Severe off-shore earthquake near Fukushima, Japan
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 30‑cm 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:
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.
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.
Tsunami.
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.
Part of a stone wall of the Aoba Castle in Sendai's Aoba Ward collapsed. The large statue of Date Masamune at the same site was also damaged, with the statue being tilted to one side as a result. The "White Cube", a concert hall and sports center in Shiroishi suffered a partial collapse of its ceiling. Shattered glass and ceiling debris was scattered across the seats in the hall. The water supply system in the hall was also damaged.
In Ichikawa, Chiba, a house caught fire immediately after power was restored. It took approximately four hours for the 14 fire trucks involved to extinguish the flames.
In an agricultural area in Yamagata City, Yamagata, a large drainage pipe was damaged and flooded, causing a section of road above it to collapse. A large sinkhole formed in place of the road, flooded with water. Nearby commercial and agricultural facilities had slight damage. At least 47 homes temporarily lost access to water. In Higashine, a shrine collapsed.
The Ministry of Land, Infrastructure, Transport and Tourism reported that there were 25 cases of people trapped in elevators in the Greater Tokyo Area. There were three reports of people trapped in elevators across Fukushima. Another nine instances of people trapped in elevators was reported in Chiba, Kanagawa, Ibaraki and Saitama prefectures.
Schools.
At least 344 schools across eleven prefectures sustained minor damage to walls and ceilings. In the prefectures of Fukushima, Miyagi, Iwate, and Yamagata, 476 schools had closed. Meanwhile, in six prefectures, lesson hours in 15 schools were reduced. Many schools in Fukushima had cracked walls and broken windows. Only a few schools had ruptured water pipes, water interruptions or collapsed ceilings.
Commercial business and industrial sector.
A number of factories suspended operations following the event. A Sapporo Breweries factory in Natori suspended beer production after damage to machines was reported. The warehouse in the factory complex also suffered a collapse, without any casualties. The company stated that shipments would experience delays due to the damage. The Kirin Company factory in Sendai would also suspend beverage production to allow inspections of equipment. In Motomiya, the Asahi Breweries also stopped production at its factory.
Four factories of Murata Manufacturing located in Fukushima and Miyagi were going through repair works on March 17. At the Tome factory, a fire caused some damage. Instances of water bursting from pipes caused significant damage at the company's Sendai, Honmiya and Koriyama factories. Three factories of Sony; two in Miyagi and one in Yamagata were closed for inspections. Only minor damage was found and operations were expected to resume soon. Productions at other factories belonging to companies including Toyota, Nissan, Kioxia, and Renesas Electronics were also impacted. One of two fires reported in Saitama occurred at Iwatsuki Ward involving a scrap facility. The fire was thought to have been caused by a battery igniting. The Onagawa fish market experienced a small switchboard fire in a room, injuring one person. It is believed the switchboard may have came into contact with water leaking from ceiling pipes and started the fire.
Extensive material damage was done to store items and storage warehouses in Miyagi, Fukushima and Niigata. Delivery bases were temporarily evacuated due to the tsunami warnings and some goods were damaged. The closure of major highways delayed the delivery of items. An audio store in Sendai was flooded by the sprinklers activating. Other stores suffered caved-in ceilings and products falling off shelves. Due to safety concerns, stores conducted business at their entrances instead of allowing customers to enter.
Power outages.
Major power outages occurred across the Tōhoku and Kantō regions. An estimated 2.2 million households from 13 prefectures and one metropolitan area were left without power. TEPCO reported 300,000 power outages as a result. An estimated 120,000 instances of power outages occurred in Tokyo, 60,000 in Kanagawa, and 50,000 in Chiba prefectures. Meanwhile, Tohoku Electric Power Co., Inc. (TEP) stated that approximately 153,200 power outages occurred, with 90,000 in Fukushima and 50,000 in Miyagi prefectures.
TEPCO was able to restore power to most households before sunrise