The number of felt earthquakes observed in Japan over the past week is significantly higher than the previous week due to the earthquake swarm in the Tokara Islands. Outside of the Tokara Islands, earthquakes were noticeable in inland areas of eastern Japan and the Pacific side of Tohoku.
Earthquakes with a seismic intensity of 3 or higher have occurred 25 times. (Tally from September 4th to September 10th at 10am)
Domestic: Earthquake swarm in Tokara Islands
Seismic activity in the Tokara Islands
Since early morning on the 8th (Friday), a swarm of earthquakes has been occurring with epicenters in the waters near the Tokara Islands in the Amami region. At around 2:28 on the 9th (Saturday), an earthquake estimated to have a magnitude of 5.0 and a depth of approximately 20km occurred, with a maximum seismic intensity of 4 observed in Toshima Village and Akusekijima.There were over 200 felt earthquakes with a seismic intensity of 1 or higher in just the two days from the 8th (Friday) to the 9th (Saturday), of which 23 earthquakes had a maximum seismic intensity of 3 or higher. (The number of earthquakes is a preliminary figure)
Looking at the scale of the earthquakes that occurred in chronological order, on the 8th (Friday), there were relatively many earthquakes with a magnitude of 2 to 3, and on the 9th (Saturday), earthquakes with a magnitude of 3 or higher became noticeable. Two magnitude 5.0 storms occurred in the early morning hours. The frequency of earthquakes decreased after the afternoon of the 9th (Saturday), and the number of felt earthquakes on the 10th (Sunday) remained at 8 by 10:00.
Earthquake swarms in the Tokara Islands often reach their peak soon after the activity begins. In May of this year, activity began to increase from May 11th, and two days later, on the 13th, an earthquake with a magnitude of 5.1 and a maximum seismic intensity of just under 5 occurred. In December 2021, the earthquake became active from December 4th, and five days later, on the 9th, an earthquake with a magnitude of 6.1 and a maximum seismic intensity of 5+ occurred.
At least for the next few days, we need to be careful about shaking with a seismic intensity of 4 to 5 lower.
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: Earthquake off the coast of Miyagi Prefecture with a seismic intensity of 4 since March
Earthquake off the coast of Miyagi Prefecture
At around 18:28 on the 8th (Friday), an earthquake estimated to have a magnitude of 5.4 and a depth of 46km occurred off the coast of Miyagi Prefecture.As a result of this earthquake, tremors with a maximum seismic intensity of 4 were observed in Kesennuma City, Miyagi Prefecture, Hanamaki City, and Ishinomaki City, Iwate Prefecture, and tremors with a seismic intensity 3 on a wide area in Miyagi Prefecture and Iwate Prefecture. The mechanism of the earthquake is analyzed to be a reverse fault type with a pressure axis running from west-northwest to east-southeast. This is the first time since March 27 of this year that an earthquake with a seismic intensity of 4 or higher was observed off the coast of Miyagi Prefecture.
This earthquake appears to have occurred near the depth of the boundary where the Pacific plate subducts into the land plate.
This earthquake occurred on the north side of the epicenter, which is called “Oki, Miyagi Prefecture.” In 2021, an earthquake with a magnitude of 6.9 and a maximum seismic intensity of 5+ occurred slightly southwest of the epicenter.
Domestic: Suonada was the epicenter of an earthquake with an intensity of 3 or higher for the first time in 9 years
At around 22:29 on the 26th (Saturday), an earthquake estimated to have a magnitude of 4.6 and a depth of approximately 80 km occurred with the epicenter in the Suonada Sea. As a result of this earthquake, a maximum seismic intensity of 3 was observed in Otake City, Etajima City, Hiroshima Prefecture, Shimonoseki City, Kaminoseki Town, and Kudamatsu City, Yamaguchi Prefecture.
This is the first time in nine years since August 2014 that an earthquake with a seismic intensity of 3 or higher occurred in the Suonada Sea.
Beneath the Suonada Sea, the Philippine Sea Plate is deeply subducting into the land plate. The depth of this earthquake’s epicenter is thought to be relatively close to the plate boundary.
Additionally, the Suonada Fault Zone is known, and the government’s Headquarters for Earthquake Research Promotion estimates that the probability of a magnitude 7.6 earthquake occurring within the next 30 years is 2-4%. The probability of an earthquake occurring due to fault activity is high.
World: M6.8 strong tremor causes major damage in Morocco
Earthquakes of M4.5 or higher around the world (USGS homepage quote/Weather News processing)
According to analysis by the U.S. Geological Survey, there have been four earthquakes with a magnitude of 6 or higher. The largest earthquake occurred in Morocco with a magnitude of 6.8.Early in the morning on the 9th (Saturday) Japan time, an earthquake estimated to have a magnitude of 6.8 and a depth of approximately 19 km occurred with its epicenter in Morocco, Africa. The mechanism of the earthquake is a reverse fault type with a pressure axis in the north-south direction, and is analyzed to include a slight strike-slip component.
It was a large earthquake that occurred in a shallow inland area, and the area around the epicenter is thought to have experienced tremors of VIII on the revised Mercalli seismic intensity scale. Although a strict comparison cannot be made, the shaking corresponds to a seismic intensity of lower 6 on the Japan Meteorological Agency’s seismic intensity scale.
The largest earthquake in the area since 1900
Situation in Morocco (AFP=current affairs)
Marrakech, one of the major cities, was also hit by strong tremors, causing many buildings to collapse. It is reported that over 1,000 people have died.Most of Morocco’s strong earthquakes occur in the north, which is the boundary between the Eurasian and African plates, and this earthquake is thought to have occurred in a fault zone distributed in the Atlas Mountains. According to data from the U.S. Geological Survey, no earthquake of magnitude 6 or greater has occurred in the vicinity of this epicenter since 1900.
In 1960, an earthquake with a magnitude of 5.9 that occurred along the coast was called the “Agadir Earthquake” and caused severe damage.
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.
2023-09-10 01:25:00
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