Due to the Great East Japan Earthquake, the assumptions of the earthquake and tsunami had to be greatly revised.
What has emerged from the investigation of past traces is that apart from the cycle of large earthquakes that occur in units of decades to 100 years, earthquakes that can be said to be "ultra" huge earthquakes that cause enormous damage over a wide area occur in units of hundreds of years. There is a cycle called "super cycle" that occurs in.
In the last 10 years of research, we have come to see the existence of "super cycles" in various parts of the coast of Japan, and where the outbreak may be imminent.
Masanobu Shishikura, head of the research group at the National Institute of Advanced Industrial Science and Technology, said that just before the Great East Japan Earthquake, a large earthquake was envisioned at that time based on a survey of tsunami deposits that estimated what kind of tsunami had hit from past geological formations. He thought that the huge earthquake and tsunami of the "super cycle", which far exceeded, was imminent on the coast of Tohoku, and called for countermeasures.
However, the Great East Japan Earthquake occurred without making use of that knowledge.
Nankai Trough ・ Is the next earthquake a super cycle?
Shishikura, who thought that there was a risk of a huge "super cycle" earthquake and tsunami in various places, examined the past traces of the Nankai Trough earthquake, which often does not know the scale and mechanism of the earthquake. It was.
Note that fossil clusters of seaside creatures such as barnacles and worms were found one after another at different heights in Kushimoto Town, Wakayama Prefecture, on the Kii Peninsula, which is located almost in the center of the epicenter.
When we examined the ages of fossils for about 5500 years, we found that it was highly possible that a huge earthquake had occurred due to the large uplift of the ground in a cycle of approximately 400 to 600 years.
Group leader Shishikura said that the last giant earthquake in the "Super Cycle" was the 1707 Hoei Earthquake, which is more than 300 years old, so the next earthquake will cause enormous damage over a wide area. I think it could be a huge "cycle" earthquake.
Is the Kuril Trench / Super Cycle Imminent?
Furthermore, it seems that a huge "super cycle" earthquake is imminent in the "Kuril Trench" off the coast of Hokkaido.
As a result of examining the tsunami deposits for the past 6500 years by the National Institute of Advanced Industrial Science and Technology, it was found that a huge tsunami-causing earthquake seems to have occurred repeatedly once every 350 years on average.
Regarding the Kuril Trench, the National Earthquake Research Promotion Headquarters also announced the assumption of a tsunami, saying that a huge earthquake of magnitude 9 class is "highly likely to be imminent", but Shishikura group leader also said "already from the previous time. It's been about 400 years, and it's safe to wake up anymore. "
Furthermore, it is possible that a huge earthquake, which is a super cycle, has occurred off the coast of the Boso Peninsula in the Kanto region, and we are planning to proceed with the investigation in the future.
Group leader Shishikura said, "I was disappointed that the Great East Japan Earthquake was'not in time.'I realized that it was necessary to quickly disseminate the research results to society and make use of them for disaster prevention measures. In the past, huge earthquakes due to super cycles had occurred repeatedly. I think it is our mission to make the facts understood by many people, including those involved in local governments, and to move forward with disaster prevention measures for society as a whole. "
Nankai Trough Super Cycle / Fossils Prove
In determining the mechanism of the "super cycle" in the Nankai Trough, the Shishikura group leader focused on fossils of seaside creatures such as barnacles and worms found at the tip of the Kii Peninsula.
At the plate boundary of the Nankai Trough, the plate on the land side usually "subducts" slowly, and when an earthquake occurs, the tip part jumps up sharply.
The ground at the tip is "raised" at this time.
Barnacles and worms live in rocky areas near the surface of the sea, so if they rise together with the rocky areas, they cannot live and become fossils.
The Shishikura group leader collects fossils at more than 30 locations, mainly in Kushimoto Town, Wakayama Prefecture.
Each fossil is piled up like layers, and fossils with three layers were also found at intervals of 90 to 150 years.
This is thought to be a record of repeated "uplifts" and "subductions" of the ground, that is, past major earthquakes.
For the "measuring rod" of fossils and super cycles in remote areas
Furthermore, the Shishikura group leader paid attention to the fact that these layered fossils were found one after another from higher altitudes.
When a huge "super cycle" earthquake occurs, the scale of the "uplift" also increases.
For this reason, I thought that the difference in fossil height was the so-called "measure" for knowing the "super cycle."
Analysis of fossils from various locations over the past 5500 years confirmed at least seven times the "uplift" of the ground of a "super cycle" giant earthquake that was clearly different from the usual large earthquakes.
We found that the cycle was about 400 to 600 years.
Furthermore, recent observations have confirmed that "slow slip", in which the plate boundary slowly shifts 20 to 30 km underground on the Kii Peninsula, has not occurred in this area, and it seems that the underground plates are firmly attached to each other. Has been done.
For this reason, it is thought that a wide range shifts at a stretch and the ground rises significantly only in the event of a huge "super cycle" earthquake.
No fossils showing traces of the Nankai Trough earthquakes that occurred in 1854 in the Edo period and 1944 and 1946 in the Showa period have been found, and the last giant "super cycle" earthquake occurred in 1707. It is believed to be an earthquake.
Shishikura, the group leader, said that since more than 300 years have already passed, the next earthquake could be a huge "super cycle" earthquake, and "it is necessary to take measures to prepare for the worst." is.
The urgency of the Kuril Trench
It is believed that the occurrence of a huge earthquake due to the "super cycle" is particularly imminent in the "Kuril Trench" off the coast of eastern Hokkaido.
In the "Kuril Trench", ▽ 1973 (Showa 48) "Nemuro Peninsula offshore earthquake" and ▽ 1952 (Showa 27) and 2003 (2003) "Tokachi offshore earthquake", etc. off Nemuro and off Tokachi On average, huge earthquakes accompanied by tsunamis occur repeatedly at intervals of about 70 years.
On the other hand, surveys of tsunami deposits have revealed that in the 17th century there were huge earthquakes that far exceeded the magnitude of these earthquakes, and large tsunamis were flooding in.
From the survey of tsunami deposits for the past 6500 years, the "super cycle" in the Chishima Trench is about 350 years on average, and about 400 years have already passed since the last huge earthquake, so the government's seismic research According to the Promotion Headquarters, the occurrence of the next huge earthquake is "highly likely to be imminent" in the Chishima Trench.
According to the assumption announced by the government last year (2020), the magnitude of the huge earthquake was 9.3 at the maximum, and the height of the tsunami was estimated to exceed 20 meters in a wide area in eastern Hokkaido.
The country is currently considering damage estimates.
In addition, Masanobu Shishikura, Group Leader of the National Institute of Advanced Industrial Science and Technology, said that in the Kanto region, huge earthquakes that hit the "super cycle" have repeatedly occurred along the Japan Trench off the east coast of the Boso Peninsula and on the Sagami Trough off the south coast. As there is a possibility, we will proceed with the investigation in the future.
The Great East Japan Earthquake also occurred in the super cycle
It is believed that there is a "super cycle" of about 600 years even off the coast of Tohoku, where a huge earthquake occurred 10 years ago.
Large earthquakes have repeatedly occurred along the "Japan Trench" from east off Aomori prefecture to off Boso, and before the Great East Japan Earthquake, there was a magnitude 7 class Miyagi prefecture offshore earthquake that occurs about once every 30 years. , Earthquakes off Sanriku, etc., have been thought to occur once every few decades to a hundred years in multiple areas.
On the other hand, based on the survey results of tsunami deposits, Shishikura group leader thought that it is highly possible that huge earthquakes that caused multiple areas to shift at once occurred repeatedly at intervals of about 600 years.
Immediately before the Great East Japan Earthquake, Group Leader Shishikura said that the last giant earthquake caused by the "super cycle" was around the 15th century, and that about 600 years had already passed and the situation was imminent, and he was appealing to add it to the assumption.
Currently, the Headquarters for Earthquake Research Promotion of the country states that a huge earthquake with a magnitude of 9 that causes the entire area off the coast of Iwate prefecture to shift from the southern part off the coast of Iwate prefecture to occur at an interval of once every 550 to 600 years on average. It is said.
On the other hand, it has been pointed out that there is a "super cycle" in the area from the south offshore of Hokkaido to the offshore of Iwate prefecture in the "Japan Trench", and the country has announced the assumption of a magnitude 9.1 earthquake.
Super Cycle ・ How to prepare
In the Nankai Trough and the Kuril Trough, the national government has already announced the damage assumption as the "worst case" and is currently considering it.
The national and local governments are also taking measures, but the scale of possible earthquakes and tsunamis is large and it is difficult to protect everything with hardware.
Recall the lessons learned from the Great East Japan Earthquake and decide on evacuation sites and evacuation methods that suit the risks of the region.
It is also important to promote "pre-reconstruction" efforts, such as deciding how to reconstruct in advance and summarizing issues, by concretely imagining the situation of the disaster.