Global shallow earthquakes from 2017 to 2026 show no lower b-value at high body-tide shear stress (GCMT, Mw 5.5 and higher, no ocean loading)

We tested one claim out of sample: the Gutenberg-Richter b-value decreases when the tidal shear stress amplitude increases (Ide et al. 2016). We used 15,100 shallow GCMT earthquakes (depth 70 km or less, Mw 5.5 or more) from 1976 to 2026. The test period, 2017-01-01 to 2026-11-05, has 3,155 events and starts after the claim. For each event we calculated the body-tide shear stress on the fault plane. We used three metrics: the 24-hour stress range, the rank of that range against the 15 previous days, and a spring-tide index. We compared b in the high and low thirds of each metric. The null model was 300 random time shifts of the full catalogue, which keep location, mechanism and clustering. In the test period, the predicted decrease did not occur: delta b was +0.062 for stress range (null mean +0.069, p = 0.40) and -0.027 for rank (null mean 0.000, p = 0.32). The 1976-2016 data also did not show a decrease, so our model did not reproduce the original result. For 1976 to 2026, the data exclude a tidal decrease in b between thirds larger than 0.010 to 0.039, depending on the metric. The largest events (Mw 7.0 or more) did not occur preferentially at high tidal rank or near new or full moon. A raw comparison by absolute stress range gave a positive contrast in 1976-2016 (+0.056, bootstrap interval +0.008 to +0.108). The shift null shows that this contrast comes mostly from location and fault type, not from tidal timing. Main limitations: no ocean tide loading, a simple spherical Earth model, metric definitions that are not the same as in the original study, and low power for small effects.

earthquake tidal triggering · b-value · Gutenberg-Richter · GCMT · earth tides · out-of-sample test · null result · time-shift null · seismicity statistics