The number of felt earthquakes observed in Japan over the past week is higher than the previous week.
There was no particular area where earthquakes were concentrated, and earthquakes occurred in various areas from the Kuril Islands to Hokkaido, Kanto, Kyushu, and Okinawa. Earthquakes with a seismic intensity of 3 or higher have occurred four times. (Tally from December 25th to 31st at 10am)
Domestic: Occurrence along the Kuril Trench, where there is concern about a huge earthquake
Earthquake off the southeast coast of Etorofu Island
At around 18:15 on the 28th (Thursday), an earthquake estimated to have a magnitude of 6.6 and a depth of 30 km occurred with its epicenter off the southeast coast of Etorofu Island. As a result of this earthquake, shaking with a maximum seismic intensity of 3 was observed in the towns of Shinhidaka and Urakawa in Hokkaido, and tremors of intensity 2 on the Pacific side of Hokkaido were observed.A magnitude 6.2 earthquake occurred off the southeast coast of Etorofu Island in September this year, with a maximum seismic intensity of 2. The mechanism of the earthquake is analyzed to be a reverse fault type with a pressure axis in the northwest-southeast direction.
The Pacific plate is subducting into the land plate around the epicenter of this earthquake, and large earthquakes often occur. Similar to this time, there was an earthquake with a magnitude of 7.3 and a depth of 40 km in 1978 at a slightly deeper location, and an earthquake with a magnitude of 8.1 and a depth of 0 km in 1963 at a shallower location.
The government’s Earthquake Research Promotion Headquarters puts the probability of an earthquake of magnitude 7.7 to 8.5 occurring off the coast of Shikotan Island and Etorofu Island within the next 30 years at around 60%, and the probability of an earthquake around magnitude 7.5 occurring at around 90%. This is an area where there is concern about the occurrence of large earthquakes.
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: M4.0 earthquake with maximum seismic intensity 3 in southeastern Hyogo Prefecture
Earthquake in southeastern Hyogo Prefecture
At around 4:42 on the 25th (Monday), an earthquake estimated to have a magnitude of 4.0 and a depth of 7 km occurred with its epicenter in the southeastern part of Hyogo Prefecture. This earthquake had a maximum seismic intensity of 3 in Tanba Sasayama City, Hyogo Prefecture. Earthquake magnitude 2 was observed in Sanda City, Hyogo Prefecture, Nakagyo Ward, Kyoto City, Kyoto Prefecture, Kameoka City, and Takahama Town, Fukui Prefecture.This is the first time in 1 year and 3 months since last September that there has been an earthquake with a seismic intensity of 3 or higher in the southeastern part of Hyogo Prefecture. The epicenter of this earthquake was in an area near the Habu Fault, which stretches from Nantan City, Hyogo Prefecture to Kameoka City, Kyoto Prefecture. If the Hanyu Fault were to activate, it is expected that the surrounding areas would experience tremors of up to 6 on the seismic intensity scale.
World: M5.3 fault activity around Lake Baikal in Russia?
Earthquakes of M4.5 or higher around the world (USGS homepage quote/Weather News processing)
According to analysis by the U.S. Geological Survey, two earthquakes with a magnitude of 6 or higher have occurred. The largest earthquake was a magnitude 6.5 earthquake that occurred off the coast of Etorofu Island. (Magnitude 6.6 according to the Japan Meteorological Agency analysis)The earthquake we are focusing on is the one that occurred in Russia. On the afternoon of the 27th (Wednesday) Japan time, an earthquake estimated to have a magnitude of 5.3 and a depth of approximately 10 km occurred north of Lake Baikal in Russia. The magnitude of the earthquake was not that large, and there were no urban areas in the area, so there was no particular damage.
Lake Baikal is a type of lake called a “fault lake” that was formed by fault activity. In Japan, Lake Biwa and Lake Suwa are formed by similar factors.
Fault activity is still observed around Lake Baikal, and Russia is in an earthquake-prone area. Recently, in 2021, there was a magnitude 6.7 earthquake on the southwest side of Lake Baikal, and if you go back to ancient times, there is a record of a magnitude 7.4 earthquake in 1957.
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.
2023-12-31 01:28:00
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