Weekly Earthquake Information 2022.2.27 Deep-focus earthquake in Enshu-nada on Monday, 21st Shake in distant Kanto and Tohoku-Weathernews

2022/02/27 09:41 Weathernews

The number of earthquakes observed in Japan in the past week has increased slightly compared to the previous week.

Earthquakes on the Pacific side from Kanto to northern Japan and around Okinawa are conspicuous. There are two earthquakes with a seismic intensity of 3 or higher during this period. (Aggregation from February 21st to 27th at 10:00)

Domestic: Earthquake at a depth of 350km in Enshu-nada Epicenter distribution in anomalous seismic area

Enshu Nada Earthquake

Around 7:12 on the 21st (Monday), there was an earthquake with a magnitude of 5.0 and an estimated depth of 356km with the epicenter in Enshu-nada. It is a so-called deep-focus earthquake with a very deep epicenter, and a seismic intensity distribution called “abnormal seismic area” is seen.

In the Tokai region near the epicenter, there were no tremors with a seismic intensity of 1 or higher, but there were many places where tremors were observed in the northeastern Kanto region and Fukushima prefecture, which are far from the epicenter. The maximum seismic intensity of 2 was in Namie Town, Fukushima Prefecture and Utsunomiya City, Tochigi Prefecture.

It is a phenomenon that occurs when shaking is transmitted to a distant area along the Pacific plate where seismic waves are easily transmitted in the epicenter distribution called anomalous seismic area. Although named “abnormal,” it is often seen during deep-focus earthquakes.

* The epicenter was off the southeastern coast of Mie prefecture in the preliminary figures, but it has been changed to Enshu-nada in the provisional values.

Many deep-focus earthquakes have occurred in the past

Deep-focus earthquake off the Kii Peninsula

Off the coast of the Kii Peninsula, many deep-focus earthquakes have occurred inside the subducting Pacific plate in the past. Recently, there was a major earthquake with a magnitude of 6.6 in July 2019 and a magnitude of 7.0 in January 1984. The same earthquake that originated in Enshu-nada occurred in October 1952.

The assumed “Nankai Trough giant earthquake” is an earthquake that occurs near the boundary where the Philippine Sea plate subducts into the Eurasian plate, and the depth of the epicenter is said to be about 10 to 40 km. The mechanism of this earthquake is a different factor from the Nankai Trough giant earthquake.

Stay alert to the tendency for activities to subside

Seismic activity in the waters near the Tokara Islands became active around noon on the 4th (Sat). The number of felt earthquakes with a seismic intensity of 1 or higher has reached 277 by 24:00 on the 10th (Friday).

Looking at the number of earthquakes by day, the number of earthquakes on the 5th (Sun) is the highest at 87. On the 9th (Thursday), when an earthquake with a seismic intensity of 5 or higher occurred, there were 36 times, which was a little high, but on the 10th (Friday), it was only 5 times, and the activity as a whole tends to be settled.

However, during the activity in October 2000, when the same tremor with a seismic intensity of 5 or higher occurred, an earthquake with a seismic intensity of 4 occurred about 3 weeks after the active state settled down. You need to be careful about strong shaking for a while.

Domestic: Earthquake swarm from Saturday, 4th in the Tokara Islands

Earthquakes have occurred frequently in the sea near the Tokara Islands in the Amami region of Kagoshima prefecture from around noon on the 4th (Sat). The first felt earthquake had a magnitude of 2.9 and a maximum seismic intensity of 1 at around 12:32 on Saturday, 4th, and an earthquake of magnitude 4.0 and a maximum seismic intensity of 3 occurred at 19:48. After that, the earthquakes continued, and as of 10 o’clock on the 5th (Sun), there were 87 felt earthquakes, of which 6 were earthquakes with a seismic intensity of 3.

These earthquake swarms often occur in the waters near the Tokara Islands. Recently, earthquakes have occurred frequently since April 9th ​​this year, and on April 10th, more than 100 felt earthquakes occurred in one day. During the three days from the 10th to the 12th, when the activity was at its peak, we observed 229 felt earthquakes, 15 seismic intensity 3 and 5 seismic intensity 4.

In many cases, the active situation from past activities lasts for several days, and the maximum seismic intensity may reach a little over 4-5. It is necessary to be careful about strong shaking for a while.

World: M6.2 human damage in Indonesia

World M4.5 or higher earthquake (USGS homepage quote / Weathernews processing)

Three earthquakes with a magnitude of 6 or greater have occurred, as analyzed by the United States Geological Survey. The largest earthquakes are magnitude 6.2 near Antarctica and Indonesia.

An earthquake with a magnitude of 6.2 and an estimated depth of about 12 km occurred on Sumatra Island, Indonesia on the morning of the 25th (Friday) of Japan time. The mechanism of the earthquake is analyzed as a strike-slip type.

In the vicinity of the epicenter, the revised Mercalli Intensity Class VI-VII was hit by strong shaking, causing damage such as the collapse of the building. According to the Indonesian National Disaster Management Agency, eight people were killed and nearly 100 were injured in the earthquake.

Damage caused by a huge earthquake at the plate boundary is conspicuous around Sumatra Island, but inland earthquakes like this one sometimes occur. In the immediate vicinity of this epicenter, there was a magnitude 6.6 earthquake in 1977. Most of the earthquakes are not so large with a magnitude of 6 units, but since the epicenter is shallow, the shaking around them is large, which leads to damage.

M5.0 earthquake in the middle of Africa

An earthquake with a magnitude of 5.0 and an estimated depth of about 10 km occurred in Congo, Africa, before noon on Wednesday, 26th Japan time. The epicenter of this earthquake is on the west side of the area called the “Great Rift Valley”.

Most African earthquakes occur around the Great Rift Valley. The African Continent is made up of African plates, and the Great Rift Valley is thought to be the area where the plates are being torn. In its activities, it has caused earthquakes, and in the past, magnitude 7 class earthquakes have also occurred.

Reference materials, etc.

* Information on the epicenter and seismic intensity in Japan is from the Japan Meteorological Agency unless otherwise stated. Information on overseas epicenters is from the United States Geological Survey (USGS) unless otherwise stated. The epicenter information may differ depending on the issuing organization.