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Location:
3 km SW of Ojiya, Japan
Period:
23 Oct 2004 09:28:48 - 24 Oct 2004 16:27:51 (1 day 6 hours 59 minutes)
Volcanoes in 100km radius:
Hiuchigatake(55km), Shiga(57km), Myokosan(62km), Kusatsu-Shiranesan(64km), Niigata-Yakeyama(67km), Nikko-Shiranesan(71km), Omanago Group(78km), Harunasan(79km), Numazawa(80km), Akagisan(80km), Nantaisan(82km), Asamayama(87km), Takaharayama(98km)
Earthquakes:
27
No swarms nearby.
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20041024.1: Analysis of Activity Near Ojiya, Japan

A seismic swarm designated S20041024.1 was recorded 3 km southwest of Ojiya in Niigata Prefecture, Japan. The sequence began at 09:28 on 23 October 2004 and concluded at 16:27 on 24 October 2004, spanning 30 hours and 59 minutes. During this interval, 27 earthquakes were registered, providing a clear example of clustered seismic activity in a tectonically active region.

The Niigata area lies within the Niigata-Kobe Tectonic Zone, where the Eurasian and Philippine Sea plates interact along reverse faults. This setting produces frequent moderate earthquakes at shallow depths, typically between 10 and 20 km. Historical records show recurrent seismic episodes linked to the same fault systems responsible for the 2004 Chuetsu earthquake sequence. The swarm occurred in the hours immediately preceding that larger event, illustrating how foreshock clusters can precede major ruptures in this geological environment.

Event magnitudes ranged from 0.0 to 4.8, with the majority occurring at depths of 10 km. Notable shocks included a 4.8 magnitude event at 09:54 on 23 October and another at 05:21 on 24 October. Several events registered at greater depths later in the sequence, reaching 20 km and 16 km, indicating possible migration along deeper fault segments. The temporal distribution showed peak activity during the first 12 hours, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or stress transfer rather than a single mainshock-aftershock pattern.

SeismoSight internal classification identifies this as the sole swarm recorded in the region since 1 January 2000, underscoring its rarity. Such episodes offer valuable data for understanding precursory signals in subduction-zone margins. Monitoring of similar swarms continues to inform probabilistic hazard assessments for central Japan, where population density and infrastructure amplify potential impacts from even moderate shaking.

Key Events Summary

  • 23 Oct 2004 09:28:48, M4.5, 10 km
  • 23 Oct 2004 09:53:52, M4.5, 10 km
  • 23 Oct 2004 09:54:35, M4.8, 10 km
  • 23 Oct 2004 09:57:04, M4.6, 10 km
  • 23 Oct 2004 10:55:19, M3.9, 10 km
  • 23 Oct 2004 11:02:39, M4.2, 10 km
  • 23 Oct 2004 11:16:55, M4.3, 10 km
  • 23 Oct 2004 11:20:44, M4.2, 10 km
  • 23 Oct 2004 11:55:16, M4.2, 10 km
  • 23 Oct 2004 13:26:48, M4.1, 10 km
  • 23 Oct 2004 13:28:18, M4.3, 10 km
  • 23 Oct 2004 14:54:07, M4.2, 10 km
  • 23 Oct 2004 22:08:11, M4.4, 10 km
  • 23 Oct 2004 22:16:06, M4.0, 10 km
  • 23 Oct 2004 23:02:04, M4.4, 20 km
  • 24 Oct 2004 00:28:03, M4.6, 16 km
  • 24 Oct 2004 04:54:24, M3.9, 10 km
  • 24 Oct 2004 05:21:34, M4.8, 10 km
  • 24 Oct 2004 07:06:31, M4.3, 13 km
  • 24 Oct 2004 08:21:40, M4.1, 10 km
  • 24 Oct 2004 15:56:14, M4.2, 10 km
  • 24 Oct 2004 16:27:51, M4.4, 10 km

Additional zero-magnitude detections occurred at 09:49, 10:49, 10:59, 14:23, and 02:49, all at 10–12 km depth.

References

Japan Meteorological Agency earthquake catalog
United States Geological Survey earthquake archive
Geological Survey of Japan active fault database