The number of felt earthquakes observed in Japan over the past week is about the same as the previous week.
Earthquakes are noticeable on land from the Kanto region to the Kinki region, and earthquakes have also occurred in the Sakishima Islands. One earthquake with a seismic intensity of 3 or higher occurred. (Tally from October 23rd to 29th at 10am)
Domestic: Abnormal seismic area due to deep earthquake in western Kanagawa Prefecture
Earthquake in western Kanagawa Prefecture
At around 21:29 on the 24th (Tuesday), an earthquake estimated to have a magnitude of 4.2 and a depth of 167 km occurred with its epicenter in western Kanagawa Prefecture. As a result of this earthquake, a maximum seismic intensity of 2 was observed in Atsugi City, Kanagawa Prefecture, Chiyoda Ward and Nakano Ward, Tokyo, Utsunomiya City, Tochigi Prefecture, and Koga City, Ibaraki Prefecture.This is the first deep earthquake with a depth of more than 100km centered in western Kanagawa Prefecture since May of this year.
Although the epicenter of this earthquake was in western Kanagawa Prefecture, there was only one location in Kanagawa Prefecture where a seismic intensity of 2 was observed, while there were two locations in Tochigi Prefecture, a little further away. No felt earthquakes have been observed in Shizuoka Prefecture. This is a seismic intensity distribution called the “abnormal seismic area.”
In many earthquakes, areas with strong shaking are distributed concentrically around the epicenter. In deep earthquakes, strong shaking is transmitted along the subducting plate, and a phenomenon in which the shaking becomes stronger in the distance close to the plate boundary is sometimes seen, and this is called an “abnormal seismic area.”
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.
Domestic: M5.9 maximum seismic intensity 3 near Yonaguni Island
Earthquake near Yonaguni Island
At around 8:05 a.m. on the 24th (Tuesday), an earthquake estimated to have a magnitude of 5.9 and a depth of 33 km occurred with its epicenter near Yonaguni Island. Due to this earthquake, a maximum seismic intensity of 3 was observed in Yonaguni Town, Okinawa Prefecture, and a seismic intensity of 2 was observed in Ishigaki City and Taketomi Town.This is the first time since April of this year that an earthquake with a seismic intensity of 3 or higher has been observed near Yonaguni Island, and the mechanism of the earthquake is analyzed to be a reverse fault type with a tension axis running from northwest to southeast.
The area near the epicenter of this earthquake is an area where the Philippine Sea plate is subducting into the land plate. The boundary is thought to be about 30 to 40 km, and this earthquake appears to have occurred at or near the boundary.
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: M5.4 earthquake in Congo, Africa
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 was a magnitude 6.0 earthquake near the Kermadec Islands in the north of New Zealand.Early in the morning on the 27th (Friday) Japan time, an earthquake estimated to have a magnitude of 5.4 and a depth of approximately 15 km occurred with its epicenter in southeastern Congo, Africa. The mechanism of the earthquake is analyzed to be a normal fault type with a tension axis running northwest-southeast.
The area to the east of the Congo is located in the Great Rift Valley, so earthquakes exceeding magnitude 5 often occur there. Although the epicenter of this earthquake is a little to the west of the area where most earthquakes occur, there was a magnitude 5.0 earthquake in January last year, and a magnitude 5.8 earthquake in 1983.
Additionally, in 2005, a large earthquake with a magnitude of 6.8 occurred across the border in Tanzania.
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.
2023-10-29 01:42:00
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