The 9.0-magnitude (MW) undersea megathrust earthquake occurred on 11 March 2011 at 14:46 JST (05:46 UTC) in the 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 nearest major city to the quake was Sendai, on the main island of Honshu, 130 km (81 mi) away. The quake occurred 373 km (232 mi) from Tokyo. The main earthquake was preceded by a number of large foreshocks, and hundreds of aftershocks were reported. The first major foreshock was a 7.2 MW event on 9 March, approximately 40 km (25 mi) from the location of the 11 March quake, with another three on the same day in excess of 6.0 MW. Following the quake, a 7.0 MW aftershock was reported at 15:06 JST, followed by a 7.4 at 15:15 JST and a 7.2 at 15:26 JST. Over eight hundred aftershocks of magnitude 4.5 or greater have occurred since the initial quake. United States Geological Survey (USGS) director Marcia McNutt explained that aftershocks follow Omori's Law, might continue for years, and will taper off in time.
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. The early warning is believed by the Japan Meteorological Agency (JMA) to have saved many lives.
Initially reported as 7.9 MW by the USGS, the magnitude was quickly upgraded to 8.8, then again to 8.9, and then finally to 9.0.
This earthquake occurred where the Pacific Plate is subducting under the plate beneath northern Honshu; which plate is a matter of debate amongst scientists. The Pacific plate, which moves at a rate of 8 to 9 cm (3.1 to 3.5 in) per year, dips under Honshu's underlying plate releasing large amounts of energy. This motion pulls the upper plate down until the stress builds up enough to cause a seismic event. The break caused the sea floor to rise by several meters. A quake of this magnitude usually has a rupture length of at least 480 km (300 mi) and generally requires a long, relatively straight fault surface. Because the plate boundary and subduction zone in the area of the rupture is not very straight, it is unusual for the magnitude of an earthquake to exceed 8.5; the magnitude of this earthquake was a surprise to some seismologists. The hypocentral region of this earthquake extended from offshore Iwate Prefecture to offshore Ibaraki Prefecture. The Japanese Meteorological Agency said that the earthquake may have ruptured the fault zone from Iwate to Ibaraki with a length of 500 km (310 mi) and a width of 200 km (120 mi). Analysis showed that this earthquake consisted of a set of three events. The earthquake may have had a mechanism similar to that of another large earthquake in 869 with an estimated surface wave magnitude (Ms) of 8.6, which also created a large tsunami. Other major earthquakes with tsunamis struck the Sanriku Coast region in 1896 and in 1933.
The strong ground motion registered at the maximum of 7 on the Japan Meteorological Agency seismic intensity scale in Kurihara, Miyagi Prefecture. Three other prefectures—Fukushima, Ibaraki and Tochigi—recorded an upper 6 on the JMA scale. Seismic stations in Iwate, Gunma, Saitama and Chiba Prefecture measured a lower 6, recording an upper 5 in Tokyo.
This earthquake released a surface energy (Me) of 1.9±0.5×1017 joules, dissipated as shaking and tsunamic energy, which is nearly double that of the 9.1-magnitude 2004 Indian Ocean earthquake and tsunami that killed 230,000 people. "If we could only harness the [surface] energy from this earthquake, it would power [a] city the size of Los Angeles for an entire year," McNutt said in an interview. The total energy released, also known as the seismic moment (M0), was more than 200,000 times the surface energy and was calculated by the USGS at 3.9×1022 joules, slightly less than the 2004 Indian Ocean quake. This is equivalent to 9,320 gigatons of TNT, or approximately 600 million times the energy of the Hiroshima bomb.
Japan's National Research Institute for Earth Science and Disaster Prevention (NIED) calculated a peak ground acceleration of 2.99 g (29.33 m/s²). The largest individual recording in Japan was 2.7g, in the Miyagi Prefecture, 75 km from the epicentre; the highest reading in the Tokyo metropolitan area was 0.16g.
The quake moved portions of northeastern Japan by as much as 2.4 m (7.9 ft) closer to North America, making portions of Japan's landmass wider than before. Portions of Japan closest to the epicenter experienced the largest shifts. A 400 km (250 mi) stretch of coastline dropped vertically by 0.6 m (2.0 ft), allowing the tsunami to travel farther and faster onto land. One early estimate suggested that the Pacific plate may have moved westward by up to 20 m (66 ft), and another early estimate put the amount of slippage at as much as 40 m (130 ft). On 6 April the Japanese coast guard said that the quake shifted the seabed near the epicenter 24 meters (79 ft) and elevated the seabed off the coast of Miyagi prefecture by 3 meters.
The earthquake shifted the Earth's axis by estimates of between 10 cm (4 in) and 25 cm (10 in). This deviation led to a number of small planetary changes, including the length of a day and the tilt of the Earth. The speed of the Earth's rotation increased, shortening the day by 1.8 microseconds due to the redistribution of Earth's mass. The axial shift was caused by the redistribution of mass on the Earth's surface, which changed the planet's moment of inertia. Because of conservation of angular momentum, such changes of inertia result in small changes to the Earth's rate of rotation. These are expected changes for an earthquake of this magnitude.
Soil liquefaction was evident in areas of reclaimed land around Tokyo, particularly in Urayasu, Chiba City, Funabashi, Narashino (all in Chiba Prefecture) and in the Koto, Edogawa, Minato, Chūō, and Ōta Wards of Tokyo. Approximately 30 homes or buildings were destroyed and 1,046 other buildings were damaged to varying degrees. Nearby Haneda Airport, built mostly on reclaimed land, was not damaged. Odaiba also experienced liquefaction, but damage was minimal.
Shinmoedake, a volcano in Kyushu, erupted two days after the earthquake. The volcano had previously erupted in January 2011; it is not known if the later eruption was linked to the earthquake. In Antarctica, the seismic waves from the earthquake were reported to have caused the Whillans Ice Stream to slip by about 0.5 m (1.6 ft).
The first sign international researchers had that the earthquake caused such a dramatic change in the Earth’s rotation came from the United States Geographical Survey which monitors Global Positioning Satellite stations across the world. The Survey team had several GPS monitors located near the scene of the earthquake, and one was directly in the epicenter. The GPS station located in the epicenter proved that Japan had gotten at least thirteen feet wider as a result of the splitting of the Earth. This motivated government researchers to look into other ways the earthquake may have had large scale effects on the planet. Scientists at NASA’s Jet Propulsion Laboratory did some calculations and determined that the Earth’s rotation was changed by the earthquake to the point where the days are now one point eight (1.8) microseconds shorter.
While the 1.8 microsecond shortening of the day is not noticeable to the average person, one way the earthquake and its effects on the Earth’s rotation and time of day is important was explained by Dr. Richard Gross, one of the head researchers working for NASA. Gross explained that even a difference of 1.8 microseconds is important to his team, because it affects the way that spacecraft being sent to Mars are navigated. Not taking the changes into account creates a greater chance for failure of the mission, resulting in millions of dollars wasted. Gross noted that the way the Earth rotates is not very smooth; he related the way the Earth moves to an old car wobbling on its axle. The earthquake was similar to if a person took a hammer and whacked the car's axle, causing it to shift and the car to drive differently. This is what occurred with the earthquake in Japan. The powerful earthquake was the hammer hitting the Earth’s axle, causing it to spin in a slightly different way.
Japan experienced over 900 aftershocks since the earthquake, with about 60 registering over magnitude 6.0 Mw and at least three over 7.0 Mw. A magnitude 7.7 Mw and a 7.9 Mw quake occurred on March 11 and the third one struck offshore on 7 April with a disputed magnitude. Its epicenter was underwater, 66 km (41 mi) off the coast of Sendai. The Japan Meteorological Agency assigned a magnitude of 7.4 MJMA, while the U.S. Geological Survey lowered it to 7.1 Mw. At least four people were killed, and electricity was cut off across much of northern Japan including the loss of external power to Higashidori Nuclear Power Plant and Rokkasho Reprocessing Plant. Four days later on April 11, another strong magnitude 6.6 Mw aftershock struck Fukushima, causing additional damage and killing a total of three people.
As of 3 June 2011 aftershocks continued; a regularly updated map showing all shocks of magnitude 4.5 and above near or off the east coast of Honshu in the last seven days showed over 20 events. By 8 June[update] shocks in the past week had dropped to 13.