The number of felt earthquakes observed in Japan over the past week is about the same as the previous week.
Earthquakes were noticeable in the land area of Kanto and the surrounding sea areas, and there were also sporadic earthquakes in Kyushu. There has been one earthquake with an intensity of 3 or higher. (Tally from December 18th to 24th at 10am)
Domestic: Four felt earthquakes occur off the eastern coast of Chiba Prefecture
Earthquake off the eastern coast of Chiba Prefecture
At around 20:29 on the 21st (Thursday), an earthquake estimated to have a magnitude of 4.8 and a depth of 11 km occurred with its epicenter off the eastern coast of Chiba Prefecture. As a result of this earthquake, a maximum seismic intensity of 2 was observed in Choshi City, Asahi City, and Katori City, Chiba Prefecture.In addition, at around 23:04 on the 21st (Thursday), an earthquake with a magnitude of 5.4 and a depth of 3 km occurred, and shaking with a maximum seismic intensity of 2 was observed over a wider area. When the date changed, there was a magnitude 4.8 earthquake at around 1:12 on Friday the 22nd, and a magnitude 5.1 earthquake at around 5:45, both of which had a maximum seismic intensity of 2.
The mechanism of the earthquake has been analyzed as a reverse fault type with a pressure axis running north-northwest to south-southeast, and is thought to be a series of seismic activities.
What is an abnormal seismic area?
This earthquake appears to be a deep earthquake that occurred in the Pacific Plate, which has subducted very deeply underground.
In an earthquake with a shallow epicenter, areas with strong shaking are distributed concentrically from the epicenter, but in deep earthquakes, strong shaking is transmitted along the subducting plate, and the shaking becomes stronger in the distance near the plate boundary. This is called an “abnormal seismic area.” Even if the shaking is small near the epicenter, strong shaking can be transmitted far away, so caution is required.
The possibility of a tsunami is small in a deep earthquake, and generally speaking, if the epicenter is deeper than 100km, there is almost no risk of a tsunami.
A deep earthquake of magnitude 6, like this one, occurs in the Sea of Japan about once every few years. Most recently, a magnitude 6.1 earthquake occurred in September 2021. Compared to this time, it occurred closer to Honshu, so a seismic intensity of 3 was observed over a wide area on the Pacific side from Hokkaido to Kanto.
Near the boundary between the Philippine Sea plate and the land plate
Earthquake off the eastern coast of Chiba Prefecture
The area around the epicenter of this earthquake, located at a depth of about 10 km, is an area where there is a boundary between the subducting Philippine Sea plate and the land plate, which became active during the super-massive earthquake that caused the Great East Japan Earthquake in 2011. On March 14, 2011, an earthquake with a magnitude of 5.7 occurred.Looking at the area off the east coast of Chiba Prefecture as a whole, there are many earthquakes near the boundary where the Pacific plate subducts, and in May of this year, a magnitude 6.2 earthquake occurred quite close to the land, with a maximum seismic intensity of just under 5.
If a large earthquake were to occur in this region, it would affect the heavily populated areas of southern Kanto, so caution is needed in activities.
World: M5.9 strong tremor causes major damage in China
Earthquakes of M4.5 or higher around the world (USGS homepage quote/Weather News processing)
According to an analysis by the U.S. Geological Survey, there have been three earthquakes with a magnitude of 6 or higher. The largest earthquake was a magnitude 6.2 earthquake that occurred in Peru, South America.The earthquake we are focusing on is the one in central China. In the early hours of the 19th (Tuesday) Japan time, an earthquake estimated to have a magnitude of 5.9 and a depth of approximately 10 km occurred in Gansu Province, China. The mechanism of the earthquake is analyzed to be a reverse fault type with a pressure axis running from west-southwest to east-northeast.
Because the epicenter was shallow, it is estimated that the land near the epicenter experienced strong shaking of up to VIII on the revised Mercalli seismic intensity scale. Although a strict comparison cannot be made, the tremor was equivalent to a seismic intensity of 5+ on the Japan Meteorological Agency’s seismic intensity scale.
Many houses were damaged by the shaking, and many people were injured.
Earthquakes exceeding magnitude 6 often occur in the inland areas of the Chugoku region. A magnitude 6.6 earthquake occurred in January last year in an area slightly north of this epicenter.
World: Tsunami of up to 40cm in Izu Islands and Hachijojima
The earthquake was large and the epicenter was relatively shallow, causing a tsunami. In the Philippines, a tidal level change of 8cm was observed in Davao, Mindanao. Because Davao is located deep in a narrow bay, it is possible that a larger tsunami could have reached coastal areas closer to the epicenter.
The tsunami reached Japan in the early hours of the 3rd (Sunday), with a maximum height of 40 cm on the Izu Islands and Hachijo Island, and 10 to 20 cm on the Pacific coast from Kanto to Kyushu and Amami. The Japan Meteorological Agency issued a tsunami advisory shortly after the earthquake occurred, and all advisories were lifted at 9:00 a.m. on the 3rd (Sunday).
Large earthquakes frequently occur around the Philippines due to the subduction of the Philippine Sea plate into the Eurasian plate. On the 17th of last month, there was a magnitude 6.7 earthquake in the south of Mindanao. Even if we restrict ourselves to areas near the epicenter of this earthquake, there were earthquakes of magnitude 7.0 in 1991 and magnitude 7.6 in 1989.
M5.5 earthquake in Bangladesh
In addition, around noon on the 2nd (Saturday) Japan time, an earthquake estimated to have a magnitude of 5.5 and a depth of approximately 39 km occurred with its epicenter in the southern part of Bangladesh. The mechanism of the earthquake is analyzed to be strike-slip type.
Because the epicenter was on land, the shaking was somewhat strong near the epicenter, and the shaking was equivalent to V on the revised Mercalli seismic intensity scale, or 4 on Japan’s seismic intensity scale, although a strict comparison is not possible. Although the epicenter was not far from the capital Dhaka, the area affected by the strong shaking was small and no major damage was reported.
Bangladesh is located near the boundary between the Indian plate and the Eurasian plate. Earthquakes with a magnitude of 6 or more occur from time to time, and the most recent one was a magnitude 6.0 earthquake in 1997. There is a very old record of a magnitude 7.1 event occurring in 1918.
Source/Reference
*Information on the epicenter and intensity in Japan is from the Japan Meteorological Agency unless otherwise specified. Information on overseas epicenters is from the United States Geological Survey (USGS) unless otherwise noted. There may be differences in the epicenter information depending on the publishing organization.
Reference materials etc.
2023-12-24 01:10:00
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