Seismic Activity Along the Japan Trench: The December 2012 M7.3 Earthquake and Regional Context

The offshore region southeast of Ofunato, Japan, lies within the Japan Trench subduction zone, where the Pacific Plate converges with and subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting produces frequent large earthquakes, including megathrust events along the plate interface and intraslab earthquakes within the descending slab. The December 7, 2012, magnitude 7.3 earthquake occurred at 08:18 UTC at a depth of 31 km, approximately 234 km southeast of Ofunato, placing its hypocenter in the vicinity of the 2011 Tohoku rupture zone.

This event formed part of the prolonged aftershock sequence following the March 11, 2011, magnitude 9.0 Tohoku earthquake. The 2011 mainshock and its immediate aftershocks, including a magnitude 7.7 event on the same day located 272 km east-southeast of Kamaishi, released substantial stored strain along the subduction interface. Subsequent activity included a magnitude 7.0 earthquake on November 14, 2005, 293 km east-southeast of Kamaishi, and another magnitude 7.0 event on July 10, 2011, 177 km east-southeast of Ishinomaki. The 2012 earthquake, occurring roughly 21 months after the Tohoku mainshock, demonstrated the extended duration of aftershock productivity in this region, with its epicenter situated near the earlier 2011 aftershock cluster.

Geologically, the Japan Trench exhibits a well-developed accretionary prism and a pronounced forearc basin system. Earthquake depths around 30 km, as recorded for the 2012 event, typically correspond to the downdip limit of the seismogenic zone or to intraslab faulting within the Pacific Plate. Historical records indicate that the Tohoku segment has hosted repeated great earthquakes at intervals of several hundred to roughly one thousand years, with the 2011 event representing the largest instrumentally recorded rupture in the area. Post-2011 geodetic and seismic monitoring has revealed viscoelastic relaxation and afterslip processes that continue to influence regional stress patterns.

The cluster of magnitude 7+ events since 2000 underscores the elevated seismic hazard along this portion of the trench. Depths consistently in the 20–40 km range reflect the geometry of the subducting slab, which steepens eastward from the trench axis. Ongoing monitoring by regional networks continues to track aftershock decay rates and the potential for additional moderate-to-large triggered events in the coming years.

References

USGS Earthquake Catalog (event data 2005–2012)
Japan Meteorological Agency seismic reports
Global CMT Project moment tensor solutions