Weekly earthquake data 2021.2.7 Second (Tuesday) Tochigi / Nikko earthquake with seismic intensity 4 Areas with many shallow earthquakes –Weathernews

Around 3:01 on the 2nd (Tuesday), there was an earthquake with a magnitude of 4.0 and an estimated depth of about 7 km, which originated in the northern part of Tochigi prefecture. A maximum seismic intensity of 4 has been observed in the city of Nikko due to this earthquake. The earthquake mechanism is analyzed as a landslide type.

Shallow earthquakes with a magnitude of around 5 occur relatively frequently around Nikko, and even if limited to earthquakes with a maximum seismic intensity of 4 or more, they occurred in December 2019 and October 2018. A little north from the epicenter, a large 6.3 magnitude earthquake occurred in 2013, with a seismic intensity of more than 5 in the city of Nikko and a seismic intensity of 3 in a wide area from Niigata and Fukushima prefectures to the prefecture of Saitama.

The Uchinogomori fault is known around Nikko, but the detailed history of past activities is unknown, and the government headquarters for the promotion of earthquake research has not calculated the probability of an earthquake.

Around 11:30 p.m. on day 1 (Monday), there was an earthquake with a magnitude of 4.0 and an estimated depth of about 7 km, which originated in the northern part of Tokushima. A maximum seismic intensity of 3 has been observed in Tokushima City, Anan City, Mima City, etc. due to this earthquake. The mechanism of the earthquake is analyzed as a landslide type.

There are not as many earthquakes with epicenters in the northern part of Tokushima prefecture, and it is the first time in four years since January 2017 that a seismic intensity of 3 or more has been observed. Most earthquakes are about 40 to 50 km deep near the Middle Tectonic Line and less than 10 km deep to the south, and this earthquake is the last.

Most of the earthquakes that cause great damage in Tokushima Prefecture are large earthquakes caused by the Nankai Canal, and the only known earthquakes that originated in the prefecture are 6.4 magnitude earthquakes that occurred in the southern part from Tokushima prefecture in 1955.

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.

An earthquake of magnitude 6 or greater has occurred once, as analyzed by the United States Geological Survey. It was an earthquake that occurred off the coast of Chile in South America on the 3rd (Wednesday) of Japan time.

An earthquake of magnitude 6.7 occurred in the Pacific Ocean off the coast of Chile, South America, after 2:00 p.m. on Wednesday 3, Japan time. Although the depth was as shallow as 10 km, the epicenter was far from the ground, so there was no effect from the shaking. In addition, the mechanism was of the lateral slide type and there was no tsunami.

Speaking of earthquakes in Chile, a large coastal earthquake caused by plate subduction is known, but this earthquake is of an entirely different type. It is believed to be an earthquake associated with a “transformation fault” that lies near the boundary between the Nazca plate and the Antarctic plate called the Ascent of Chile.

The “transformation faults” develop around the ridges where the plates are separated from each other, causing slip earthquakes. Even if the scale is large, it is often not accompanied by a tsunami, which is a big difference from the earthquake on the Chilean coast.