Weekly Earthquake Information 12/27/2020 Third Seismic Intensity 5 Minor on December 21 (Monday) M6.5 off the East Coast of Aomori Prefecture

Around 2:23 a.m. on the 21st (Monday), an earthquake with a magnitude of 6.5 and an estimated depth of about 43 km occurred off the east coast of Aomori Prefecture, causing strong tremors mainly in the Tohoku region. The maximum seismic intensity is less than 5 in the city of Morioka, Iwate prefecture. Seismic intensity 4 has been observed in Hakodate City, Hokkaido, Hachinohe City, Aomori Prefecture, Towada City, Nito City, Iwate Prefecture, and Yawatahira City.

This is the seventh earthquake to observe a seismic intensity of 5 less or greater this year, and three of them are concentrated in December. The mechanism of this earthquake is a type of reverse fault with a pressure axis in the west-northwest-east-southeast direction, and is believed to have occurred in the area after the great earthquake that caused the Great East Japan Earthquake. On Saturday December 12, a 5.6 magnitude earthquake struck off the coast of Iwate Prefecture on the southwest side, and a maximum seismic intensity of 5 minus has just been observed.

Around 2:55 AM on the 24th (Thursday), an earthquake with a magnitude of 3.6 and an estimated depth of about 28 km occurred in Nara prefecture. A maximum seismic intensity of 3 was observed in Yoshino city, Nara prefecture. The source of the earthquake was near the prefectural border between Nara and Mie prefectures, and preliminary figures showed the source was the southern part of Mie prefecture, but provisional values ​​have been revised for Nara prefecture. . It is the first time in five years that a seismic intensity of 3 or more has been observed in an earthquake whose source is Nara prefecture.

Nara prefecture has several known fault zones in the north, and the magnitude 6.7 earthquake in 1952 called the “Yoshino Earthquake” killed Nara prefecture and the surrounding prefectures. Although there are no known faults near the border with Mie Prefecture, a 7.0 magnitude earthquake is believed to have occurred in 1899.

In addition to these inland earthquakes, even the expected giant Nankai Trough earthquake is believed to cause tremors in a wide area of ​​Nara prefecture with a seismic intensity of 6 or more, so regular countermeasures are indispensable.

Two earthquakes with a magnitude of 6 or greater occurred, as analyzed by the American Geological Survey. One is the earthquake off the east coast of Aomori prefecture (USGS has a magnitude of 6.3) and the other is the earthquake that occurred in the southern part of Luzon Island, Philippines.

The depth of the seismic source is as deep as about 109 km, and the earthquake mechanism is analyzed as a type of reverse fault with a pressure axis in the northeast-southwest direction. It is believed to have occurred near the boundary between the subducting Philippine Sea plate and the Eurasian plate.

On the South China Sea side of Luzon Island, earthquakes with a depth of more than 100 km often occur, and near the source of this earthquake, earthquakes with a magnitude of 6 or more and a depth of 100 km or more in 2017 and 2004.

In the Philippines, a 6.1 magnitude earthquake occurred off the southern coast of the island of Mindanao on Wednesday 16, but there seems to be little direct relationship because it is far from the origin of this earthquake.