Countermeasures for the Nankai Trough Giant Earthquake Preparing “Custom Made” Assumption March 17 at 17:46

Some municipalities, which are expected to suffer damage from the Nankai Trough Earthquake, have begun to take measures to simulate specific damage according to the characteristics of the area. Experts say that with limited staffing and budgets, it is important to have a path for efficient measures.

When the largest earthquake in the Nankai Trough occurs, it is expected that a tsunami of more than 10 meters will sweep over a wide area from Kanto to Kyushu, and it will exceed 30 meters in high places.

The research group of Prof. Toshihiro Noda of Nagoya University responded to the conditions of each region according to the conditions of each region such as the strength of the largest earthquake shaking and the height of the tsunami, as well as the detailed ground characteristics that were not taken into account so far. We are working on making custom-made assumptions.

Hekinan City, Aichi Prefecture, is expected to inundate about one-third of the city. Noda focused on a river embankment that could sink due to the severe shaking of the earthquake.

Until now, the national index assumed that the embankment would sink uniformly by 75%, and assumed the inundation area due to the tsunami, but a simulation based on detailed ground data showed that the amount of subsidence was up to 50%. %, It was found that the inundation area was expected to be slightly narrower.

On the other hand, factors that could increase the damage have also become apparent. Many small boats, such as pleasure boats, are moored in the river, but they may flow out to the residential area in the town from the right bank, where the embankment subsides, causing the damage to spread.

If the sluice in the middle of the river could not be closed, the small boat would go upstream along with the tsunami and then flow out to the left bank of the river. Came into view.

The research team plans to share the results of these simulations with the city so that specific measures can be considered.

Professor Noda says that it is important to prepare for the largest earthquakes and tsunamis based on the idea of ​​`` eliminating unexpectedness '' based on the lesson learned from the Great East Japan Earthquake, but `` What to do if we assume the largest class It's a kind of hand-lifting or very overkill. It's important to have a path for efficient measures, given the limited number of people and budgets. "

Expert "Do not be prepared not only for tsunami but also for shaking"

Regarding the earthquake that occurred along the Nankai Trough, a tsunami research expert said, `` I don't know if the next major earthquake will be large enough to involve a huge tsunami, but it is certain that it will be accompanied by severe shaking, and we will prepare not only for the tsunami but also for the shaking. Don't be lazy. "

Masanobu Shishikura's group from the National Institute of Advanced Industrial Science and Technology (AIST) is a group of people on the Pacific Ocean side from the Tokai to Kyushu, where past earthquakes occurred from fossils of coastal organisms and sediments such as sand brought inland by the tsunami. And research to determine the scale.

Past earthquakes that occurred in the Nankai Trough have been known up to about 1300 years ago by ancient documents, but recent research can date back to 5500 years ago, such that a huge tsunami reaches deep inland It has become clear that the earthquakes occurred repeatedly at intervals of 400 to 600 years.

However, no trace of a huge tsunami caused by a magnitude 9 class earthquake, which is assumed by the country to be the largest, has not been found at this time.

On the other hand, the research group is also investigating sediments on the sea floor, and now it is possible to go back to an earthquake about 40,000 years ago.

The latest analysis shows that the Nankai Trough has an average of 100 to 150 years of major earthquakes, with evidence of some 70-year intervals.

The last major earthquake in the Nankai Trough was the Showa Nankai earthquake of magnitude 8.0 that occurred in 1946, and more than 70 years have already passed.

Mr. Shishikura did not know whether the next earthquake would be accompanied by a giant tsunami, and said, "If the tsunami is not huge, the shaking will surely increase. The Nankai Trough Earthquake should always be prepared for shaking, "he said.