TOKYO - A magnitude 5.9 earthquake centered in southern Ibaraki Prefecture shook a wide area of the Kanto region at around 2 a.m. on August 23, registering a maximum intensity of lower 5 on Japan's seven-level seismic scale in parts of Ibaraki, Saitama, Chiba and Tokyo.
The earthquake was one of nine quakes registering an intensity of 3 or higher in Japan during the week through 10 a.m. on August 23. The overall number of perceptible earthquakes declined from the previous week, although seismic activity remained elevated in the area affected by the recent Kumamoto earthquake and earthquakes on land across Honshu were also prominent.
The August 23 earthquake had a focal depth of 68 kilometers. Lower 5 shaking was recorded in locations including Tsukuba and Toride in Ibaraki Prefecture, Kawaguchi and Toda in Saitama Prefecture, Urayasu in Chiba Prefecture and Tokyo's Adachi Ward.
Intensity 4 and 3 shaking was observed across a much broader area.
It was the first earthquake centered in southern Ibaraki to produce shaking of lower 5 or stronger since June 16.
The underground structure beneath the Kanto region is particularly complex. The Pacific Plate is subducting beneath Kanto from the east, while the Philippine Sea Plate is pushing beneath the region from the south.
The August 23 earthquake is believed to have occurred near the boundary between the Pacific and Philippine Sea plates.
The zone at depths of roughly 50 to 70 kilometers beneath southern Ibaraki and northwestern Chiba is known for frequent earthquakes and is sometimes described as an earthquake cluster because of the large number of tremors that occur there.
Earthquakes capable of producing intensity 5-level shaking are therefore not considered unusual in the area.
Aftershock activity following the August 23 quake remained limited. As of 1 p.m., no additional earthquakes strong enough to be felt had been recorded following the main shock at around 2 a.m.
The region nevertheless requires continued routine preparedness because of its complex tectonic setting and history of repeated earthquakes.
Elsewhere in Japan, a magnitude 4.2 earthquake struck the Fukuoka area of Fukuoka Prefecture at around 6:37 a.m. on August 17.
The quake occurred at a very shallow depth of 6 kilometers and registered an intensity of 4 in Itoshima, Fukuoka Prefecture.
It was the first earthquake centered in the Fukuoka area to produce shaking of intensity 4 or stronger since May 2005, a gap of about 21 years.
Several active faults are located near the epicenter, although any connection between them and the August 17 earthquake remains unclear.
The epicenter was some distance from the Kego Fault near Fukuoka City, which was associated with the major earthquake off western Fukuoka Prefecture in 2005, making a direct relationship with that fault unlikely.
Another notable earthquake occurred at around 9:02 p.m. on August 21 in waters east of Hachijojima.
The magnitude 5.4 quake had a focal depth of 63 kilometers and registered an intensity of 3 in Miyake and Mikurajima villages in Tokyo.
It was the first earthquake centered east of Hachijojima to produce shaking of intensity 3 or higher since July 2024, about two years ago.
The area lies near a plate boundary where the Pacific Plate subducts beneath the Philippine Sea Plate and has a history of relatively large earthquakes.
A magnitude 6.6 earthquake occurred there in 2009, while a magnitude 7.2 quake struck in 1972. The 1972 earthquake produced intensity 6 shaking on Hachijojima under the seismic scale then in use.
Globally, two earthquakes of magnitude 6 or greater were recorded during the week, based on data compiled by the U.S. Geological Survey.
The largest was a magnitude 6.7 earthquake in Peru at around 3 a.m. Japan time on August 21. Its focal depth was 99 kilometers.
Peru lies along the boundary between the Nazca Plate and the South American Plate, making it one of the world's most seismically active regions.
Earthquakes there generally become deeper farther inland from the coast as the Nazca Plate descends beneath the South American Plate.
Other large earthquakes at depths of around 100 kilometers have occurred in the region in recent years, including a magnitude 6.5 quake in northern Peru in 2022 and a magnitude 7.5 earthquake in 2021.
Even relatively deep earthquakes can cause damage when their magnitude is sufficiently large, making the region one where strong seismic activity continues to require close attention.
Source: ウェザーニュース
