Weekly Earthquake Information 2021.3.7 Giant M8.1 Earthquake in South Pacific on Friday, 5th Tidal Level Change in Ogasawara Islands –Weathernews

Around 6:23 on the 3rd (Wednesday), there was an earthquake with a magnitude of 5.9 and an estimated epicenter at a very shallow epicenter off the east coast of Hokkaido. Due to the influence of this earthquake, we have observed tremors mainly in eastern Hokkaido, and we have observed a maximum seismic intensity of 3 in Nemuro City.

Off the east coast of Hokkaido is the area where the Pacific plate is subducting towards the North American plate, but since the epicenter of this earthquake is shallow, less than 10 km, it appears that the interior of the North American plate was destroyed in place of the plate boundary. Recently, there was a 6.3 magnitude earthquake near the epicenter in July 2015.

In the vicinity of the epicenter, this time, the 8.2 magnitude “1994 Hokkaido Toho-oki Earthquake” occurred in October 1994, although the depth is slightly different from about 28 km.

Along the Kuril Trench, there is concern about shallow plate earthquakes such as 1994 and huge plate boundary earthquakes. Over the next 30 years, the government’s Earthquake Research Promotion Headquarters has estimated that the probability of occurrence of magnitude 7.5 off Shikotan Island and off Chotoku Island in the area that includes the epicenter is approximately 90%, and that the magnitude of 7.7 to 8.5 is approximately 60.%. If an earthquake of magnitude greater than 7 occurs, a tsunami can occur, so caution is essential in this area.

Around 16:09 on the 23rd (Tuesday), there was an earthquake with a magnitude of 4.2 and a depth of about 53 km with the epicenter of Iyo-nada. A maximum seismic intensity of 3 has been observed in Seiyo City and Ikata City, Ehime Prefecture, due to this earthquake.

In Iyo-nada, earthquakes that occur within the subducting Philippine marine plate occur frequently. Recently, in March 2014, a magnitude 6.2 earthquake occurred slightly northwest of the epicenter, and a seismic intensity of 5 or more was observed in the city of Seiyo.

The same type of earthquake also includes the Geiyo earthquakes that occurred in 2001 and 1905, respectively.

The government headquarters for the promotion of seismic research is located in the area from Aki Nada to Iyo Nada and the Bungo Canal, and the probability of an earthquake with a magnitude of 6.7 to 7.4 will occur in the next 30 years is about 40%. If the epicenter occurs close to the ground, there is a risk of a strong earthquake with a seismic intensity of 6 lower than 6 higher, making it an area that requires caution not only for large earthquakes caused by the Nankai Canal but also also for such earthquakes.

In shallow earthquakes, areas with strong shaking are often concentrically distributed from the epicenter, but in deep-focus earthquakes, the seismic intensity is called the “abnormal seismic area” where the shaking is transmitted to distant areas along the Pacific plate where seismic waves occur. it is easily transmitted. It can be a distribution.

Similar deep-focus earthquakes often occur where the Pacific plate is deeply subducted, and earthquakes of magnitude 6 and above occur once every few years. On the other hand, it is also characterized by the fact that there are almost no aftershocks in a single earthquake.

There have been 13 earthquakes of magnitude 6 or greater, as analyzed by the United States Geological Survey. In the waters near New Zealand, earthquakes of magnitude 7 and above occurred one after another from Thursday the 4th to Friday the 5th.

The first earthquake occurred around 10:27 p.m. Thursday, Japan’s fourth time, with a magnitude of 7.3 in the waters near New Zealand’s North Island. Just four hours later, around 2:41 am on Friday the 5th, there was a 7.4 magnitude earthquake in the waters near the Kermadec Islands about 1000 km to the north, and a 8.1 magnitude earthquake at approximately 4 o’clock. : 28. The earthquake of magnitude 8 and higher analyzed by the USGS is the first in about two years since the May 2019 earthquake in Peru.

The earthquake mechanism was a type of reverse fault with a pressure axis in the west-northwest-east-southeast direction, and the epicenter was as shallow as about 27 km, which is why the tsunami occurred. A tsunami of about 50 cm was observed in Kingston, Norfolk Island, Australia. The tsunami has reached not only Australia and New Zealand, which are close to the epicenter, but also Mexico and Chile. Also in Japan, tidal level fluctuations of up to 20 cm were observed in the afternoon of the 5th (Friday) on the Ogasawara and Chichijima islands, and the tsunami is believed to have arrived.

The area from New Zealand to the Kermadec Islands is the plate boundary where the Pacific plate subducts into the Australian plate. Earthquakes with a magnitude greater than 7 occur frequently, and even near the epicenter this time, there were earthquakes with a magnitude of 7.3 in June 2019, a magnitude of 7.4 in October 2011, and a magnitude of 7.6 in July 2011.

The tsunami caused by the earthquake in this area tends to spread in an east-west direction and does not have a significant impact on Japan. However, it cannot be said that there will be no effect when it comes to the magnitude 8 class like this time, so caution is required.