M 7.0; 30 km E of Ishinomaki, Japan; (20 Mar 2021) (20km from the earthquake)
M 7.1; 73 km ENE of Namie, Japan; (13 Feb 2021) (63km from the earthquake)
M 9.1; 2011 Great Tohoku Earthquake, Japan; (11 Mar 2011) (68km from the earthquake)
M 7.0; 107 km E of Namie, Japan; (19 Jul 2008) (97km from the earthquake)
M 7.2; 66 km ESE of Ishinomaki, Japan; (16 Aug 2005) (39km from the earthquake)
M 7.0; 27 km SSW of ?funato, Japan; (26 May 2003) (63km from the earthquake)
Seismic Activity in Japan's Tohoku Region
The Tohoku region of northeastern Japan lies along the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate at a rate of approximately 8–9 cm per year. This convergent margin produces frequent megathrust earthquakes and associated aftershock sequences. The area has experienced repeated large-magnitude events throughout recorded history, reflecting long-term strain accumulation and release along the plate interface. On 11 March 2011, the M9.1 Great Tohoku Earthquake struck 68 km from the reference location near Ishinomaki, generating a devastating tsunami and triggering widespread aftershocks. One notable aftershock was the M7.1 event that occurred at 14:32 on 7 April 2011, centered 29 km ESE of Ishinomaki at a depth of 42 km. This earthquake caused additional damage in already affected coastal communities. Instrumental records since 2000 document several other strong earthquakes within 100 km of the 7 April 2011 epicenter. These include an M7.3 event 57 km ENE of Namie on 16 March 2022, an M7.0 shock 30 km E of Ishinomaki on 20 March 2021, an M7.1 event 73 km ENE of Namie on 13 February 2021, an M7.0 earthquake 107 km E of Namie on 19 July 2008, an M7.2 event 66 km ESE of Ishinomaki on 16 August 2005, and an M7.0 quake 27 km SSW of Ōfunato on 26 May 2003. These events cluster along the downdip and along-strike extensions of the 2011 rupture zone, consistent with stress redistribution following the mainshock. Geological studies of the margin indicate that the 2011 rupture extended approximately 500 km along strike and reached the trench axis, producing unusually large shallow slip. Post-2011 geodetic data show ongoing viscoelastic relaxation and afterslip, contributing to elevated seismicity rates that persist more than a decade later. The region remains capable of generating additional M7-class earthquakes as residual stress continues to adjust.
References
USGS Earthquake Catalog (event parameters and locations) Japan Meteorological Agency seismic database (regional history)