On 11 March 2011, at 14:46:24 local time (UTC+9), a  9.0–9.1 undersea megathrust earthquake occurred in the Pacific Ocean, east of the Oshika Peninsula of the Tōhoku region of Japan. It lasted approximately six minutes and caused a tsunami. It is sometimes known in Japan as the , among other names. It is often referred to by its numerical date, 3.11 (read in Japanese).

The earthquake was the most powerful ever recorded in Japan, and the fourth most powerful recorded in the world since modern seismography began in 1900. It triggered powerful tsunami waves that may have reached heights of up to in Miyako in Tōhoku's Iwate Prefecture,Yomiuri Shimbun evening edition 2-11-04-15 page 15, nearby Aneyoshi fishery port (姉吉漁港)(Google map E39 31 57.8, N 142 3 7.6) 2011-04-15, 大震災の津波、宮古で38.9 m...明治三陸上回る by okayasu Akio (岡安 章夫) and which, in the Sendai area, traveled at and up to inland. Residents of Sendai had only eight to ten minutes of warning, and more than a hundred evacuation sites were washed away. The snowfall which accompanied the tsunami and the freezing temperature hindered rescue works; for instance, in Ishinomaki, the city with the most deaths, the temperature was as the tsunami hit. Official figures released in 2021 reported 284 firefighters dead from attempts to close preventative fire gates, and A report in 2015 found that 228,863 people were still living away from their home either temporarily or permanently.

The tsunami caused the Fukushima Daiichi nuclear disaster, primarily the meltdowns of three of its reactors, the discharge of radioactive water in Fukushima and the associated evacuation zones affecting hundreds of thousands of residents. Many electrical generators ran out of fuel. The loss of electrical power halted cooling systems, causing heat to build. Without ventilation, hydrogen gas accumulated within the upper refueling hall and exploded, forcefully ejecting the refueling hall's blast panels. Residents within a radius of the Fukushima Daiichi Nuclear Power Plant and a radius of the Fukushima Daini Nuclear Power Plant were evacuated.

Early estimates placed insured losses from the earthquake alone at US$14.5 to $34.6 billion. The Bank of Japan offered ¥15 trillion (US$183 billion) to the banking system on 14 March 2011 in an effort to normalize market conditions. The estimated economic damage amounted to over $300 billion, making it the costliest natural disaster in history. According to a 2020 study, "the earthquake and its aftermaths resulted in a 0.47-percentage-point decline in Japan's real GDP growth in the year following the disaster".

Earthquake

The magnitude 9.1 (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 , with its epicenter approximately 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. On 11 July 2016, the USGS further upgraded the earthquake to 9.1. Sendai was the nearest major city to the earthquake, from the epicenter; the earthquake occurred northeast of Tokyo.

The main earthquake was preceded by a number of large foreshocks, with hundreds of aftershocks reported. One of the first major foreshocks was a 7.2 Mw event on 9 March, approximately 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 7.4 Mw aftershock was reported at 15:08 JST (6:06 UTC), succeeded by a 7.9 Mw at 15:15 JST (6:16 UTC) and a 7.7 Mw at 15:26 JST (6:26 UTC). Over 800 aftershocks of magnitude 4.5 Mw or greater occurred after the initial quake. Aftershocks followed Omori's law, which states that the rate of aftershocks declines with the reciprocal of the time since the main quake. The aftershocks thus tapered off over time, but continued for years after the initial quake, including one on 26 October 2013 (local time) of magnitude 7.1 Mw.

The earthquake moved Honshu east, shifted the Earth on its axis by estimates of between , increased Earth's rotational speed by 1.8 μs per day, and generated infrasound waves detected in perturbations of the low-orbiting Gravity Field and Steady-State Ocean Circulation Explorer satellite. Initially, the earthquake caused part of Honshu's Pacific coast to sink by roughly a meter, but after about three years, the coast rose back and then kept on rising to exceed its original height.未来を考える力を 気仙沼復興レポート㉘ 地盤の沈下と隆起 11 June 2016 今川 悟地盤が隆起 漁業者困惑 最大40センチ、積み荷に影響 三陸沿岸で地殻変動か Nikkei 9 March 2015東北沿岸、地震で沈んだ地盤が謎の隆起 マントル影響か Nikkei 27 February 2017

Geology

This megathrust earthquake was a recurrence of the mechanism of the earlier 869 Sanriku earthquake, which has been estimated as having a magnitude of at least 8.4 Mw, which also created a large tsunami that inundated the Sendai plain. Three tsunami deposits have been identified within the Holocene sequence of the plain, all formed within the last 3,000 years, suggesting an 800-to-1,100-year recurrence interval for large tsunamigenic earthquakes. In 2001 it was reckoned that there was a high likelihood of a large tsunami hitting the Sendai plain as more than 1,100 years had then elapsed. In 2007, the probability of an earthquake with a magnitude of Mw 8.1–8.3 was estimated as 99% within the following 30 years.

This earthquake occurred where the Pacific plate is subducting under the plate beneath northern Honshu. The Pacific plate, which moves at a rate of per year, dips under Honshu's underlying plate, building large amounts of elastic energy. This motion pushes the upper plate down until the accumulated stress causes a seismic slip-rupture event. The break caused the sea floor to rise by several meters. The magnitude of this earthquake was a surprise to some seismologists. A quake of this magnitude usually has a rupture length of at least and generally requires a long, relatively straight fault surface. Because the plate boundary and subduction zone in the area of the Honshu rupture is not very straight, it is unusual for the magnitude of its earthquake to exceed 8.5 Mw. The hypocentral region of this earthquake extended from offshore Iwate Prefecture to offshore Ibaraki Prefecture. The Japan Meteorological Agency said that the earthquake may have ruptured the fault zone from Iwate to Ibaraki with a length of and a width of . Analysis showed that this earthquake consisted of a set of three events. Other major earthquakes with tsunamis struck the Sanriku Coast region in 1896 and in 1933.

The source area of this earthquake has a relatively high coupling coefficient surrounded by areas of relatively low coupling coefficients in the west, north, and south. From the averaged coupling coefficient of 0.5–0.8 in the source area and the seismic moment, it was estimated that the slip deficit of this earthquake was accumulated over a period of 260–880 years, which is consistent with the recurrence interval of such great earthquakes estimated from the tsunami deposit data. The seismic moment of this earthquake accounts for about 93% of the estimated cumulative moment from 1926 to March 2011. Hence, earthquakes in this area with magnitudes of about 7 since 1926 had only released part of the accumulated energy. In the area near the trench, the coupling coefficient is high, which could act as the source of the large tsunami.

Most of the foreshocks are interplate earthquakes with thrust-type focal mechanisms. Both interplate and intraplate earthquakes appeared in the aftershocks offshore Sanriku coast with considerable proportions."Spatial distribution and focal mechanisms of aftershocks of the 2011 off the Pacific coast of Tōhoku Earthquake" by Y. Asano, T. Saito, Y. Ito, K. Shiomi, H. Hirose, T. Matsumoto, S. Aoi, S. Hori, and S. Sekiguchi.

Energy The surface energy of the seismic waves from the earthquake was calculated to be joules, which is nearly double that of the 9.1 Mw 2004 Indian Ocean earthquake and tsunami that killed 230,000 people. If harnessed, the seismic energy from this earthquake would power a city the size of Los Angeles for an entire year. The seismic moment (M0), which represents a physical size for the event, was calculated by the USGS at 3.9×1022 joules, slightly less than the 2004 Indian Ocean quake.

Ground motion The strong ground motion registered at the maximum of Shindo 7 on the Japan Meteorological Agency seismic intensity scale in Kurihara, Miyagi Prefecture. Three other prefectures—Fukushima, Ibaraki and Tochigi—recorded a 6 upper on the JMA scale. Seismic stations in Iwate, Gunma, Saitama and Chiba Prefectures measured a 6 lower, while Tokyo and five other prefectures recorded 5 upper.

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). A USGS seismic installation at Kurihara recorded 2.751 g in ground acceleration (pga) and in ground velocity (pgv); the station data corresponded to a Modified Mercalli intensity (MMI) of IX (Violent), which was also recorded at five other installations at Ōsaki, Onagawa and Iwanuma, Miyagi, Haga, Tochigi, and the Fukushima Daiichi Nuclear Power Plant; tremors exceeding MMI VI (Strong) or higher were felt in the Greater Tokyo Area, with a seismic station in Kōtō recording a pga of 0.2552 g and a pgv of , corresponding to MMI VIII (Severe) shaking. Tremors from the earthquake were also felt in the Northern Mariana Islands, the Korean Peninsula, Taiwan, Northeast China and the Russian Far East. IntensityPrefectureMiyagiFukushima, Ibaraki, TochigiIwate, Gunma, Saitama, ChibaAomori, Akita, Yamagata, Tokyo, Kanagawa, YamanashiNiigata, Nagano, ShizuokaHokkaido, Gifu, AichiToyama, Ishikawa, Fukui, Mie, Shiga, Kyoto, Osaka, Hyōgo, NaraWakayama, Tottori, Shimane, Okayama, Tokushima, Kochi, Saga, KumamotoHiroshima, Kagawa, Ehime, Yamaguchi, Fukuoka, Nagasaki, Oita, Kagoshima IntensityPrefectureLocation(s)MiyagiKuriharaMiyagiWakuya, Tome, Misato, Osaki, Natori, Zao, Kawasaki, Yamamoto, Sendai (Miyagino), Ishinomaki, Shiogama, Higashimatsushima, OhiraFukushimaShirakawa, Sukagawa, Kunimi, Kagamiishi, Ten-ei, Naraha, Tomioka, Okuma, Futaba, Namie, ShinchiIbarakiHokota, Hitachi, Takahagi, Omitama, Naka, Kasama, Chikusei, HitachiomiyaTochigiOtawara, Utsunomiya, Mooka, Ichikai, TakanezawaIwateOfunato, Rikuzentakata, Kamaishi, Takizawa, Yahaba, Hanamaki, Ichinoseki, ŌshūMiyagiKesennuma, Minamisanriku, Shiroishi, Kakuda, Iwanuma, Ogawara, Watari, Sendai (Aoba, Wakabayashi, Izumi), Matsushima, Rifu, Taiwa, Ōsato, TomiyaFukushimaFukushima, Koriyama, Nihonmatsu, Koori, Kawamata, Nishigō, Nakajima, Yabuki, Tanagura, Tamakawa, Asakawa, Ono, Tamura, Date, Motomiya, Iwaki, Soma, Hirono, Kawauchi, Iitate, MinamisomaIbarakiMito, Kitaibaraki, Hitachinaka, Ibaraki, Tokai, Hitachiota, Tsuchiura, Ishioka, Toride, Tsukuba, Kashima, Itako, Miho, Bando, Inashiki, Kasumigaura, Namegata, Sakuragawa, Joso, Tsukubamirai, ShirosatoTochigiNasu, Nasushiobara, Haga, Nasukarasuyama, NakagawaGunmaKiryuSaitamaMiyashiroChibaNarita, Inzai

Geophysical effects