Seismic Swarm PS20041023.1 Near Ojiya, Japan

The seismic swarm designated PS20041023.1 was recorded 2 km northeast of Ojiya in Niigata Prefecture, Japan. Activity commenced at 08:56 on 23 October 2004 and concluded at 21:05 on 24 October 2004, encompassing 23 earthquakes over 36 hours and 9 minutes. This sequence unfolded in a region shaped by ongoing compressional tectonics along the eastern margin of the Sea of Japan, where reverse faulting predominates due to the interaction between the Eurasian and North American plates. The initial event reached magnitude 6.6 at a depth of 16 km, followed rapidly by a magnitude 5.7 shock at 10 km depth three minutes later. Subsequent events included a magnitude 6.1 at 10 km and a magnitude 6.3 at 10 km within the first hour, illustrating the clustered release of strain typical of swarm behavior in this setting. Depths varied considerably, ranging from shallow crustal levels around 10 km to deeper occurrences exceeding 100 km, reflecting the complex fault network and possible involvement of both upper crustal and sub-crustal structures. Later activity on 23 and 24 October featured repeated magnitude 5.2 to 5.8 events at depths between 10 km and 33 km, with isolated deeper shocks at 80 km and 121 km. The final pair of events, magnitudes 5.7 and 5.8, occurred at 11 km and 100 km respectively, marking the swarm's termination. Such temporal and spatial clustering highlights the swarm's role in redistributing stress across multiple fault segments rather than a single mainshock-aftershock cascade. Niigata Prefecture lies within one of Japan's most seismically active zones, with historical records documenting destructive earthquakes linked to active folding and thrust faults. The geological framework features Neogene sedimentary basins overlying basement rocks, facilitating the propagation of seismic energy and the occurrence of both shallow and intermediate-depth events. Updated assessments of regional tectonics confirm persistent strain accumulation, consistent with the observed swarm characteristics. This sequence provides insight into swarm dynamics, where multiple events of comparable magnitude occur without a dominant mainshock, often signaling fluid involvement or distributed fault slip in compressional regimes. Depths exceeding 30 km in several cases suggest activation of deeper structures that may influence long-term seismic hazard in the Ojiya area.

References

• SeismoSight internal swarm classification PS20041023.1
• Geological Survey of Japan, Active Fault and Seismic Source Database (updated regional tectonic summaries)