The March 9, 2011 Magnitude 7.3 Foreshock off Ōfunato, Japan

On March 9, 2011, at 02:45 local time, a magnitude 7.3 earthquake struck approximately 120 km southeast of Ōfunato, Japan, at a depth of 32 km. This event occurred in the offshore region of the Japan Trench, where the Pacific Plate subducts beneath the Okhotsk Plate. The hypocenter placed the rupture within the seismogenic zone typical of interplate thrust faulting in this convergent margin. The Tohoku region of northeastern Honshu lies along one of the most active subduction zones on Earth. The Japan Trench accommodates convergence rates of approximately 8–9 cm per year, generating repeated megathrust earthquakes. Historical records document major events in 1896 and 1933, both producing devastating tsunamis along the Sanriku coast. The March 9 foreshock formed part of a precursory sequence that preceded the Mw 9.1 Great Tohoku Earthquake by two days. Seismic data since 2000 indicate elevated activity in the vicinity. Notable events include the Mw 7.0 earthquake of July 10, 2011 (177 km ESE of Ishinomaki), the Mw 9.1 mainshock of March 11, 2011 (201 km ESE of Ishinomaki), the Mw 7.2 event of August 16, 2005 (66 km ESE of Ishinomaki), and the Mw 7.0 shock of October 31, 2003 (133 km ESE of Ishinomaki). The March 9 foreshock itself was located 0 km from its reported epicenter, while the mainshock occurred 43 km away and the July 2011 aftershock 57 km distant. Geodetic and seismological observations show that the March 9 rupture likely loaded adjacent fault segments, contributing to the stress conditions that culminated in the March 11 megathrust event. Aftershock distributions following the foreshock aligned with the plate interface, consistent with the broader tectonic framework of the Japan Trench.

References

USGS Earthquake Catalog (provided event parameters).
Japan Meteorological Agency seismic records.