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, three people died and 247 were injured.
Tectonic setting
The Pacific Plate, made of oceanic lithosphere, subducts beneath the Okhotsk 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 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. 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.