南海なんかいトラフ巨大きょだい地震じしん 30さんじゅう年ねん以内いない発生はっせい確率かくりつ 2つの確率かくりつを新あらたに算出さんしゅつ

Regarding the possibility of a massive earthquake with a magnitude of 8 or higher occurring in the Nankai Trough, the Earthquake Research Committee has stated that as of January this year, there is about an 80% chance of it happening within the next 30 years.

This is calculated based on the idea that factors such as the interval between earthquakes and the magnitude of the previous earthquake affect the timing of the next earthquake.

One of the bases for the calculation was the crustal movement data of Muroto City’s port in Kochi Prefecture, which rises with each earthquake. The amount of uplift is determined from historical records, such as old documents that recorded the port’s water depth.

However, there has been ongoing debate regarding the accuracy of the records, and a paper has also been published that examined the uncertainty of the recorded water depth at the port.

For this reason, after carefully examining the records and taking the margin of error into account, the calculated probability of occurrence ended up being a wide range, from around 60% to over 90%.

Furthermore, as with earthquakes that occur along other trenches, when calculations were made using only the average interval at which earthquakes occur, without using crustal deformation data, the probability was found to be 20% to 50%.

The Earthquake Research Committee stated that neither of the two methods could be considered superior, but said, Both indicate that the urgency of a massive earthquake is already extremely high. Accordingly, they classified the likelihood of earthquakes occurring along the trench as the highest Rank III.

When explaining probabilities, it is recommended to emphasize the higher range, around 60% to over 90%, from the standpoint that disaster prevention measures need to be continued.

It has already been about 80 years since the Nankai earthquake in the Showa era, and if we were to express in numbers how likely we feel it is to happen at any time, it would be a high value of over 90%.

However, as the figure of 20% suggests, it’s not something that will definitely happen after 30 years, and I want you to understand that there is that much variation and uncertainty.

It remains classified as Rank III, which means there is a very high probability of a massive earthquake occurring, and if it does happen, extensive damage is expected.

Its not surprising if an earthquake happens at any time, so I want measures against earthquakes and tsunamis to be taken as soon as possible.

Even if the probabilities change, the fact that there are risks of earthquakes and tsunamis and the need for daily preparedness remain unchanged. If we do not clearly communicate how to interpret these probabilities, it could lead to confusion.

The probability of occurrence published by the Earthquake Research Committee scientifically conveys the results of academic studies, but just being shown the numbers makes it difficult to understand how to use them for disaster prevention actions or preparedness, and the current information alone is insufficient.

It is necessary not only to emphasize the numbers, but also to share information after considering measures that will ultimately lead to disaster prevention.

Why is the probability of a massive Nankai Trough earthquake being reassessed? Q1

It was said that there is about an 80% chance that a massive Nankai Trough earthquake will occur within the next 30 years.

Why was it reviewed this time? The Earthquake Research Committee says it is because there were various criticisms regarding the method of calculating the probability and the data used for the calculation.

The probability of a massive Nankai Trough earthquake occurring had been estimated using the amount of crustal movement ground uplift observed at Murotsu Port in Muroto City, Kochi Prefecture, during the three earthquakes that occurred in the Nankai Trough from the Edo period to the Showa period.

The data on this crustal movement had been estimated based on information such as the water depth of the port recorded in an old document called the Kubono Family Documents housed at a museum in Kochi Prefecture, as well as surveying data from that time. However, the latest research has pointed out that there are errors in these records, since at the time, work was being done to deepen the seabed around the port and multiple instruments were used for measurement, among other reasons.

Therefore, in this case, the calculation was performed using data that took errors into account.

Furthermore, the method for calculating probabilities has also been changed.

Until now, the calculation has been based on the time prediction model, which assumes that the interval until the next earthquake varies depending on the magnitude of the previous earthquake.

The Showa Nankai Earthquake that occurred in 1946 was smaller in scale compared to other Nankai Trough earthquakes, so it was calculated that the interval until the next earthquake would be shorter, resulting in a higher probability.

A new calculation method was also used for this estimation, taking into account the variability in the magnitude and occurrence intervals of earthquakes.

As a result, a probability range of about 60% to over 90% was obtained.

This time, two probabilities are listed: about 60% to 90% or higher and 20% to 50%.

In the Nankai Trough, large-scale earthquakes have repeatedly occurred since ancient times, so research has progressed, and as mentioned earlier, probabilities have been calculated using data such as old documents, past surveys, and crustal deformation data.

On the other hand, for earthquakes that occur along other trenches, there is little data available, so the probability is calculated solely based on the intervals between past earthquakes.

Regarding the Nankai Trough megaquake, as with other earthquakes along ocean trenches, the figure of 20% to 50% is a result calculated solely based on the occurrence interval.

A similar calculation method was used during the previous review in 2013, but due to opinions such as a low probability could reduce disaster preparedness awareness, only the high probabilities were emphasized in the public announcement.

This time, it was decided to list the probabilities calculated by both methods, due to comments from committee members such as Only presenting the high probability is not scientifically correct.

Naoshi Hirata, the chairman of the Earthquake Research Committee and Professor Emeritus at the University of Tokyo, said, In this revision, because both the way earthquakes occur and our knowledge about them are uncertain, we presented two probabilities in order to properly indicate the uncertainty of these probabilities.

On the other hand, regarding the presentation of two probabilities of occurrence, Professor Naoya Sekiya of the University of Tokyo Graduate School, who specializes in disaster information, said, From a general perspective, it is difficult to understand.

Even if the probability changes, the fact that there are risks of earthquakes and tsunamis and the need for daily preparedness do not change. He points out that if the way we interpret probability is not communicated clearly, it could cause confusion.

On top of that, he said, Its important not just to emphasize the numbers, but to share information after considering measures that will ultimately lead to disaster prevention.

How should we interpret these different probabilities? Regarding the two probabilities, the Earthquake Research Committee stated, Based on current scientific knowledge, we cannot determine which is more reliable, and added, Both probabilities indicate that the urgency of a major earthquake is already very high.

Furthermore, from the perspective of wanting countermeasures to be implemented as soon as possible, it is considered desirable for the national and local governments to emphasize the higher probability, such as about 60% to 90% or more, when presenting probabilities.

The probability of an earthquake occurring is indicated using four ranks, and when looking at the two probabilities with these evaluations, both are in Rank III, which is 26% or higher, making them the highest rank as before.

Chairman of the Earthquake Research Committee and Professor Emeritus at the University of Tokyo, Dr. Nao Hirata, said, It has already been about 80 years since the Showa Nankai Earthquake, so if I were to express in numbers the feeling that it wouldn’t be surprising if an earthquake occurred at any time, it would be a high value of over 90%.

However, as the figure of 20% suggests, it’s not something that will definitely happen after 30 years, and I want you to understand that there is that much variation and uncertainty.

On top of that, the Rank III, which indicates a very high probability of a massive earthquake occurring, remains unchanged, and if it does occur, extensive damage is expected.

It wouldnt be surprising if an earthquake happened at any time, so I urge you to take measures against earthquakes and tsunamis as soon as possible.

This time, although the probabilities have been revised, the situation remains unchanged: a massive Nankai Trough earthquake could occur at any time.

In addition to securing furniture and making it earthquake-resistant, it is necessary to be aware of evacuating to higher ground immediately when shaking occurs, and to steadily make preparations.

At that time, instead of referring to it as the Nankai Trough Megaquake, the focus was on the Nankai Earthquake and the Tonankai Earthquake, with their source regions off the coast of Shikoku and off the coast of Lake Hamana in Shizuoka Prefecture, respectively. The probability of occurrence for each was calculated and published, and both earthquakes were assumed to be of magnitude 8 class.

After assuming that the largest possible earthquake in the Nankai Trough would be around magnitude 9, it was announced in 2013 that the probability of such an event occurring within the next 30 years was 60% to 70%.

This probability gradually increases year by year if no earthquake occurs, so it was raised to about 80% in January this year.

Other Rank III earthquakes... The Earthquake Research Committee has also announced the probability of earthquakes other than the Nankai Trough megaquake occurring within the next 30 years.

As of January this year, the data shows that most earthquakes occurring along the trench are classified as Rank III, indicating the highest likelihood of occurrence.

The probability of earthquakes off the coast of Shikotan Island and off the coast of Etorofu Island with a magnitude around 5 is about 60%, etc.

The probability of an earthquake of around magnitude 9 off the eastern coast of Aomori Prefecture and the northern part of the coast off Iwate Prefecture is 20% to 40%.

The probability of an earthquake with a magnitude around 4 occurring near the coast off Miyagi Prefecture is 80% to 90%.

The Earthquake Research Committee explains how to understand the probability of occurrence by comparing it to the probability of being involved in a traffic accident.

They are calling for people to be prepared for earthquakes on a daily basis, such as by securing furniture and making buildings earthquake-resistant, as the risk is higher than, for example, 2%.