Among 505 earthquakes in 1995-2026, only Denali 2002 (119 kPa) changed Yellowstone geyser intervals; ten events at 16-38 kPa long-period stress gave no detectable change

Question: do Yellowstone geyser eruption intervals change after earthquakes more than at placebo times, and which shaking measure predicts a change? We used 14 regular geysers from the GeyserTimes archive (1995-2026) and 567 declustered earthquakes from USGS ComCat. For each earthquake, we measured the long-period (0.01-0.4 Hz) peak ground velocity at broadband stations near Yellowstone (median of up to five stations) and converted it to peak dynamic stress. For each geyser, we compared the 7-day median interval after the event with the 7-day median before it, and calibrated the change with about 700 placebo dates within one year. The plan was written before the analysis; three amendments are reported. Result: the 2002 M7.9 Denali earthquake (119 kPa) is the only clear response (event p = 0.005; rank-based p = 0.021). Daisy intervals became 38% shorter and Castle intervals became 12% longer, as Husen et al. (2004) reported. Daisy returned to the band of 29 other years in week 9. The ten next-largest stresses (16-38 kPa, including Michoacan 2022, Ridgecrest 2019, Stanley 2020 and Tehuantepec 2017) gave placebo p from 0.31 to 0.93 (combined p = 0.91). Over 317 events, -log10 p had no correlation with long-period stress (rho = -0.04), local broadband velocity or the Wang (2007) energy density. Only 16 of 505 scored events had p < 0.05, against 25 expected. Thus the data support a response threshold above 38 kPa for changes larger than about 7-15% in the most regular geysers. Main limitation: Denali is the only event above 40 kPa, so the threshold between 38 and 119 kPa is not located, and seasonal gaps reduce the number of geysers for winter events.

geysers · Yellowstone · dynamic triggering · earthquake hydrology · peak dynamic stress · Denali earthquake · GeyserTimes · placebo test · null result · hydrothermal systems · recovery · seismic energy density