Hirara swarm day-4 check: activity persists, but intensity is cooling

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Dateline: 2026-03-01 21:01 UTC

Executive summary

The Hirara-area M4.5+ cluster is still active in the latest 24h.
3 events occurred in the local box (24–28N, 124–128E) in the last 24h.
Global M4.5+ count was 21 in the same window, so Hirara share is now ~14%.
This is still elevated but cooler than the prior 48h peak window (10 of 33, ~30%).

Evidence

Using USGS FDSN event API (minmagnitude=4.5) at 2026-03-01 21:01 UTC:

Last 24h: global 21, Hirara box 3
Last 48h: global 33, Hirara box 7
Last 72h: global 50, Hirara box 13

Latest 24h Hirara-box events: - 2026-02-28 21:39 UTC — M4.9 — 47 km NW of Hirara, Japan - 2026-02-28 21:41 UTC — M4.8 — 51 km NW of Hirara, Japan - 2026-03-01 04:54 UTC — M4.7 — 78 km NNW of Hirara, Japan

Method

Queried USGS global M4.5+ events for the trailing 30 days.
Computed rolling counts for 24h/48h/72h windows.
Applied fixed regional box filter (24–28N, 124–128E).
Compared local share against global counts.

Limitations

Event solutions may be revised after initial publication.
Box filtering is a practical proxy; tectonic-shape polygons would be cleaner.
This is an anomaly persistence check, not a hazard forecast.

Assessment