Deep-focus earthquake in Ise Bay, abnormal seismic area Different factors from Nankai Trough earthquake-Weathernews

2022/05/09 17:53 Weathernews

There was a deep earthquake in Ise Bay around 17:33 on May 9 (Monday). The depth of the epicenter is estimated to be about 340km, and the magnitude of the earthquake is estimated to be 5.1. Due to this earthquake, shaking with a seismic intensity of 2-1 was observed in various places from Miyagi prefecture to Kanagawa prefecture.

The seismic intensity distribution is called the “abnormal seismic area” where the shaking is transmitted to the Pacific side of the Kanto and Tohoku regions, but it happens frequently and the seismic activity itself is not abnormal.

What is an abnormal seismic area due to a deep earthquake?

Schematic diagram of abnormal seismic area due to deep earthquake

This earthquake is called a “deep-focus earthquake” that occurs in a very deep place.

In many earthquakes, areas with strong tremors are distributed concentrically from the epicenter, but in deep-focus earthquakes, strong tremors are transmitted along the subducted plate, and there is a phenomenon that the tremors increase in the distance near the plate boundary. This is called the “abnormal seismic zone”. Even if the shaking is small in the vicinity of the epicenter, strong shaking may be transmitted in the distance, so caution is required.

It is probable that the quake propagated along the Pacific coast of northern and eastern Japan along the Pacific plate, where seismic waves are easily transmitted.

The possibility of a tsunami is small in a deep-focus earthquake, and it is generally considered that there is almost no concern about a tsunami in an earthquake with an epicenter deeper than 100 km.

Similar deep-focus earthquakes often occur in the Sea of ​​Okhotsk, the Sea of ​​Japan, off the southeastern coast of Mie Prefecture, off the southern coast of Tokaido, and in the sea near Torishima, and earthquakes of M6 or larger 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.

On January 1, 1984, there was a M7.0 earthquake off the southeast coast of Mie Prefecture, and seismic intensity 4 was observed in Chiyoda-ku, Tokyo and Yokohama. Although these earthquakes do not generate tsunamis, they can be accompanied by large tremors, so caution is required.

Factors different from the Nankai Trough giant earthquake

Plates around the Japanese archipelago

The depth of the epicenter of this earthquake is analyzed to be about 340km in breaking news. It is believed that the earthquake occurred around the Pacific Plate, which sinks westward from the Japan Trench / Izu-Ogasawara Trench. According to the breaking news analysis of the National Research Institute for Earth Science and Disaster Prevention, it seems to be a reverse fault type earthquake with a pressure axis in the east-west direction.

On the other hand, a general “Nankai Trough giant earthquake” is an earthquake that occurs near the boundary where the Philippine Sea plate sinks into the Eurasian plate, and the expected epicenter depth is about 10 to 40 km. The mechanism of this earthquake is considered to be a different factor from the Nankai Trough giant earthquake.

Also, considering the magnitude of the earthquake, the risk of induced earthquakes is considered to be small.

Observing long shaking in an area far from the epicenter

Looking at the data of “Yure Station”, which is installed by Weathernews to observe the shaking of the living environment, after a strong rolling of about 40 seconds on the 9th floor of the rebar building in Itabashi-ku, Tokyo. , I caught a slight shaking for more than 1 minute.

Live broadcasting of the latest information

https://www.youtube.com/watch?v=tMvMWlpzUAs

* Depending on the scale and impact of the earthquake, the broadcast content may be normal after breaking news.

Reference materials, etc.