In the Great East Japan Earthquake, a large tsunami of 10 meters or more caused enormous damage, but the risk of a tsunami can be reached quickly.
A national research project has compiled new findings on the Sea of Japan, which is particularly at high risk.
There are a total of 185 faults that can cause a tsunami, and experts say that there are many "sea and land faults" in which the fault extends across land and sea and the tsunami arrives immediately after the earthquake. I want you to use it for disaster prevention in the area. "
In response to the Great East Japan Earthquake, the Earthquake Research Institute of the University of Tokyo, JAMSTEC = Japan Agency for Marine-Earth Science and Technology, and Niigata University have been investigating possible earthquakes and tsunamis on the Sea of Japan side as a project of the Ministry of Education, Culture, Sports, Science and Technology in eight years. , The results of the whole country are summarized.
Based on the results of structural surveys conducted on land and sea and the latest geological surveys, it is estimated that there are a total of 185 faults that may cause tsunamis.
In particular, from Hokkaido to Hokuriku, it was found that there are more than 30 faults that straddle land and sea, which are also called "land-sea faults," and faults that border the coast.
The maximum height of the tsunami is several meters, which is not as high as the big tsunami 10 years ago, but the time it takes for the maximum tsunami to reach is several minutes to 10 minutes, and it is about 30 to 40 minutes. It is extremely fast compared to Tohoku 10 years ago.
Repeated damage in the Sea of Japan in the past
The Sea of Japan has suffered heavy damage from repeated earthquakes and tsunamis, such as the 1983 Central Japan Sea Earthquake and the 1993 Hokkaido Nansei-oki Earthquake.
However, the problem is that there are many things that cannot be understood due to the lack of past records.
The tsunami assumption of the national study group announced seven years ago is centered on faults with a length of 40 km or more offshore for the purpose of extracting the largest class as "height". Summarized the whole picture including the fault near the land.
Expert "First of all, risk recognition"
Professor Masahisa Shinohara of the Earthquake Research Institute, the University of Tokyo, who is the research representative of the "Japan Sea Earthquake and Tsunami Survey Project," said, "Evacuation is particularly difficult because the tsunami comes immediately after a large tremor near the land and sea fault. I hope it will be useful for discussions on local disaster prevention and town planning. "
The project will soon compile the findings into a report and submit it to the Ministry of Education, Culture, Sports, Science and Technology.
There are more than 30 land and sea faults
In this project, the existence of "sea-land faults" that straddle the sea and land or run along the land has been pointed out, mainly from Hokkaido to Hokuriku, and there are more than 30 in total.
The fault that became the epicenter during the 1964 Niigata earthquake was on the sea side, but in this survey it was estimated that there is also a fault near the mouth of the Agano River to the southwest.
It means that there is a high possibility of "land-sea faults" including those on the sea side.
Strong shaking at the land and sea fault
Since the epicenter of the land-sea fault is near the land or directly below it, the damage caused by the violent shaking is serious in addition to the early tsunami.
Professor Tomotaka Iwata of the Disaster Prevention Research Institute, Kyoto University, who specializes in strong vibrations, simulated shaking at land and sea faults in Niigata and Akita prefectures, and found that the maximum seismic intensity could be 7.
Professor Iwata says that in addition to being close to the epicenter, the ground is weak in the coastal areas, so the shaking is easily transmitted to a wide range, and if the location where the fault rupture begins changes, the range in which the shaking becomes stronger also changes.
Large-scale liquefaction due to violent shaking
There is a risk that the building will be damaged by violent shaking, but there is also the risk of liquefaction as it occurred in the Great East Japan Earthquake.
The Niigata earthquake caused large-scale liquefaction, causing great damage such as the collapse of apartments and the large-scale damage to bridges.
According to Professor Atsushi Urabe of the Institute of Disaster and Reconstruction Science, Niigata University, who is a member of the project and specializes in geology, the risk of liquefaction is widespread because the ground is often weak in the lowlands on the Sea of Japan side and coastal areas. There is.
Professor Urabe said, "If an earthquake occurs on a fault over land, it is expected that the building will be seriously damaged and liquefaction will occur over a wide area. It could have a big impact. "
Response to "early tsunami" Making an evacuation map
How to prepare for an "early tsunami" that is difficult to evacuate.
Efforts have begun to find out about the characteristics of the city and the tsunami and escape to a safe place as soon as one minute or one second.
The Nagaoka Plain western margin fault zone, which is one of the major active fault zones in Japan, is one of the "land-sea faults", and a new tsunami simulation was carried out in this project as well.
In Chuo-ku, Niigata City, which may be severely damaged by this tsunami, we are creating an evacuation map for each district under the supervision of Professor Atsushi Urabe of Niigata University, who is participating in this project.
The feature of the map is that the time until the tsunami arrives is color-coded, and the optimal evacuation site and method for each person are set in detail.
In the Numadare area facing the harbor, there are no seawalls and low land spreads along the road, so it is expected that the tsunami will inundate in a short time.
Some people live more than 300 meters away from the tsunami evacuation building designated by the city, and there is a risk that they will not be in time. Residents and others took the lead in negotiating so that they could be evacuated.
In addition to being displayed as a "high building" on the map, the direction in which water easily flows and the route for evacuation are also written so that you can escape in a safe place in a short time.
Mr. Yoshinori Takeda, who was involved in the map making, said, "I think we have created a good map that shows where in the area the impact will start and how the damage will spread and where to go to help in the event of a tsunami. I would like to continue thinking so that I can evacuate safely and securely. "
Chuo-ku, Niigata City will create such a "voluntary evacuation map" in a total of seven districts and distribute it to residents to help ensure evacuation.
Professor Urabe of Niigata University said, "Because the tsunami is approaching soon, it is not" there is nothing to do ", but by searching for and sharing risks and usable resources in the area, residents' understanding and awareness of disaster prevention will increase, so the Pacific Ocean I think this is a very useful initiative in other regions, including the side. I hope that each region will be better prepared for the tsunami that is approaching soon. "