On March 16, 2022, at 23:36 JST, a strong earthquake struck off the coast of Fukushima, Japan. The earthquake had a magnitude of 7.4 according to the Japanese Meteorological Agency (JMA), while the United States Geological Survey (USGS) gave an estimate of 7.3. Immediately after the event a 30cm tsunami was reported. The event is known in Japanese as . As a result of this natural disaster, four people died and 247 were injured.
Tectonic setting
The Pacific plate, made of oceanic lithosphere, subducts beneath the Okhotsk microplate 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 70 mm/yr, subducting beneath Japan at the Japan Trench. This subduction zone can produce megathrust earthquakes with magnitudes greater than 8.5, as evident in historical records. It was on the subduction interface where the 2011 Tōhoku earthquake and tsunami nucleated. That event involved a rupture of 220 × 400 km on the subduction zone.
The 2022 earthquake coincidentally occurred near the dates of the first anniversaries of the February 2021 Fukushima and March 2021 Miyagi earthquakes besides being just five days after the eleventh anniversary of the aforementioned megaquake. An analysis of the February 2021 earthquake suggest reverse faulting within the downgoing Pacific plate beneath the subduction interface. Japanese scientists said that the quake ruptured along a 45 km-long, north–south striking fault that dips towards the east.
Earthquake
According to the JMA, the earthquake had a magnitude of 7.4 and struck at a depth of 57 km. The USGS stated that the quake had an of 7.3 at a depth of 41.0 km. It was preceded by a 6.0 foreshock at a depth of 48.1 km. The foreshock also measured 6.1 by the JMA and was recorded with a maximum intensity of Shindo 5-.
This shallow-focus earthquake occurred as a result of reverse faulting at a depth of 57 or 63.1 km within the Pacific plate. The focal mechanism by the USGS suggest it occurred on a plane striking north northeast–south southwest. The USGS added that earthquakes of reverse mechanism with similar magnitudes are usually the result of rupture on a fault plane measuring 55 km × 30 km in size. A finite fault model by the USGS indicate the main rupture patch was an oval-shaped area situated at a depth of 50–70 km (40 × 20 km2). It produced a maximum slip of 1.9 meters.
A source process analysis of body waves indicate:
The mainshock ruptured an area measuring along the strike direction and along the dip direction. Most of the slip occurred near the hypocenter, initiation of the rupture. The maximum slip was . The rupture occurred over a period of 20 seconds.
Based on seismic inversion using strong motion data, the seismic moment was calculated to be 1.3×1020 Nm, corresponding to a moment magnitude of 7.4. The greatest slip occurred north of the hypocenter, measuring .
The earthquake may have been an aftershock of the disastrous 2011 earthquake. According to an official from the JMA, the quake had an epicenter very close to that of another earthquake in February 2021, and there may be a link between the two. Although the 2021 event was considered an aftershock, the large time span after 2011 has made determining it difficult. Determining whether the 2022 quake was an aftershock was also challenging as the earthquake had a different mechanism of faulting. The earthquake differs from the 2011 event in that the 2022 quake may have occurred within the subducting Pacific plate while the 2011 event occurred on the boundary between the Pacific and Okhotsk plates.
It is thought that earthquakes of magnitude 7.0–7.5 are expected to occur off the coast every 40 years, even after the 2011 earthquake aftershocks sequence have ended. It was estimated prior to the occurrence of the earthquake, that there was a 60–70% chance that a magnitude 7.0–7.5 earthquake could occur within the Pacific plate as it subducts beneath the Tōhoku region. After the 2011 earthquake, the chances of earthquakes of a similar origin have increased.
According to seismologists, the earthquake formed part of a sequence of three large events, including the April 2011 and February 2021 earthquakes. The epicenter of the 2022 earthquake occurred just 7 km to the southwest of the 2021 shock. While the 2021 shock ruptured in a southernly direction along a 45 km fault, the 2022 shock ruptured northwards along a fault twice as large. The two shocks have been categorized as a doublet earthquake due to their close location, 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. Coulomb stress transfer inferred from calculations showed that the gap off of the Central Miyagi coast has been increasingly stressed and is the likely source for a future rupture.
Positioning stations around northern Honshu found small crustal deformation following the earthquake. Observation points located at Ishinomaki and on the Oshika Peninsula were displaced 3 cm to the north. In the areas of Sendai and northern Fukushima, the crust moved 1 cm east, closer to the epicenter. Meanwhile, in southern the crust moved 1 cm southwest.
An earthquake light manifested in downtown Sendai, coinciding with a magnetic disturbance detected by a geomagnetic observatory in Kakioka, Ibaraki Prefecture. Researchers ruled out "the possibility of the flashes being caused by explosions in transformers or power supply facilities" by checking the maintenance reports of nearby power stations, none of which had malfunctioned within the field of view of two CCTV cameras that had recorded it.
Intensity The Japan Meteorological Agency stated that a maximum intensity of 6+ on the JMA seismic intensity scale was recorded in Miyagi and Fukushima prefectures, while the USGS reported a maximum Mercalli intensity of VIII (Severe). It was felt as far as the cities of Nanao and Aomori. The deep focal depth of the event, as well as its magnitude, resulted in strong shaking across the Tōhoku and Kantō regions. The earthquake was felt as far north as Hokkaido to the south in Kyushu with intensities of at least Shindo 1.
According to the National Research Institute for Earth Science and Disaster Prevention, a seismic intensity of close to Shindo 7 was recorded at Soma and Kunimi, Fukushima. Shindo 7 may have been recorded in Kunimi and Koori, both in Fukushima Prefecture. A peak ground acceleration of 1,233 gal was recorded at Kawasaki, Miyagi.
+Japan Meteorological Agency seismic intensity Shindo 5− and higherIntensityPrefectureLocation6+MiyagiTome, ZaōFukushimaSōma, Kunimi, Minamisōma6-MiyagiKurihara, Ōsaki, Wakuya, Natori, Iwanuma, Kakuda, Ishinomaki, Higashimatsushima, Kawasaki, Watari, OgawaraFukushimaNihonmatsu, Tamura, Fukushima, Ten-ei, Naraha, Tomioka, Ōkuma, Futaba, Namie, Shinchi, Date, Koori, Iitate5+MiyagiShikama, Kami District, Misato, Shiroishi, Murata, Shibata, Marumori, Aoba-ku, Miyagino-ku, Wakabayashi-ku, Taihaku-ku, Shiogama, Tagajo, Tomiya, Matsushima, Shichigahama, Rifu, Taiwa, Osato, Ohira, Onagawa FukushimaKōriyama, Shirakawa, Sukagawa, Motomiya, Kawamata, Ōtama, Kagamiishi, Izumizaki, Nakajima, Yabuki, Tanagura, Tamakawa, Asakawa, Furudono, Kawauchi, Katsurao, Iwaki, HironoIwateIchinoseki, Yahaba, ŌshūYamagataNakayama5-MiyagiKesennuma, Minamisanriku, ShichikashukuFukushimaYamatsuri, Ishikawa, Hirata, Miharu, OnoIwateMorioka, Hanamaki, Kitakami, Tōno, Kanegasaki, Hiraizumi, Fudai, Noda, Ōfunato, Kamaishi, SumitaAkitaYokote, DaisenYamagataKaminoyama, Tendō, Yamanobe, Kahoku, Sakata, Mogami, Yonezawa, Takahata, Kawanishi, ShiratakaIbarakiMito, Hitachi, Hitachiōta, Kitaibaraki, Kasama, Hitachinaka, Hitachiōmiya, Naka, Omitama, Ibaraki (town), Shirosato, Tōkai, Daigo, Tsuchiura, ChikuseiTochigiNasu, Ichikai, Takanezawa, NakagawaNiigataMinamiuonuma
Long period ground motion Class IV was assigned to northern Miyagi on the JMA long period ground motion scale. Long period ground motions are felt on upper floors of tall buildings where the period of seismic waves resonate with a building's natural resonance. The effect is large short frequency movements on the top floors of buildings. Class IV intensity ground motion would make standing impossible. Ground motion of this type would cause unsecured furniture to fall over and crack partition walls.
+JMA long period ground motion scaleClassPrefectureLocationIVMiyaginorthern regionIIIMiyagicentral and southern regionsFukushimaHamadōri, Nakadōri, AizuYamagataMurayama RegionIIAomoriNorthern Tsugaru RegionIwatenorthern coast, southern coast, northern inland regionsAkitanorthern inland regionYamagataShōnai Region, Mogami Region, Okitama RegionIbarakinorthern and southern regionTochigisouthern regionChibanortheastern and northwestern regionTokyo23 wards of TokyoKanagawaEastern KanagawaNiigataChūetsu, KaetsuYamanashiFuji Five Lakes, eastern regionNaganocentral regionShizuokaeastern regionIHokkaidoCentral Tokachi, eastern Oshima, southern Ishikari, eastern Shiribeshi, central Sorachi, central and eastern Iburi, central NemuroAomorisouthern Tsugaru, Sanpachi Kamikita, ShimokitaIwatesouthern coastAkitanorthern coastTochiginorthern regionGunmanorthern regionSaitamanorthern and southern region, ChichibuChibasouthern regionTokyoeastern TamaKanagawaeastern and western regionNiigataJoetsuNaganonorthern regionShizuokacentral and western regionAichiwestern region
Tsunami + Tsunami observations Location Arrival time (local) Recorded height (m) Iwaki, Fukushima March 17, 00:36 "small" Ayukawa, Ishinomaki, Miyagi March 17, 01:41 0.1 Sendai Port, Miyagi March 17, 01:46 0.2 Ishinomaki Port, Miyagi March 17, 02:14 0.3 Soma, Fukushima March 17, 03:15 0.2
A tsunami measuring 20 cm was recorded by the JMA at Ishinomaki Port, Miyagi at 00:29 local time. It was followed by another wave measuring 30 cm at 02:14. At Sendai Port, the tsunami measured 20 cm and arrived at 03:15. In Ōfunato, a tsunami of 0.4 ft was detected. The earthquake did not generate a large tsunami due to its deep depth, which caused little uplift of the seafloor and only generated small displacements of seawater.
Impact Across the affected region, 217 buildings were completely destroyed, 4,556 others were severely damaged and 52,162 others suffered minor damage, with the vast majority of them in Miyagi and Fukushima Prefectures, with damage also occurring in five other prefectures.
In Kunimi, a building collapsed, causing its roof to slide off and scatter roof tiles onto an adjacent empty lot. A vehicle was trapped underneath the fallen roof. A shrine in Minamisōma sustained significant damage and partially collapsed. Shelves in offices toppled over. The exterior walls of some homes partially collapsed. Broken windows were also reported. In Yabuki, the exterior walls and doors of a hotel were damaged.
A horse racing facility in Fukushima City suffered water leakage and damage to the walls. The Japan Racing Association stated that horse racing events in Fukushima City had to be cancelled as inspections were ongoing. The facility also suffered damage during the earthquake on February 13, 2021. The Inari Shrine in the city was also affected; three stone lanterns restored following last year's earthquake were destroyed. Shattered glass and broken bricks were reported in the city center. Minor damage including cracked tiles and floods occurred at Fukushima Station.