The focal depth of the 2011 Tōhoku earthquake was 29 kilometers. This is the depth of the earthquake’s hypocenter—the underground point where rupture began—not the height or depth of the tsunami that followed.
The magnitude-9.0–9.1 megathrust struck offshore Japan on 11 March 2011, east of the Tōhoku region. It occurred at the convergent boundary where the Pacific Plate moves beneath the plate associated with northern Japan. The rupture lifted or lowered parts of the seafloor, displacing seawater and generating a devastating tsunami.
A common mix-up is to confuse the focal depth with the earthquake’s offshore location, the length of its rupture, or the tsunami’s run-up height. The earthquake’s 29-kilometer depth helped classify it as a relatively shallow megathrust event, allowing strong seafloor displacement to affect the ocean above. In Miyako, Iwate, tsunami waves reached about 40.5 meters, a separate measurement from the 29-kilometer focal depth. The disaster also triggered the Fukushima Daiichi nuclear accident and caused catastrophic damage across northeastern Japan.