Aftershocks of 26 onshore Japanese crustal mainshocks (JMA Mj 5.0-7.3, 1999-2023) did not go below the background seismicity base in days 1-365; the first day is unresolved

We tested if aftershocks of onshore crustal mainshocks in Japan go below the base of the local background seismicity, as reported for large California sequences. We used the JMA unified hypocenter catalog for 1998-2024 and 47 isolated mainshocks of Mj 5.0-7.3 at depths of 20 km or less. For each sequence, we compared aftershock depths with background depths from the 10 years before, in the same 2 km map cells. We calibrated tail shares against a finite-sample null and used a sequence bootstrap. 26 mainshocks gave usable windows. In days 1-10, 10-100 and 100-365, the median change of D90 was -0.39 km (95% interval -0.84 to -0.03, 19 sequences), -0.54 km (-2.02 to -0.03, 17) and -0.29 km (-0.79 to -0.13, 12). The share of aftershocks below the background D90 was 4 to 7 percentage points lower than the null, with upper bounds of +3, +1 and -3 points. Sequences of Mj 6.0 or more, 14 other design variants and a control annulus gave the same result. Instead, the depth range became wider at the top: in days 10-100, 9.7 points more aftershocks than expected were above the background D10 (+1.2 to +19.9). All changes went back near zero after one year. Mainshock hypocenters were not clearly near the base of the background seismicity (median 41.5% of background events deeper, 35 mainshocks). Limitations: few sequences, few events of M 6.7 or more, routine JMA locations, and incomplete catalogs in the first day. Thus the first-day intervals are wide and we cannot exclude a short early deepening. Code, per-sequence tables, provenance and figures are included. The raw catalog is not bundled; a hash-checked retrieval script is included.

aftershocks · seismogenic depth · brittle-ductile transition · D90 · JMA catalog · Japan · crustal earthquakes · null result