Where is the current earthquake risk?

Direct earthquake approaching due to strain September 24, 20:24

"No way ... Hokuriku!"

On the night of September 16th, the sound of the Earthquake Early Warning rang loudly at the NHK Broadcasting Center in Shibuya, Tokyo, and people rushed around.

Among them, I (Oikubo) was fascinated by the position of the flickering epicenter on the monitor.

Noto Peninsula.

Only about three weeks ago, it was the place where experts were conducting an extraordinary investigation on site.

Now, a certain analysis method has highlighted the risk of earthquakes in each region.

We approach the forefront of its possibilities and limits.

(Reporter, Social Affairs Department, Disaster Officer Yuta Oikubo)

Map the probability of a direct earthquake

"Special feature of the earthquake that is being done on NHK. The probability of an earthquake near the place where you newly started living is high ..."

"I experienced the Great Hanshin Earthquake, but the probability of hitting the second big earthquake while I am alive is quite high."

During the broadcast of NHK Special's "MEGA QUAKE Giant Earthquake 2021", the most responsive on Twitter was when we introduced a map of the probability of a direct earthquake.

It shows the susceptibility to large earthquakes with a magnitude of 6.8 or more in approximately 20 km square, centering on western Japan, for which analysis has been completed so far.

The redder the color, the more likely it is to occur within 30 years.

From Hokuriku to Shiga, Kyoto, Osaka, Nara, etc., you can see high areas in a strip shape.

In addition, the Chugoku-Shikoku region is also red in places, and Kyushu is also red in the areas around Oita, Kumamoto, and Kagoshima, indicating a high probability.

The analysis is being carried out by Associate Professor Takuya Nishimura of the Kyoto University Disaster Prevention Research Institute.

An expert in crustal movements.

GPS is used for analysis.

This technology is also used in car navigation systems and mobile phones.

Observation devices at approximately 1,300 locations nationwide communicate with artificial satellites to capture the movement of the earth at that location on a daily basis in millimeters.

From this fluctuation, we analyze the "strain" that causes an earthquake and calculate the likelihood of an earthquake.

Noto Peninsula "Extremely rare incident"

Most of the Noto Peninsula was out of the range of the map released this time because the analysis had not been scrutinized, but Hokuriku showed a tendency to be totally distorted, and for the Noto Peninsula, Recently, unusual fluctuations have been detected.

Since December last year, fluctuations have been detected in the area where the ground rises by 2 cm.

2 cm is extremely rare in areas without volcanoes.

Seismic activity was also active.

Therefore, Mr. Nishimura was conducting an extraordinary investigation.

Associate Professor Nishimura

"Originally, it is thought that earthquakes are more likely to occur, and earthquakes are occurring in the place that we are paying attention to. We will analyze the relationship between crustal movements and earthquakes in this place as well."

Calculate "strain" from GPS

Since the earthquake happened to be right after the program, there were a series of reactions on SNS such as "By the way, Ishikawa was red" and "Mr. Nishimura was too scared".

However, the Noto Peninsula, which has undergone rapid fluctuations in a short period of time, is rather exceptional.

By nature, GPS crustal movements are understood by analyzing long-term trends so that we are completely unaware that crustal movements are occurring in our daily lives.

The map introduced this time investigates the movement of the earth, its direction and difference at each observation point, and analyzes the "strain" that causes an earthquake.

From there, we calculate the probability that a major earthquake will occur in the future.

The maximum value displayed is 1%.

You might think it's low.

However, this means that the numbers are unlikely to be high because they are calculated for each narrow range of 20 km square.

It should be noted that the color coding is relative and the probability of an earthquake is not zero even where the probability is low.

The yellow line on the map is the "major active fault" that the country has recognized as having a large socio-economic impact in the event of a major earthquake.

From Hokuriku to Kinki, it is called the "strain concentration zone".

In Kinki, it also overlaps with the area where major active faults are concentrated.

Those who live around Kobe, especially those who have experienced the Great Hanshin-Awaji Earthquake, may think "Mataka", but there are many major active faults in the urban areas of Kinki, and there are also distortions. It can be said that it is accumulated.

There is no big difference between the presence and absence of active faults in the Chugoku region, but there are places where the probability is high on the Sea of ​​Japan side.

In Shikoku, there are high probabilities in Ehime, Tokushima, and Kagawa near the active fault and Median Tectonic Line fault zone that run east and west.

On the other hand, there are places where the probability is high even in areas where major active faults in Kochi Prefecture have not been found.

In Kyushu, there are areas such as Fukuoka, Oita, and Kumamoto in the north where many active faults are found with high probability.

There are major active fault zones in Oita prefecture, but the probability of an earthquake occurring within 30 years is not so high, such as "almost 0%".

However, Nishimura's analysis turns red, indicating that the probability is relatively high.

In addition, the probability is particularly high in areas where no active faults have been found in Kagoshima and Miyazaki in southern Kyushu.

Don't end with "Where an earthquake can occur"

Mr. Nishimura has analyzed the risk of earthquakes from GPS data and sounded a warning bell.

In the NHK special "Mega Disaster MEGA DISASTER II" broadcast in April 2016, Mr. Nishimura pointed out that "the amount of strain accumulated in the San'in region is large, and there is a possibility that a larger earthquake will occur in the future." ..

According to the analysis, the movement of the earth was large in the eastern direction in the coastal area of ​​the San'in region, while the movement of the earth was small in the inland area.

Mr. Nishimura concluded that from the difference in the direction and magnitude of the movement of the earth indicated by GPS,

"the strain is accumulated in the San'in region and earthquakes are likely to occur

."

In October 2016, six months after the broadcast, there was a magnitude 6.6 earthquake in central Tottori prefecture.

Fortunately, no one was killed, but 25 were injured and 330 were completely destroyed.

(From "October 21, 2016 Tottori-ken Chubu Earthquake Record Magazine")

"It was not wrong as a method for long-term earthquake prediction."

Mr. Nishimura, who deepened his conviction, embarked on this earthquake probability map. Was created.

The background is "I want to move the earthquake prediction forward even if I can't make it perfect right away."

Associate Professor Nishimura

"It's up to that point to say,'Where inland earthquakes can occur in Japan,' but that doesn't mean that they occur evenly throughout the country, and there are places where earthquakes are likely to occur, for example. I think that such a map is also effective for prioritizing when the government promotes earthquake resistance etc. with a limited budget. "

The country also has risk assessment issues based on active faults

Risk assessments for direct earthquakes have traditionally been conducted by country, but forecasting methods are different.

The Earthquake Research Committee of the Headquarters for Earthquake Research Promotion of the government estimates the scale and interval of earthquakes that may occur in the future for the "major active faults" that have repeatedly caused large earthquakes in the past, and publishes the probabilities.

The basis for this is the location of active faults, the intervals between earthquakes, and the scale of displacement, which were investigated from aerial photographs and excavation surveys by experts.

In other words, it is a “record of past earthquakes”.

However, there are major earthquakes even in places that are not certified as active faults, and there are also issues in how to convey risks.

Earthquakes caused by active faults are characterized by long intervals of thousands to thousands of years, unlike "plate boundary type" earthquakes that occur in the Nankai Trench and the Japan Trench.

Therefore, if you calculate the probability of occurrence within the next 30 years, the value will be small.

Even the S rank, which is said to be "high", is 3% or more.

It's hard to feel the urgency.

For this reason, the Headquarters for Earthquake Research Promotion is currently working to determine the probability of a major earthquake occurring in each region, including other active faults in addition to individual major active faults.

Therefore, Mr. Nishimura decided to calculate the probability based on the crustal movement by GPS data in the same area division and compare the results.

Probability of large earthquakes in Kyushu, Shikoku, and China What is the difference from the countries?

This is a comparison between the probability based on GPS data in Kyushu, Shikoku, and the Chugoku region, for which regional evaluations have already been published, and the probability published by the national government.

The area classification is the same as that of the Headquarters for Earthquake Research Promotion.

Overall, we found that the probabilities calculated from GPS data tended to be higher than the probabilities based on active faults.

Especially in southern Kyushu, it has more than doubled, and Shikoku is also higher.

On the other hand, it is low in central Kyushu and western China.

According to Mr. Nishimura, the probability is higher than the probability calculated by the government in the area where active faults are rarely found and the strain is analyzed by GPS.

Mr. Nishimura has in mind the earthquake risk assessment in California, USA.

Here, the risk is evaluated by combining both GPS data analysis and individual active fault evaluation.

Mr. Nishimura thinks that GPS data should be incorporated into the prediction of direct earthquakes in Japan as well.

Associate Professor Nishimura

"Relying solely on data based on active faults is also a weakness. In recent years, the amount of data that knows the movement of the Japanese archipelago, including GPS, has increased significantly and has come to be understood at a considerable level. I think it is important to improve the prediction based on active faults, GPS, and multiple data. "

In Kanto and northern Japan, the impact of the huge earthquake 10 years ago is still large, and it is difficult to analyze, but we will proceed in the future.

“All my research is correct” is a big deal

"Dangerous city ranking!" "The next active fault earthquake is here!" Prediction of earthquake risk, which is often crowded in magazines and SNS.

Although people are interested in it, there is a risk that numbers and place names will be taken up in a way that is different from the thoughts of the parties concerned.

Moreover, Mr. Nishimura who cooperated with the publication of the results on the way.

Despite the conflict, I decided to put it out into the world.

It is said that the background to this is the experience of the Great East Japan Earthquake.

A huge earthquake of magnitude 9 that was repeated as "unexpected".

The offshore area of ​​Tohoku was also the place where Mr. Nishimura had been studying for a long time.

In fact, GPS analysis revealed that a large amount of strain had accumulated off the coast of Tohoku, and in 2000, the paper also summarized that "an earthquake may be larger than expected."

However, it was completely unpredictable that such a huge earthquake would occur.

Mr. Nishimura, who spent his school days in Tohoku and knows the streets well, visited the disaster area and lost his language.

Associate Professor Nishimura

"There are no researchers in Tohoku's GPS analysis in Japan, and I was confident that I had achieved some results, so I felt a shock or responsibility that did not help reduce the damage. What should I do? I felt that I really needed to reconsider. "

The path of researchers who started their career with the desire to contribute to disaster prevention.

While I was absorbed in the joy of pure science, I think that the consciousness of returning the results to society was gradually diminishing.

Mr. Nishimura said that he decided to announce the result to the world and think about how to prepare for it, even if the result could be expected to some extent.

Associate Professor Nishimura

"Of course, research is the result of all my efforts, but maybe there are some things that are wrong. But if I wait until the perfect thing is made, it will take hundreds of years. I may not be able to tell you the results. After 3.11, I strongly felt that "I am proud that all my research is correct." I think it's important to have a broad discussion about what we can do. "

The current seismic activity is also described as "entering the unknown Japanese archipelago" after a huge earthquake 10 years ago.

However, it is not possible at present to predict earthquakes that specify the date and time of occurrence and the scale of the earthquake.

How do you make use of the "ambiguous" data of probability?

With the spread of the new coronavirus infection, the way science and society face each other is being questioned, and I am keenly aware that the way we communicate is also being questioned.

Social section reporter

Oikubo Yuta

, 2012. He joined

the current affiliation through the Morioka office, Kagoshima stations

in addition responsible for the Ministry of Internal Affairs and Communications Fire and Disaster Management Agency of disaster