Generic USGS aftershock forecasts were calibrated for 779 shallow M6+ mainshocks in 2016-2025 and too high for 1990-2015; catalog small-event ratios follow the change

Question: USGS operational aftershock forecasts outside California start from generic Reasenberg-Jones parameters for 15 tectonic regimes (Page et al. 2016; OpenSHA OAF file dated 2016-03-21). Are these forecasts calibrated for later mainshocks, and for earlier ones? Approach: we used the USGS ComCat catalog (M 4.5 or more, 1985 to November 2026). We selected shallow (depth 70 km or less) M 6.0 or more mainshocks, and counted M 4.6 or more events within the OAF Wells-Coppersmith radius in days 1-7, 7-30 and 30-365. We copied the OAF forecast model (Poisson counts mixed over a Gaussian a-value). We wrote the plan before scoring. Results: for 779 mainshocks in 2016-2025 (2,337 window forecasts), 2.9% of counts were below the 2.5% forecast quantile and 2.8% above the 97.5% quantile; the PIT histogram did not differ clearly from uniform (p = 0.18); the best-fit shift of the mean a-value was +0.02 (95% CI -0.04 to +0.08). For 2,413 mainshocks in 1990-2015 the forecasts were too high: shift -0.16 (-0.18 to -0.12), observed counts 75% of the forecast mean. The difference, +0.18 (0.12 to 0.24), remained at M 5.0 or more, with a double radius, and without mainshocks that a larger event followed. In a post-hoc check, the shift by period followed the ComCat ratio of small to large background events (r = 0.95, slope 1.08, 10 periods); the yearly number of M 4.6 or more events rose by about 50% while M 5.5 or more did not change. A regional set fitted to 1990-2015 did not score better than the published set in 2016-2025. Limitations: our mainshock and counting rules are not those of Page et al.; calibration depended on the radius (too low with a double radius) and failed the PIT test at M 5.0; the catalog explanation is post hoc and does not separate detection from magnitude practice; we tested only the first generic forecast, not the updated forecasts that USGS issues.

aftershock forecasting · Reasenberg-Jones model · forecast calibration · operational earthquake forecasting · USGS ComCat · tectonic regionalization · catalog completeness · probability integral transform · aftershock productivity · out-of-sample test