Late on the 7th, there was an earthquake with an epicenter in the northwestern part of Chiba prefecture, and strong shaking with a seismic intensity of over 5 was observed in Adachi Ward, Tokyo and Kawaguchi City, Saitama Prefecture.


It is the first time that a tremor with a seismic intensity of 5 or higher has been observed in the 23 wards of Tokyo since the Great East Japan Earthquake that occurred 10 years ago, and the Japan Meteorological Agency is calling for attention to earthquakes with similar tremors for the next week or so.



I will tell you in detail how the characteristics of this earthquake are different from the expected earthquake directly under the Tokyo metropolitan area.

Is it different from the expected earthquake directly under the capital?

Regarding the relationship between this earthquake and the "earthquake directly under the capital" assumed by the national government, the Japan Meteorological Agency explained that "this earthquake was deeper than the expected earthquake directly under the capital and was smaller in scale."



The "earthquake directly under the capital" is a magnitude 7 class earthquake that the government's Earthquake Research Committee estimates with a 70% probability of occurring within the next 30 years.



According to the Cabinet Office's assumptions, Tokyo will be hit by a violent tremor with a maximum seismic intensity of 7, causing enormous damage in the southern part of the Kanto region, and in the worst case, it is estimated that about 23,000 people will die.

Japan Meteorological Agency "Earthquake directly under the capital is a little shallower and bigger"

According to the Japan Meteorological Agency,


▼ this earthquake was estimated to have a magnitude of 5.9 and the maximum seismic intensity was 5 or

higher, while

▼ the earthquake


directly below

the

Tokyo metropolitan area

is estimated to have a maximum seismic intensity of

7 in the magnitude 7 class, which is one size smaller. ..



Regarding the relationship between this earthquake and the earthquake directly beneath the Tokyo metropolitan area, Shinya Tsukada, chief of the Earthquake and Tsunami Monitoring Division of the Japan Meteorological Agency, said, "The assumed earthquake directly beneath the Tokyo metropolitan area was a little shallower and larger, and this time it was a deeper earthquake and a little smaller. I'm thinking about it. "

Is it an earthquake that occurred near the plate boundary?

Shinya Tsukada, chief of the Earthquake and Tsunami Monitoring Division of the Japan Meteorological Agency, said about the mechanism of this earthquake, "It is an earthquake called" reverse fault type "that occurred when the bedrock was compressed from east and west. I suspect it was an earthquake near the boundary between the plate and the Pacific plate. "

On top of that, "I don't know if this earthquake is the mainshock or the foreshock of a larger earthquake until the whole seismic activity is over. However, from the evaluation of past earthquakes, it will be about the next week, especially 2-3 During the day, please be careful of earthquakes with the same strong shaking as this time. "

Why seismic intensity 5+ in a place away from the epicenter

In this earthquake, the largest tremors with a seismic intensity of 5 or higher were observed not in Chiba Prefecture, which is the epicenter of the northwestern part of Chiba Prefecture, but in Adachi Ward, Tokyo and Kawaguchi City, Saitama Prefecture.



Regarding this, Shinya Tsukada, chief of the Earthquake and Tsunami Monitoring Division of the Japan Meteorological Agency, said, "The seismic intensity was higher because the ground was along an old river or the ground was softer than the surroundings, rather than due to a special earthquake mechanism." It explains.



According to the Japan Meteorological Agency, a magnitude 6.0 earthquake occurred 16 years ago in July 2005 in the northwestern part of Chiba prefecture, which was the epicenter of this earthquake. Was observing.

How do you know the "goodness" of the ground?

"It was said that the ground was loose, but


it wouldn't

shake so much ..."

"It wasn't a big deal because the ground was solid


."



In addition to the position from the epicenter, it was the ground, especially the "surface ground" near the ground, that influenced the shaking.



But did you know that the "goodness" of the ground is different even if it is a little far away?

Check out the detailed data based on the latest research.

"Surface ground" Sedimentary shaking such as soft clay layer is likely to be amplified

The website "J-SHIS" (Earthquake Hazard Station) operated by the National Research Institute for Earth Science and Disaster Prevention was updated to more realistic data for the Kanto region in March.



In general, earthquakes sway as the distance that seismic waves reach is closer.



However, it is known that the shaking is likely to be amplified if a soft clay layer is deposited on the "surface ground" near the ground.

Survey conducted in Kanto reveals "easiness to shake"

Until now, the analysis was based on topography, but the National Research Institute for Earth Science and Disaster Prevention conducted a survey in the Kanto region from 2014 to 2017 in order to bring it closer to the actual situation.



I used a high-performance seismograph called a "fine movement array".

As a result of analyzing the survey conducted at a total of 14,000 locations, taking into account the results of the boring survey, the geology of the surface layer and the way seismic waves are transmitted, that is, "easiness of shaking" was clarified.



Reflecting detailed data, the characteristics of the region that had not been seen before emerged.

Amplification rate is also large around Adachi Ward where seismic intensity 5 upper was observed

Compared to the conventional one, you can see that the amplification factor of the shaking is larger in the areas along the river such as the Tone River and the Arakawa River.



It can be confirmed that the amplification factor is also large around Adachi Ward, where the seismic intensity of 5 upper was observed this time.



On the other hand, it was also found that the shaking is not so easy even along the river, and the shaking is amplified even on the plateau where the ground is generally considered to be hard.



There are big differences depending on the region around central Tokyo, Bunkyo Ward, Taito Ward, and Arakawa Ward.



The National Research Institute for Earth Science and Disaster Prevention will continue to investigate in other areas in the future.

Viewpoints and precautions

It should be noted that the amplification factor does not directly lead to the shaking of all buildings.



Buildings have a "cycle" that is easy to shake.

For example, low buildings are generally prone to sway with short, rattling cycles, and tall buildings are liable to sway with slow, long cycles.



Earthquake tremors contain various components, and the tremors that are amplified differ depending on the nature of the ground. If the soft ground is thick, slow tremors with a long cycle are likely to be amplified.



In other words, even if the ground is hard, if the period of shaking and the period of the building match = resonance occurs, the shaking of the building will increase.



It is also important to understand the characteristics of the building.

"Long-period ground motion" Observed class 2 in Tokyo's 23 wards and northwestern Chiba prefecture

According to the Japan Meteorological Agency, this earthquake caused a "long-period ground motion" in which skyscrapers and other buildings sway slowly and greatly, and the second class "2" from the bottom was observed in the 23 wards of Tokyo and northwestern Chiba prefecture. Did.



On the upper floors of skyscrapers in these areas, it is difficult to walk without catching something, and tableware on the shelves and books on the bookshelf may fall off, which may have caused a big shake.

"Embankment" can also affect damage

In addition to the surface ground, there are other things that affect the damage.



It is a residential land created by embankment.



Damage has been confirmed one after another in large earthquakes, including a huge earthquake 10 years ago, on residential land that was filled mainly in the old age.



If certain conditions are met, such as large-scale ones, the local government will investigate and publish it.



Just because an embankment does not mean that everything is dangerous.



Also, please note that there is a limit to the damage caused by the embankment not shown on the map.



Please check what is happening around your home, where you work, and the school you attend.

Japan Meteorological Agency "Watch out for earthquakes in the next few weeks, especially for the next few days"

A magnitude 3.6 earthquake occurred near the epicenter on the 8th after 5 am, and we are observing shaking with a seismic intensity of 2 in Nerima Ward, Tokyo.



The Japan Meteorological Agency is


calling for measures such as fixing furniture that is prone to fall, paying attention to earthquakes

with the same strong shaking as this time, for the next week or so, especially for the next

few days.



Also, be aware that there may be an increased risk of rockfalls and landslides in areas with strong shaking.