Seismic Swarm in the Hokkaido Region: Analysis of the November 2004 Event

Hokkaido lies at the convergent boundary between the Pacific Plate and the Eurasian Plate, forming part of the Pacific Ring of Fire. This tectonic setting produces frequent seismic activity through subduction processes, with the Pacific Plate descending beneath the overriding plate at rates of several centimeters per year. The region has a well-documented history of large earthquakes, including the magnitude 8.1 Tokachi-Oki event of 25 September 2003, whose epicenter was located approximately 12 km from the center of the later swarm.

On 14 November 2004, a seismic swarm occurred in the Hokkaido region, registering five earthquakes within a span of one hour and six minutes. The sequence began at 17:37 and concluded at 18:44 local time. Magnitudes ranged from 5.0 to 5.7, with focal depths between 2 km and 25 km. The events clustered tightly in both space and time, consistent with swarm behavior often observed in subduction-zone settings where fluid migration or stress redistribution can trigger multiple moderate shocks without a single dominant mainshock.

This swarm followed significant seismic unrest in the same area. Historical records since 2000 indicate only two such swarms in the region, the first occurring in 2003. The 2003 Tokachi-Oki earthquake (magnitude 8.1) and an associated magnitude 7.4 event 119 km ESE of Shizunai-furukawachō both occurred within tens of kilometers of the 2004 swarm epicenters, highlighting the persistent aftershock zone and ongoing stress adjustment along the plate interface.

Such swarm activity provides valuable insight into the mechanics of subduction-zone fault systems. The variable depths recorded during the 2004 sequence suggest involvement of both shallow crustal faults and deeper portions of the megathrust. Post-2003 monitoring in Hokkaido has shown that clusters of moderate events can occur years after major ruptures as residual stresses relax.

Continued seismic surveillance remains essential for the Hokkaido region given its tectonic position. The 2004 swarm, though moderate in scale, underscores the dynamic nature of the plate boundary and the value of dense observational networks in refining hazard assessments.

References
USGS Earthquake Catalog (historical event parameters)
Japan Meteorological Agency seismic records (regional tectonics)
SeismoSight internal swarm classification (PS20041114.1)