Static Coulomb stress shadows did not decrease JMA earthquake rates in the first year after 11 shallow Mw 6.5+ Japanese earthquakes (1999-2024), with simple slip models

We tested whether static Coulomb stress decreases (stress shadows) lower later earthquake rates. We used the JMA unified hypocentre catalogue for 1998-2024 and GCMT mechanisms for 11 isolated shallow Mw 6.5+ mainshocks near Japan. For each mainshock, we modelled a tapered rectangular rupture in an elastic half-space. We calculated the Coulomb stress change at each hypocentre, with mainshock-like receivers and friction 0.4. We compared rates in days 30-365 with the year before in shadow (<= -10 kPa), positive (>= +10 kPa) and control (|dCFS| < 1 kPa) regions. Placebo mainshock times at the same places gave the null distribution. Positive regions had a larger rate increase than control regions: the log rate-ratio difference was +1.09 above placebo (90% interval +0.74 to +1.43). Shadow regions showed no decrease relative to control regions: the difference was +0.10 (90% interval -0.31 to +0.52; one-sided p = 0.67). The data exclude a mean relative decrease of more than 27%. Absolute shadow-region rates increased by about 1.7 times. In 12 variants of the days 30-365 test (friction, receiver plane, source plane, distance, thresholds, Mc, selection) and in two other windows, no variant showed a shadow decrease. A year-2 decrease (-0.42, p = 0.084) did not remain in other versions. Tohoku 2011, a separate case study, had about 7.5 times higher shadow-region rates. Limitations: simple scaling-law slip models, mainshock-like receivers, 11 mainshocks, and one change to the isolation rules after a first look. Code, aggregate counts, placebo statistics, hashes and figures are included. The raw catalogues are not included.

Coulomb stress · stress shadow · aftershocks · JMA catalogue · Japan · static stress triggering · placebo test · seismicity rate · GCMT