Seismic Swarm PS20121207.1: Analysis of Activity off Ishinomaki, Japan

On 7 December 2012, a seismic swarm designated PS20121207.1 was recorded approximately 221 km east-southeast of Ishinomaki, Japan. The sequence began at 08:18 and concluded by 23:00, encompassing 10 earthquakes within a span of 14 hours and 41 minutes. This event occurred in the tectonically active Japan Trench region, where the Pacific Plate subducts beneath the Okhotsk Plate, generating frequent seismic activity along the subduction interface.

The swarm initiated with a magnitude 7.3 earthquake at a depth of 31 km, followed by a magnitude 6.2 event 13 minutes later at 32 km depth. Subsequent shocks included magnitudes of 5.2, 4.7, 5.5, 5.0, 5.1, 5.0, 5.1, and a final 5.2, with depths ranging from 20 to 35 km. These events clustered tightly in both time and space, characteristic of swarm behavior driven by fluid migration or stress transfer along fault segments rather than a single mainshock-aftershock sequence.

This swarm aligns with the broader aftershock regime following the 2011 Tohoku earthquake (magnitude 9.0), which ruptured a vast portion of the subduction zone. The December 2012 activity lies within an area of elevated post-seismic stress adjustment. Historical records since 2000 indicate 10 prior swarms in the vicinity, with one occurring in 2008 and nine in 2011, underscoring episodic clustering in this segment of the trench.

Notable strong earthquakes since 2000 include the magnitude 7.3 event of 7 December 2012 itself, located 23 km from the swarm center; a magnitude 7.0 on 10 July 2011 (53 km distant); the magnitude 7.7 on 11 March 2011 (81 km distant); and a magnitude 7.0 on 31 October 2003 (99 km distant). These occurrences reflect the persistent seismic hazard posed by the subduction zone, where megathrust events can exceed magnitude 8.

Geologically, the Japan Trench exhibits high convergence rates of approximately 8–9 cm per year, facilitating both interplate and intraplate seismicity. Depths of 20–35 km place the swarm events near the downdip limit of the seismogenic zone, where temperature and pressure conditions influence rupture behavior. Updated monitoring by regional networks confirms ongoing low-level activity in the area, consistent with viscoelastic relaxation following major ruptures.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Database
Global Centroid Moment Tensor Project