Off northeast Japan, M4+ rates in 2019-2024 stayed above the 1998-2010 rate without a higher ETAS background, and the 2011 high-slip zone background decreased (JMA catalog)

We tested if earthquake rates in northeast Japan returned to their 1998-2010 level after the 2011 Mw 9.0 Tohoku-oki earthquake. We used the JMA unified hypocenter catalog for 1998-2024 in five source volumes and one inland normal-fault area. We compared annual counts and fitted temporal Omori-ETAS models at M 4.0 or larger. In the forearc outside the high-slip core, the 2019-2024 M4+ rate was 1.79 times the 1998-2010 rate (95% year-bootstrap interval 1.32-2.37). But the ETAS background did not increase: the post-2011 to pre-2011 background ratio was 0.78 (95% profile interval 0.44-1.05). In the model, the excess came from slow Omori decay of the mainshock term (p0 = 1.02) and from later cascades, which include the 2021 M7.3 and 2022 M7.4 earthquakes. A model fitted to 2011-2016 with the pre-2011 background forecast 2017-2024 correctly: 1,190 events occurred against a median of 1,320 (95% range 1,077-2,667). In the high-slip core, the M4+ rate fell to 0.17 times the pre-2011 rate, and an unchanged background was rejected (deviance 15.8). There, 22 events occurred in 2017-2024 against a 95% forecast range of 38-130. In the outer rise and the 70-200 km slab, an unchanged background agreed with the data. The lower inland-arc rate was not resolved as a background change. In the forearc, the JMA b-value rose by 0.11 (95% CI 0.07-0.15) and did not return to pre-2011 values through 2024. But JMA magnitude-type selection changed after 2011, and GCMT Mw at 5.3 or larger shows no clear change. Limitations: background and slow decay trade off in 14 years of data, the models are temporal only in box regions, several small-region fits reached parameter bounds, and the host load limited the ETAS analysis to M4+ with 500 simulations. The package contains code, a derived event table, result tables, figures, logs and input hashes.

Tohoku-oki 2011 · aftershock duration · ETAS · background seismicity · Omori-Utsu · b-value · JMA catalog · seismic quiescence · forecast test · magnitude type · Japan Trench · GCMT