GRACE seasonal water load gave a weak thrust-timing signal that did not replicate, and no fault-type sign contrast, in global shallow GCMT Mw 5+ mainshocks, 2002-2026

Question: does the annual cycle of surface water mass change the timing of moderate earthquakes worldwide, with the sign that Coulomb theory gives for each fault type? A surface load decreases Coulomb stress on thrust faults and weakly increases it on normal faults. Approach: we used 15,140 declustered GCMT mainshocks (Mw 5.0 and higher, April 2002 to June 2026). For each event we found the annual phase of the CSR RL06.3 GRACE/GRACE-FO mascon water load at its centroid cell. We wrote the test plan before we calculated any outcome. Inference used 5-degree tile bootstraps and tile phase randomisation. Main finding: 609 shallow thrust mainshocks in cells with annual amplitude of 3 cm or more occurred slightly more often at low load (modulation alpha -0.12, 95% CI -0.22 to -0.01, p = 0.043). This agrees in sign with Coulomb theory, but the evidence is not robust. A post hoc calendar-preserving null gave p = 0.067. The effect was present in GRACE years (alpha -0.23) but not in GRACE-FO years (alpha +0.13, CI -0.05 to +0.33); the era difference was significant. Removal of 5 tiles (central Chile, southern Mexico, Hokkaido, Zagros) made the effect 60% smaller. Normal events did not show the predicted opposite sign (normal minus thrust mean cos +0.02, CI -0.07 to +0.11). The effect did not increase clearly with load amplitude. Deep events gave no signal. Conclusion: in this global Mw 5 and higher sample, there is no robust evidence that the GRACE seasonal load (median Coulomb amplitude 0.26 kPa on thrust faults) modulates earthquake timing. The data exclude thrust modulation stronger than about 22%. Limitations: 300 km GRACE resolution, a simple confined-load Coulomb model without pore-pressure diffusion, low power below 10% modulation, approximate declustering, and quick GCMT solutions after 2020. Code, derived event table, GCMT 2021-2026 monthly files and retrieval hashes are included.

GRACE · GRACE-FO · mascon · hydrological loading · seasonal seismicity · GCMT · Coulomb stress · thrust faults · earthquake triggering · null result · terrestrial water storage