The 9.0 magnitude (MW) undersea megathrust earthquake occurred on 11 March 2011 at 14:46 JST (05:46 UTC) in the north-western Pacific Ocean at a relatively shallow depth of 32 km (19.9 mi), with its epicenter approximately 72 km (45 mi) east of the Oshika Peninsula of Tōhoku, Japan, lasting approximately six minutes. The earthquake was initially reported as 7.9 MW by the USGS before it was quickly upgraded to 8.8 MW, then to 8.9 MW, and then finally to 9.0 MW. Sendai was the nearest major city to the earthquake, 130 km (81 mi) from the epicenter; the earthquake occurred 373 km (232 mi) from Tokyo.
The main earthquake was preceded by a number of large foreshocks, with hundreds of aftershocks reported. The first major foreshock was a 7.2 MW event on 9 March, approximately 40 km (25 mi) from the epicenter of the 11 March earthquake, with another three on the same day in excess of 6.0 MW. Following the main earthquake on 11 March, a 7.0 MW aftershock was reported at 15:06 JST (6:06 UTC), succeeded by a 7.4 MW at 15:15 JST (6:16 UTC) and a 7.2 MW at 15:26 JST (6:26 UTC). Over eight hundred aftershocks of magnitude 4.5 MW or greater have occurred since the initial quake, including one on 26 October 2013 (local time) of magnitude 7.3. Aftershocks follow Omori’s Law, which states that the rate of aftershocks declines with the reciprocal of the time since the main quake. The aftershocks will thus taper off in time, but could continue for years.
One minute before the earthquake was felt in Tokyo, the Earthquake Early Warning system, which includes more than 1,000 seismometers in Japan, sent out warnings of impending strong shaking to millions. It is believed that the early warning by the Japan Meteorological Agency (JMA) saved many lives. The warning for the general public was delivered about 8 seconds after the first P wave was detected, or about 31 seconds after the earthquake occurred. However, the estimated intensities were smaller than the actual ones in some places in Kanto and Tohoku regions. This was thought to be because of smaller estimated earthquake magnitude, smaller estimated fault plane, shorter estimated fault length, not having considered the shape of the fault, etc. There were also cases where large differences between estimated intensities by the Earthquake Early Warning system and the actual intensities occurred in the aftershocks and triggered earthquakes.