The number of felt earthquakes observed in Japan over the past week is about the same as the previous week.
There is no particular place where earthquakes are concentrated, and the epicenters are evenly distributed from Hokkaido to Okinawa. Two earthquakes with a seismic intensity of 3 or higher occurred. (Tally from October 16th to 22nd at 10am)
Domestic: M6.0 maximum seismic intensity 4 near Miyakojima
Earthquake near Miyakojima
At around 7:42pm on Monday the 16th, an earthquake estimated to have a magnitude of 6.0 and a depth of 33km occurred with its epicenter near Miyakojima. In this earthquake, a maximum seismic intensity of 4 was observed in Miyakojima City, Okinawa Prefecture, and a large area of the Sakishima Islands and parts of the main island of Okinawa were affected by shaking with an intensity of 1 or higher.This is the first time since December 2021 that an earthquake with a magnitude of 4 was observed near Miyako Island, and the mechanism of the earthquake is analyzed to be a normal fault type with a tension axis in the northeast-southwest direction.
The earthquake in December 2021 had almost the same epicenter as this earthquake, the scale was the same at magnitude 6.1, and the mechanism was the same type.
The Philippine Sea Plate is subducting in the Nansei Islands, but the epicenter of this earthquake occurred shallower than the plate boundary, and is thought to be an activity within the land plate.
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 subducted 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.
Domestic: Earthquake with maximum seismic intensity 3 in Tsugaru Strait
Tsugaru Strait Earthquake
At around 9:47 a.m. on the 19th (Thursday), an earthquake estimated to have a magnitude of 2.9 and a depth of 11 km occurred with the epicenter in the Tsugaru Strait. This earthquake was observed in Hakodate City, Hokkaido, with a maximum seismic intensity of 3.This is the first time in 15 years since 2008 that an earthquake with a seismic intensity of 3 or higher was observed with an epicenter in the Tsugaru Strait, and the first time in 41 years since 1982 that an earthquake with a shallow epicenter like this one occurred.
Earthquakes with a magnitude of around 3 occur occasionally in the Tsugaru Strait, and tremors with a magnitude of 1 to 2 are observed several times a year. In the past, magnitude 5 class earthquakes have occurred, and it is possible that earthquakes with magnitude 4 or higher may be involved.
Domestic: Abnormal seismic area for earthquake off the west coast of Aomori Prefecture
At around 19:00 on the 13th (Friday), an earthquake estimated to have a magnitude of 5.0 and a depth of 171 km occurred with its epicenter off the western coast of Aomori Prefecture. As a result of this earthquake, a maximum seismic intensity of 2 was observed in Samani Town and Urahoro Town in Hokkaido, Hachinohe City in Aomori Prefecture, Miyako City in Iwate Prefecture, and Kuji City in Iwate Prefecture.
Although the epicenter was in an area near the Sea of Japan, most of the shaking with a seismic intensity of 1 or higher was observed on the Pacific side. Like the Kunashiri Island earthquake mentioned above, the epicenter was deep, making it an “abnormal seismic area.”
On the Sea of Japan side, the Pacific plate is deeply subducting, and large deep earthquakes often occur. In 2009, there was an earthquake with a magnitude of 5.4 near the epicenter of this earthquake, and a maximum seismic intensity of 3 was also observed on the Pacific side.
In deep earthquakes, even if the shaking is small near the epicenter, strong shaking can be transmitted far away, so care must be taken.
World: M6.4 earthquake in Aleutian Islands
Earthquakes of M4.5 or higher around the world (USGS homepage quote/Weather News processing)
According to analysis by the U.S. Geological Survey, one earthquake with a magnitude of 6 or higher has occurred. The largest earthquake occurred in the Aleutian Islands with a magnitude of 6.4.On the night of the 16th (Monday) Japan time, an earthquake estimated to have a magnitude of 6.4 and a depth of approximately 187 km occurred with its epicenter near the Aleutian Islands.
Because the epicenter was deep, so-called a deep earthquake, the magnitude of the earthquake was somewhat large, but no tsunami was generated.
In the Aleutian Islands, the Pacific plate is subducting under the North American plate, and large earthquakes often occur. Earthquakes exceeding magnitude 7 often have relatively shallow epicenters, and most deep earthquakes like this one are magnitude 6 class.
World: Rare M5 earthquake in Slovakia
In the early morning hours of the 10th (Tuesday, Japan time), an earthquake estimated to have a magnitude of 5.0 and a depth of approximately 8 km occurred with its epicenter in eastern Slovakia. The mechanism of the earthquake is analyzed to be a reverse fault type, with the pressure axis running in the east-west direction.
Slovakia is on the Eurasian plate, so large earthquakes do not occur often. There are only a handful of earthquakes with a magnitude of around 5 similar to this one.
According to records from the U.S. Geological Survey, this is the first time since 1930 that an earthquake of magnitude 5 or higher has occurred in Slovakia. It was the largest earthquake in eastern Slovakia since 1900.
Reference materials etc.
*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.
2023-10-22 02:00:00
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