Japan box (30–46°N, 128–146°E), M ≥ 3.5, 2010–2025, split by local (JST) origin hour.
| Window | Mc | N | b | SE |
|---|---|---|---|---|
| Day (06–18) | 4.4 | 5,329 | 1.167 | 0.016 |
| Night (18–06) | 4.5 | 4,830 | 1.272 | 0.018 |
Difference (night − day): +0.105 ± 0.024, z = 4.36.
The result is significant and the mechanism is not
The prediction was straightforward: cultural noise is lower at night, more small events are detected, completeness improves, and apparent b rises. Half of that happened — b is higher at night.
But completeness moved the wrong way. Mc is higher at night (4.5 vs 4.4), not lower. If detection were better at night, Mc should fall. So the simple detection-bias account does not fit these numbers, and the difference cannot be attributed to it on this evidence.
Three candidate explanations remain, none tested here:
- Mc estimator instability. Maximum curvature is a crude mode estimator; with ~5,000 events and 0.1-magnitude binning, a one-bin shift is not informative on its own.
- Aftershock timing. Sequences are not declustered. If large sequences fall disproportionately in one window, both b and Mc move together for reasons unrelated to noise.
- Genuine catalogue heterogeneity in how the network processes events at different hours.
Why it is still worth recording
A 0.1 shift in b is larger than most regional differences reported in the literature (Japan 1.30 vs California 1.05 in the companion file came from the same estimator). If b varies by 0.1 with the clock, then cross-region and cross-epoch comparisons inherit a bias of that order unless detection is controlled.
That is a caution, not a discovery. The observation is real; the explanation is open.
Limits
- One region, one threshold, one estimator.
- No station-level noise data; local hour is a proxy.
- No declustering.
- USGS
timeis origin time, not detection time. - The analysis was run once, not pre-registered.