In order to clarify the risk of inland earthquakes that occur directly under the city, experts in crustal movement analyzed underground strains in areas centered on western Japan from GPS data and summarized the probability of earthquake occurrence.



Even in areas where there are few active faults, earthquakes are likely to occur, and experts are aiming to utilize them for future earthquake countermeasures.

In order to reduce the damage caused by large inland earthquakes, the national government will proceed with the investigation of active faults based on traces, etc. Is published.



However, there is a limit to clarifying all risks, and there are a series of earthquakes in which previously unknown faults shift, such as the Hokkaido Eastern Iburi Earthquake and the Northern Osaka Earthquake three years ago.



Takuya Nishimura, an associate professor at the Disaster Prevention Research Institute of Kyoto University, who specializes in crustal movements, analyzes the amount of "strain" that causes inland earthquakes from GPS observation data and calculates the probability of a major earthquake in western Japan. Did.



The probability of a major earthquake with a magnitude of 6.8 or greater within 30 years was 31% to 42% in southern Kyushu, which consists of parts of Kagoshima and Miyazaki prefectures, as well as Kumamoto and Oita prefectures.



The result of the government's seismic investigation committee, which is calculated in the same area based on the investigation of active faults, is 7% to 18%, which is more than double this time.



Also, in Shikoku, the probability of being based on GPS has increased in many areas, with 20% to 28%, which is almost double the result of the Earthquake Research Committee.

According to Associate Professor Nishimura, even if active faults are not often found in southern Kyushu, the probability tends to be relatively high because GPS shows the area where strain is accumulated.



Associate Professor Nishimura says, "Risks that could not be seen on active faults are becoming visible on GPS. It is important to utilize GPS data in addition to surveying active faults and connect them to earthquake countermeasures." ..

The probability of occurrence in each region is

It is the probability that a large earthquake with a magnitude of 6.8 or more, calculated for each region, will occur within 30 years.



Even if the probability is low, it does not mean that it is zero, and it does not mean that it is "safe because it is low".



It's Kyushu.



▽ 11% to 16% in "Northern Kyushu" to Fukuoka and Saga prefectures.


▽ 12% to 17% in "Central Kyushu" from Oita prefecture to northern Kumamoto prefecture and Nagasaki prefecture,


▽ 31% in "Southern Kyushu" consisting of parts of Kagoshima prefecture and Miyazaki prefecture, and Kumamoto prefecture and Oita prefecture respectively It is ~ 42%.



In the Chugoku region,



▽ 8% to 12% in "North China" centered on Tottori and Shimane prefectures.


▽ 6% -9% in "Eastern China" centered on Hiroshima and Okayama prefectures.


▽ It is 11% to 16 in "Western China" in Yamaguchi prefecture, Shimane prefecture and part of Hiroshima prefecture.



▽ "Shikoku" is 20% to 28%.



On the other hand, the probability that the national government has calculated based on the investigation of active faults is


▽ "Northern Kyushu" 7% to 13%,


▽ "Central Kyushu" 18% to 27%,


▽ "Southern Kyushu" 7% to 18 %,


▽ "Northern China" is 3% to 7%,


▽ "Eastern China" is 2% to 3%,


▽ "Western China" is 14% to 20%,


▽ "Shikoku" is 9% to 15% I am.

What is GPS-based prediction?

Associate Professor Nishimura uses GPS data with observation points at approximately 1,300 locations nationwide to predict inland earthquakes.



The movement of the earth can be measured in millimeters by capturing the position of the observation point with an artificial satellite and collecting data for a certain period of time.



We analyze the difference in the direction and size of movement at each observation point for a certain period of time, and estimate where and how much "strain" that causes the earthquake is accumulated.



This time, we announced the probability that a large earthquake with a magnitude of 6.8 or more will occur within 30 years at each location in about 20 km square, centering on western Japan, which has been analyzed and verified so far. doing.



After that, the probabilities were calculated by summarizing the areas.



Although active fault data is not used in the calculation, Associate Professor Nishimura analyzed GPS data in advance for the earthquake of magnitude 6.6 that occurred in central Tottori prefecture in October 2016, although no active fault was found. It was an area where I thought that the strain had accumulated.



In California, USA, earthquake risk assessment is performed by combining active faults and GPS data.



Associate Professor Nishimura plans to proceed with analysis in Kanto and northern Japan, and hopes to encourage the use of GPS data in addition to active fault data in Japan as well.

Prediction by active faults

The National Headquarters for Earthquake Research Promotion recognizes active faults that are approximately 20 km or more in length and have the potential to have a significant social and economic impact if an earthquake occurs.



So far, 114 have been certified, and we have published evaluation results such as the probability and scale of future earthquakes.



The degree of urgency for an earthquake to occur is divided into four ranks, and active faults with a probability of 3% or more are designated as the highest "S rank".



The "Itoigawa-Shizuoka Tectonic Line Fault Zone" in Nagano and Yamanashi prefectures is applicable.



The next highest risk was "A rank" of 0.1 or more and less than 3%, and the "Futagawa fault zone" that caused the Kumamoto earthquake in 2016 was evaluated as "A rank" immediately before the earthquake.



On the other hand, large earthquakes frequently occur even in places where no active faults have been found in advance, such as the 2008 Iwate-Miyagi Nairiku Earthquake and the earthquake that observed shaking with a maximum seismic intensity of 7 in Hokkaido three years ago.



For this reason, the national government is working not only on individual evaluations of major active faults, but also on "regional evaluations" that summarize the regions, including other active faults. The evaluation has been published.