Weekly Earthquake Information 2022.10.9 Two earthquakes with seismic intensity of 3 occurred on the Tohoku Pacific side – Weather News

2022/10/09 10:09 Weather news

The number of felt-earthquakes observed in Japan over the past week has slightly decreased compared to the previous week.

Earthquakes from the coast of Fukushima Prefecture to the coast of Miyagi Prefecture are conspicuous. Overall, there are not many earthquakes with epicenters on land. There were two earthquakes with a seismic intensity of 3 or higher. (Tally from October 3rd to October 9th at 10:00)

Japan: Seismic intensity 3 in the early morning off Miyagi Prefecture and off Fukushima Prefecture every day

Miyagi offshore earthquake

At around 4:04 on the 6th (Thursday), an earthquake estimated to have a magnitude of 4.5 and a depth of 57 km occurred off the coast of Miyagi prefecture. A maximum seismic intensity of 3 was observed in Kesennuma City and Ishinomaki City in Miyagi Prefecture, Kitakami City in Iwate Prefecture, and Ichinoseki City in Iwate Prefecture.

It was the first time in regarding three months since July 6 that an earthquake with a seismic intensity of 3 or higher centered off the coast of Miyagi Prefecture. The mechanism of the earthquake is analyzed as a reverse fault type with a pressure axis in the northwest-southeast direction.

At around 3:14 on the following day, Friday, July 7, an earthquake with a magnitude of 4.4 and an estimated depth of regarding 30 km occurred off the coast of Fukushima Prefecture. In this earthquake, a maximum seismic intensity of 3 was observed in Tamura City, Iwaki City, and Naraha Town in Fukushima Prefecture.

This is the first earthquake with a seismic intensity of 3 or higher with an epicenter off the coast of Fukushima Prefecture since August 18. The earthquake mechanism is analyzed as a normal fault type with a tension axis in the north-northwest-southeast direction.

Although there were many earthquakes from the coast of Miyagi prefecture to the coast of Fukushima prefecture this spring, activity has calmed down a little since summer. Both of these earthquakes are magnitude 4 class, so they are not large-scale earthquakes, so they are not considered to be particularly active.

However, the Tohoku Pacific Ocean side is an area where a large earthquake might occur at any time, so it is essential to take countermeasures on a daily basis.

Occurs near plate boundaries?

Epicenter distribution off the east coast of Osumi Peninsula

Similar to Hyuga-nada, the area near the epicenter of this earthquake is where the Philippine Sea plate is subducting into the Eurasian plate. Although it is out of the hypothetical focal region of the so-called Nankai Trough megathrust earthquake, the same mechanism is used to generate earthquakes.

This is the first time since 1919, when the Japan Meteorological Agency has detailed statistics, that an earthquake with an epicenter off the east coast of the Osumi Peninsula recorded a seismic intensity of 5 Lower or higher. However, due to the fact that there were few observation points in the past, earthquakes with a magnitude of 6 or more, which are larger than this time, often occur.

In 1948, an earthquake with a magnitude of 6.4 occurred near this epicenter, and a seismic intensity of 4 was observed in Miyazaki City and Kumamoto City.

Looking at the distribution of past hypocenters, in this region, earthquakes were concentrated at a depth of 20 to 30 km, which is the plate boundary, and deeper.

Japan: A series of earthquakes with a seismic intensity of 1 occur in Tachibana Bay, Nagasaki Prefecture

Tachibana Bay earthquake

At around 16:10 on the 3rd (Monday), an earthquake estimated to have a magnitude of 2.0 and a depth of 13km occurred with an epicenter in Tachibana Bay. In this earthquake, a seismic intensity of 1 was observed in Unzen City and Minamishimabara City, Nagasaki Prefecture.

After that, a magnitude 2.3 earthquake occurred at around 18:32 on the 3rd (Monday) and a magnitude 2.2 earthquake occurred on the 4th (Tuesday), both of which had a maximum seismic intensity of 1.

Earthquakes related to the volcanic activity of Mt. Unzen have occurred in Tachibana Bay. In addition, the Unzen fault group is distributed from Shimabara Peninsula to Tachibana Bay, and earthquakes unrelated to volcanic activity have also occurred.

Recently, an earthquake with a magnitude of 4.3 and a maximum seismic intensity of 4 occurred in June 2017, and according to old records, a large earthquake with a magnitude of 6.9 occurred in 1922, causing great damage.

There have been earthquakes as large as M6.0 in the past.

Shallow earthquakes associated with volcanic activity are well known when it comes to earthquakes in the Izu Islands, such as Izu Oshima and Miyakejima. Looking at the hypocenter distribution of the earthquakes that actually occurred around the Izu Islands, most of them occurred in the area from the northwest of Izu Oshima to the Izu Peninsula, and in the area from Miyakejima to Kozushima.

Most of the earthquakes are concentrated at a depth of around 10 km even in the depth of the east-west cross section. There are few occurrences at depths of 20 km or more, but there are a few at depths of 100 km or more. Earthquakes that occur near the boundary between the Pacific plate and the Eurasian plate or within the Pacific plate.

Most deep earthquakes are around magnitude 4, but in May 2014, a large earthquake with a magnitude of 6.0 occurred. In this earthquake, a maximum seismic intensity of 5 lower was observed in Chiyoda Ward, Tokyo, and a seismic intensity of 3 to 4 was observed in a wide area of ​​the Kanto region, causing damage such as injuries.

World: M5.8 earthquake in northern Peru

Earthquakes of M4.5 or higher around the world (quoted from USGS website/processed by Weathernews)

There have been no earthquakes of magnitude 6 or higher according to analysis by the United States Geological Survey. The largest quake of magnitude 5.8 occurred in South America.

On the evening of the 5th (Wednesday) Japan time, an earthquake estimated to have a magnitude of 5.8 and a depth of regarding 48 km occurred with an epicenter in the northern part of Peru in South America. The mechanism of the earthquake is analyzed as a reverse fault type with a pressure axis in the east-northeast-west-southwest direction.

Peru is located on the plate boundary where the Nazca Plate is subducting the South American Plate, and is an area with many large earthquakes.

From northern Peru to Ecuador, where this earthquake occurred, shallow coastal earthquakes along the plate boundary and deep inland earthquakes occur frequently. A large earthquake of magnitude 8.0 occurred.

Magnitude 6.5 earthquake hits southern Taiwan

An earthquake estimated to have a magnitude of 6.5 and a depth of regarding 10 km occurred with an epicenter in southern Taiwan on the night of the 17th (Saturday) Japan time. According to the Taiwan Meteorological Bureau, the earthquake had a maximum seismic intensity of upper 6. (Taiwan’s seismic intensity scale is almost the same as Japan’s)

The earthquake mechanism is analyzed as strike-slip type.

Aftershocks followed one following another, and an earthquake of magnitude 5.5 occurred regarding an hour following the first one. There are concerns regarding damage due to strong tremors around the epicenter.

Reference materials, etc.

*Information on earthquake sources and seismic intensity in Japan is from the Japan Meteorological Agency unless otherwise specified. Overseas epicenter information is from the United States Geological Survey (USGS) unless otherwise specified. There may be differences in hypocenter information depending on the publishing organization.