Japan Earthquake VLF Window
Scope
This exploratory check covers the 14 days before the 28 July 2026 Kyushu earthquake. The local Japan seismic refresh found a main event at 2026-07-28T07:27:15Z, magnitude 6.8, depth 10 km, at 32.6817, 130.722, followed by a M5.6 event and smaller aftershocks. NIED Hi-net lists the corresponding local-time event at 2026/07/28 16:27:15 JST; contemporary reporting described it as approximately M7.1. See Hi-net and AP News.
The VLF sample now contains the earlier Moshiri captures plus the newly validated 2026-07-27T23:00Z CDF, each with 8,646 spectral rows and both channels. The archive listing still ends at 27 July 23:00 UTC, so there is no direct VLF observation of the mainshock or its immediate aftermath yet.
Method
For each daily file, the analysis used median channel active fraction and median log-power in three non-floor spectral bands. Baselines were the daily medians from 15–24 July. A robust deviation score used the median absolute deviation of those baseline days. This avoids treating the CDF power-floor bands as meaningful amplitude changes.
Results
| Date | Composite deviation | Interpretation |
|---|---|---|
| 15 Jul | 0.66 | ordinary baseline variation |
| 18 Jul | 2.67 | elevated |
| 20 Jul | 1.69 | moderately elevated |
| 23 Jul | 1.47 | moderate |
| 24 Jul | 2.15 | elevated |
| 25 Jul | 2.26 | elevated |
| 26 Jul | 3.77 | highest in this sample |
| 27 Jul | 1.52 | moderate, not continuing upward |
July 26 is worth retaining as a candidate anomaly day, but it is not conclusive. Similar deviations occurred earlier, and the score fell on July 27 rather than forming a clear sustained pre-event progression. With one station, sparse daily sampling, no 28 July file, and no matched non-earthquake control period, this result does not establish a VLF precursor.
New 27 July sample
Using the same robust baseline idea with power-floor values excluded, the 27 July 23:00 sample has a supplementary composite deviation score of 2.322. It is elevated, and falls about 8 hours before the main event, but it is not the largest score: 15 July 12:00 scored 2.657 and 16 July 12:00 scored 2.345. The timing is therefore compatible with a candidate anomaly but does not distinguish the earthquake from ordinary or instrumental variation. The detailed values are in data/derived/reports/japan_2026-07-28_vlf_anomaly_check.csv.
Next Check
When the archive exposes 28 July, acquire the full event-day and post-event files. Then repeat the analysis with hourly coverage, a longer seasonal baseline, and control days matched for receiver operation and local-time distribution. Keep the Japan CDF and derived products restricted to the permitted scientific research use.