Koshima earthquake

The 2011 Tōhoku (M 9.0–9.1) earthquake’s hypocenter lay roughly 14 km west‑north‑west of the town of Naze in Miyagi Prefecture, a point that sits on the shallow, active megathrust where the Pacific Plate is subducting beneath the North American (or Okhotsk) Plate. At a depth of about 30 km, the fault rupture slipped by up to 20 m, releasing a colossal amount of strain that triggered a devastating tsunami and a swarm of aftershocks extending hundreds of kilometers along the trench‑forearc system. The region’s tectonics are characterized by a near‑vertical, oblique convergence that transfers tens of millions of tons of tectonic load into the megathrust, making it one of the most powerful seismic sources on Earth. Subsequent geodetic and seismic studies have mapped the complex fault geometry, including multiple fault segments, inter‑plate locking zones, and the presence of a shallow‑crustal “secondary” fault system that contributed to the event’s large magnitude and extensive surface rupture.