An intense and deadly seismic event struck offshore east of Tōhoku, Japan on 13 February 2021. The 7.3 or 7.1 earthquake occurred on a Saturday night at 23:07 JST (14:07 UTC) at a focal depth of . It had a maximum JMA intensity of Shindo 6+ to Shindo 7 while on the Mercalli intensity scale, it registered a rating of VIII (Severe). The earthquake was followed by multiple aftershocks within less than an hour, three of which registering magnitude 5.3. The earthquake itself has been considered an aftershock of the 2011 Tōhoku earthquake which had occurred almost ten years prior.
The earthquake left three people dead, and at least 186 injured. It also inflicted significant structural damage across the Tōhoku and Kanto regions. This earthquake resulted in both insurance claims and losses exceeding ¥138billion (US$1.2billion). Small tsunami waves were also observed without any damage. Because of its proximity to the Fukushima Daiichi Nuclear Power Plant, the event provoked concerns of radiation leaks but was dismissed soon after. Although there were no changes to the level of radiation, cooling water used in two of the reactor units was discovered leaking.
Background
The 11 March 2011 Tōhoku earthquake struck off the coast of eastern Japan, with an epicenter north of Sendai, and a hypocenter depth of 29 km. It was the largest instrumentally recorded earthquake in the country, and the fourth largest ever recorded worldwide. This earthquake generated massive tsunamis measuring up to 41 meters in height, the highest in Japan. It swept through the Kantō Plain, destroying towns and cities. The tsunami also caused a series of catastrophic nuclear meltdowns at the Fukushima Daiichi Nuclear Power Plant shortly after. In all, an estimated 20,000 people lost their lives due to both the earthquake and tsunami. The disaster resulted in economic losses of US$360 billion, making it the costliest natural disaster in history.
Tectonic setting
The Pacific plate, made of oceanic lithosphere, subducts beneath the Okhotsk Sea plate along a convergent boundary located off the east coast of the northern half of Japan. It runs from the Boso triple junction and ends near Hokkaido, where it joins the Kuril–Kamchatka Trench. At this location, the Pacific plate moves approximately westward relative to the North American plate at a velocity of /yr, subducting beneath Japan at the Japan Trench. This subduction zone is capable of producing megathrust earthquakes with magnitudes greater than 8.5, evident in the historical records. It was on the subduction interface where the 2011 earthquake nucleated. That event involved a rupture by on the subduction zone.
The most recent similar-sized comparable to the 2021 quake was the 6.9 2016 Fukushima earthquake, which occurred just southwest of where the February 2021 earthquake struck. That earthquake was the result of normal faulting within the overriding Okhotsk Sea Plate at a depth of . The Japan Meteorological Agency (JMA) observed a maximum intensity of Shindo 5 Lower, or VII (Very strong) according to the US Geological Survey. A moderate tsunami of struck the Sendai area shortly after, the largest since the 2011 disaster. Minimal damage and few casualties were reported.
Earthquake The earthquake occurred as the result of reverse faulting within the subducting Pacific plate; not on the subduction interface itself. Hence, it could be considered an intraplate earthquake. Moment tensor solutions indicate slip occurred either on a moderately dipping fault striking to the south, or a moderately dipping fault striking to the north-northeast, consistent with the east–west oriented compression expected in this region. Meanwhile, the Earthquake Research Committee from the Japanese government said the earthquake ruptured along a 45 km-long, north–south striking fault that dips towards the east. According to the US Geological Survey, the earthquake struck at a depth of , while the JMA placed its depth at .
According to the National Research Institute for Earth Science and Disaster Resilience (NIED), a preliminary source inversion using peak ground acceleration data indicate a reverse fault measuring 32 km × 24 km ruptured during the quake. The rupture caused a maximum coseismic slip of 2.7 meters southwest of the hypocenter. Much of the coseismic slip which was detected southwest of the epicenter occurred five to ten seconds into the mainshock. The total seismic moment released is estimated at 4.7 × 1019 Nm.
A back-projection analysis of the event revealed that the earthquake occurred in a coseismic gap of the 2011 mainshock rupture. Stress transfer from the 2011 event would have increased compressional strain around the subduction interface (which ruptured in 2011). The subducting Pacific plate beneath the rupture experienced an increase in compressional strain, causing the earthquake. The 2021 event may have released strain within the Pacific plate as a response to the increased stress brought by the 2011 shock. It is thought to be the largest thrust faulting event located well within the 2011 rupture area.
According to seismologists, this earthquake formed part of a sequence of three large events including the March 2022 and April 2011 events. The epicenter of the 2021 earthquake occurred close to that of the larger 2022 shock and has been categorized as a doublet earthquake due to their close location, the timing of occurrence, and magnitude. A seismic gap exists between the northern extent of the 2022 shock and the southern extent of the 2011 shock, located off the coast of Central Miyagi Prefecture, including Sendai. Coulomb stress transfer inferred from calculations showed that the gap has been stressed and is the likely source for a future rupture.
Seismic intensity According to the JMA, the earthquake had a maximum intensity of Shindo 6 Upper. These intensities were recorded in Fukushima and Miyagi prefectures. However, the Earth Science and Disaster Prevention Research Institute suggested the earthquake may have also caused Shindo 7 shaking at Yamamoto, Miyagi. At the same location, the maximum peak acceleration caused by the earthquake was recorded at 1,432 gals, far exceeding the 980 gals and 1,362 gals recorded in Mashiki, Kumamoto during the 2016 Kumamoto earthquakes which were assigned Shindo 7. This event which occurred near the deeper portion of the subduction interface generated short-period seismic energy which caused severe shaking at the coast.
+Japan Meteorological Agency seismic intensity Shindo 5 Lower and higherIntensityPrefectureLocation6+MiyagiZaōFukushimaSōma, Kunimi, Shinchi6-MiyagiIshinomaki, Kawasaki, Iwanuma, Tome, Watari, YamamotoFukushimaFukushima, Koriyama, Sukagawa, Minamisōma, Motomiya, Koori, Kawamata, Ten-ei, Naraha, Okuma5+MiyagiSendai, Ishinomaki, Shiogama, Shiroishi, Kurihara, Osaki, Ogawara, Marumori, Matsushima, Shichigahama, RifuFukushimaFukushima, Tomioka, Inawashiro, Kagamiishi, Motomiya, MinamisōmaTochigiTakanezawa, Nasu5-MiyagiIshinomaki, Tagajo, Kurihara, Osaki, Tomiya, Marumori, Taiwa, Shikama, Misato, KamiFukushimaIwaki, Shirakawa, Yugawa, Aizumisato, Nishigo, Tanagura, Yamatsuri, Hirata, Furudono, MiharuTochigiŌtawara, NakagawaIwateIchinoseki, YahabaYamagataYonezawa, Kaminoyama, Nakayama, ShiratakaIbarakiKasama, Hitachi, Tsuchiura, Chikusei, Shirosato, TokaiSaitamaKazo
Long period ground motion In addition to violent ground motions, extreme long period ground motion was also recorded. The intensity of the Japan Meteorological Agency long period ground motion scale reached a maximum of Class IV in Fukushima's Nakadori Region—the first time that level of intensity was observed after the 2018 Hokkaido Eastern Iburi earthquake. Class IV intensity has been observed only four times since the scale was first used in 2013 and the first time it was measured since full time operation of the scale began in 2019.
Class IV Long Period Ground Motion (LPGM) results in people finding it impossible to move without crawling, fixtures on casters will move wildly and may fall over, unsecured furniture will be more likely to fall over and there will be many cracks in partition walls. Class III LPGM results in people finding it hard to stand, fixtures on casters will move about and unsecured furniture may fall over, and partition walls will crack. Class II LPGM will result in people finding it difficult to walk without holding onto something stable, fixtures on casters will move slightly and objects on shelves may fall. Class I LPGM will result in people feeling slight shaking and may feel startled, and hanging items like lamps and window blinds will move significantly. +JMA long period ground motion scaleClassPrefectureLocationIVFukushimaNakadōriIIIMiyagiSouthern Miyagi, Northern MiyagiFukushimaHamadōriIIIwateInland area of Southern IwateMiyagiCentral MiyagiAkitaInland area of Southern AkitaYamagataShōnai (Shonai Region), Mogami (Mogami Region), Murayama (Murayama Region), (Okitama Region)FukushimaAizuIbarakiNorthern Ibaraki, Southern IbarakiTochigiNorthern TochigiChibaNorthwestern ChibaKanagawaEastern KanagawaNiigata(Kaetsu Region), (Chuetsu Region)YamanashiFuji Five Lakes, Eastern YamanashiNaganoCentral NaganoIHokkaidoCentral Tokachi, Eastern OshimaAomoriNorthern Tsugaru, Southern Tsugaru, Sanpachi Kamikita, ShimokitaIwateInland area of Northern Iwate, Coastal area of Northern IwateAkitaCoastal area of Southern Akita, Coastal area of Northern Akita, Inland area of Northern AkitaTochigiSouthern TochigiGunmaSouthern GunmaSaitamaChichibu Region, Northern Saitama, Southern SaitamaChibaNortheastern Chiba, Southern ChibaTokyo23 Special wards, Eastern TamaKanagawaWestern KanagawaYamanashiCentral and Western YamanashiNaganoSouthern NaganoShizuokaCentral Shizuoka, Eastern ShizuokaAichiWestern Aichi
Foreshocks and aftershocks +2021 Miyagi–Fukushima earthquakeTime (UTC)LocationMagnitudeDepth (km)MMIRef.2021-01-28 18:24:08near the east coast of Honshu4.339.4 -2021-02-01 08:34:4778 km east of Namie4.644.7II2021-02-03 06:47:0634 km south of Ōfunato4.266.8 -2021-02-04 03:40:1310 km north northeast of Kitaibaraki4.354.9I2021-02-13 14:07:4872 km east southeast of Namie7.144.0VIII2021-02-13 14:36:4260 km east northeast of Namie4.959.8V2021-02-13 14:51:4269 km east northeast of Namie5.450.3IV2021-02-13 16:14:4171 km east northeast of Namie4.352.0 -2021-02-13 16:32:5870 km east northeast of Namie4.456.5 -2021-02-13 18:25:2853 km east southeast of Kamaishi5.248.2IV2021-02-14 07:31:4944 km east of Namie5.138.2IV2021-02-14 07:59:2367 km east northeast of Namie4.553.6 -2021-02-14 09:26:5869 km east of Namie4.650.9 -2021-02-15 11:27:1877 km east northeast of Namie4.235.0 -2021-02-15 11:59:5052 km east northeast of Namie4.435.0 -2021-02-15 12:26:0451 km east of Namie5.335.6 III2021-02-15 12:26:1834 km east of Namie5.340.1 III2021-02-16 13:59:32Kakuda4.354.1 -2021-02-16 13:59:3256 km east of Ishinomaki4.645.0 -2021-02-18 06:02:4359 km northeast of Namie4.558.8 -2021-02-18 12:43:0058 km east northeast of Namie4.359.9 -2021-02-19 16:16:28111 km east northeast of Hachinohe5.046.1 II2021-02-19 19:01:0666 km east northeast of Namie4.560.3 -2021-02-20 01:17:0542 km east of Ishinomaki4.368.5 -2021-02-22 21:22:3646 km east northeast of Namie4.450.6 -2021-02-26 04:04:2456 km east northeast of Namie4.658.0 -2021-02-26 10:18:3153 km east northeast of Namie4.459.9 -2021-02-26 15:33:2935 km southeast of Namie4.679.5 -2021-02-26 17:03:4889 km east northeast of Namie4.751.4II2021-03-07 03:42:1915 km east southeast of Funaishikawa4.254.3 -2021-03-07 12:44:2761 km east northeast of Namie4.260.5 -2021-03-08 14:36:0665 km east northeast of Namie4.659.2 -2021-03-08 23:29:5834 km south southwest of Ōfunato4.983.3III2021-03-17 08:28:4958 km east northeast of Namie5.045.9IV2021-03-20 21:56:3680 km east of Namie4.754.5 -2021-05-13 23:58:1476 km east northeast of Namie6.032.0IV
Other events