Seismic Activity in the Tohoku Region Near Ishinomaki, Japan

The coastal area east of Ishinomaki in Miyagi Prefecture lies within the tectonically active Japan Trench subduction zone. Here the Pacific Plate descends beneath the overriding plate at a rate of approximately 8–9 cm per year, generating frequent megathrust earthquakes and associated aftershock sequences. The region’s geology features a well-developed accretionary prism and forearc basin, with crustal faults that accommodate both interplate and intraplate deformation. Historical records and modern instrumentation confirm that this segment of the trench has repeatedly produced great earthquakes, often with magnitudes exceeding 7.0. Between 2000 and 2022, ten events of magnitude 7.0 or greater occurred within roughly 80 km of the 16 August 2005 M7.2 earthquake (66 km ESE of Ishinomaki at 36 km depth). These include the 11 March 2011 M9.1 Great Tohoku Earthquake (29 km from the 2005 epicenter), the 7 April 2011 M7.1 aftershock (39 km), the 20 March 2021 M7.0 event (39 km), the 13 February 2021 M7.1 (65 km), the 16 March 2022 M7.3 (74 km), and earlier shocks in 2003, 2008, and 2011. The 2005 M7.2 itself was a significant intraslab or interface event that contributed to the accumulating strain release pattern preceding the 2011 mainshock. The 2011 M9.1 earthquake remains the largest recorded in Japan’s instrumental era and produced extensive coseismic slip along the plate interface, extensive tsunami inundation, and widespread aftershock activity that continued for more than a decade. Subsequent large events, such as the 2021 and 2022 shocks, occurred on or near the same megathrust and demonstrated the ongoing seismic productivity of the Miyagi-oki segment. Depths of these events typically range from 20 km to 80 km, consistent with the geometry of the subducting slab. Seismic hazard assessments for the Ishinomaki region therefore emphasize both the potential for future great earthquakes and the likelihood of damaging aftershocks and triggered events in the decades following 2011. Continuous monitoring by dense networks of seismometers and GPS stations provides real-time data that refine rupture models and improve long-term forecasts for this portion of the Japan Trench.

References

USGS Earthquake Catalog (events 2000–2022)
Japan Meteorological Agency seismic database