Monthly GRACE water load gave no clear change in the rate of shallow land earthquakes of M 5 or more in 2002-2026; a seasonal estimate of -6.8% per kPa failed the planned null test

We tested if monthly changes in water mass on land change the rate of shallow earthquakes. We used GRACE and GRACE-FO mascons and the USGS ComCat catalog for 2002 to 2026. The primary sample has 1,942 declustered events of M 5.0 or more at depth 40 km or less, in 623 land cells of 2 degrees. A Poisson model with cell and year fixed effects separated seasonal and non-seasonal load, and load rate. The analysis plan was written before the first fit. No primary term passed the planned test (smallest Holm p = 0.31). Seasonal load gave -6.8% per kPa (95% bootstrap interval -13.5 to -1.5, null p = 0.078). The bootstrap interval excludes zero but the null test does not reject zero, so we do not report a detection. Non-seasonal load gave +2.6% per kPa (-5.1 to +11.4). The seasonal estimate changed with the magnitude floor (-2.0 at M 4.5, +0.9 at M 5.5). The design can detect approximately 10 to 12% per kPa, so smaller effects are not excluded. Limits: the test covers land cells only, GRACE resolution is approximately 300 km, regional opposite signs can cancel, and lags were not tested. An injection test showed possible attenuation of the seasonal rate term, and a longer repeat stopped at a time limit before it could resolve this.

earthquake rate · hydrological loading · GRACE · terrestrial water storage · seasonal seismicity · null result · Poisson fixed effects