On an active fault with an imminent 31 degree of earthquake, "same or more than immediately before the earthquake" January 17 5:04

Based on the lessons learned from the Great Hanshin-Awaji Earthquake 25 years ago, the country has conducted active fault surveys nationwide and evaluated risks such as the probability of occurrence. At present, there are 31 active faults that have an imminent degree of earthquake that is equal to or greater than that immediately before the Great Hanshin-Awaji Earthquake, and it is important to prepare for an active fault earthquake again.

The Great Hanshin-Awaji Earthquake was caused by a large earthquake with a magnitude of 7.3 in the southern part of Hyogo Prefecture, which caused a part of the active fault located from northwestern Osaka Prefecture to Awaji Island in Hyogo Prefecture to shift. .

Based on this lesson, the government established the Earthquake Research Promotion Headquarters and established 114 active faults nationwide, which are approximately 20 km long and have a major social impact if an earthquake occurs. We have focused on surveys and assessed risks such as the probability of an earthquake occurring in the next 30 years.

The degree of urgency of earthquake occurrence is divided into four ranks. Active faults with a probability of 3% or more are regarded as the highest "S rank", and the probability of occurrence immediately before the Great Hanshin-Awaji Earthquake occurs is 0.02% to 8%. Applies to the S rank.

As of January 1, last year, 31 active faults nationwide were rated as “S rank”, of which “Itoigawa-Shizuoka Tectonic Line Fault Zone”, “Chuo Tectonic Line Fault Zone”, “Miura” In some of the eight active fault zones, such as the "Peninsula Fault Group", the occurrence probability exceeds 8%, and the degree of urgency is higher than before the Great Hanshin-Awaji Earthquake.

In addition, there are 35 active faults of “A rank” with the next highest risk nationwide, 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, there have been a number of cases where earthquakes have occurred, such as the Niigata Chuetsu Earthquake in 2004 and the Iwate-Miyagi Nairiku Earthquake in 2008, which were previously unknown active faults shifted.

"We would like to continue to consider ways to understand the risks that actually occur in the future. Rather, I want you to think again that there is a big earthquake while you are alive. "

In particular, the S-rank active fault with high urgency

Earthquakes caused by active faults, unlike plate-bound earthquakes such as the Nankai Trough and the Japan Trench, have long intervals of about several thousand years, so the probability of occurrence for 30 years is not a large numerical value.

However, as the probability just before the occurrence of the great earthquake that caused the Great Hanshin-Awaji Earthquake was 0.02% to 8%, even if the probability is small, caution is required.

For this reason, 114 major active faults are classified into four ranks, such as "S" and "A", indicating the degree of imminence that an earthquake will occur within 30 years.

Active faults with an earthquake probability of 3% or more are classified as the highest S rank, and there are 31 in the whole country as of January 1, last year.

Of these, the probabilities exceeded 8% just before the Great Hanshin-Awaji Earthquake occurred, and the areas of particularly high urgency are part of the following eight active fault zones.
In order of urgency,
「Itoigawa-Shizuoka Tectonic Line Fault Zone in Nagano and Yamanashi Prefectures
富士 Fujikawa Estuary Fault Zone in Shizuoka Prefecture
日 Hinagu Fault Zone in Kumamoto Prefecture
「Sakaitoge Kamiya Fault Zone in Nagano Prefecture
「The“ Chuo Tectonic Line Fault Zone ”located from Kinki to northern Shikoku and northern Kyushu
「The“ Atera Fault Zone ”in Gifu and Nagano prefectures
「Miura Peninsula Fault Group in Kanagawa Prefecture
「Akinada Fault Zone off the coast of Hiroshima and Yamaguchi prefectures
It has become.

Risks other than major active faults

In the last 25 years, 114 major active faults have been surveyed and evaluated, but earthquakes due to active faults that have not been evaluated have also been occurring one after another.

The Niigata Chuetsu Earthquake with a magnitude of 6.8 in 2004 and the Iwate-Miyagi Nairiku Earthquake with a magnitude of 7.2 in 2008 are said to have caused active faults that had not been identified before.

Experts have also pointed out that an unknown active fault also caused the 6.7 magnitude Eastern Hokkaido Iburi Earthquake, which occurred when an adult fell.

For this reason, the Earthquake Research Promotion Headquarters has added not only major active faults, but also active faults that cause earthquakes with a magnitude of 6.8 or more, and evaluated the risk of earthquakes not for each active fault but for each region. , Published in Kanto.

The probability that an earthquake of magnitude 6.8 or more will occur in the next 30 years,
5050% to 60% in the entire Kanto region
5050% in China
3030% to 42% throughout Kyushu
99% to 15% in Shikoku
It is estimated.

In the evaluation of each area, multiple active faults are included, so the probability is inevitably increased, but there is a disadvantage that the range is widened and the alert is weakened, and how to communicate the risks to the residents and prepare them is an issue. Has become.