025 SCEC Annual Meeting.

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