Nankai Trough Earthquake Doubts on the probability of occurrence within 30 years “70 to 80%” Need for re-examination Soil heaving of land that may be the origin of construction: Tokyo Shimbun TOKYO Web

Murotsu Port in Muroto City, Kochi Prefecture, which serves as the basis for seismic predictions by the government’s earthquake research committee, which indicates the likelihood of a mega-earthquake occurring along the trench of Nankai within 30 years is 70-80%. A paper and research by Designated Professor Manabu Hashimoto (Seismology) of Tokyo Denki University and others revealed that the change in ground uplift in the area may have been caused by the construction of the port during the period. Edo, and not by seismic activity. (Keiichi Ozawa)

A survey by this article and specially appointed professors at Tokyo Denki University, announced at a university conference on 19

The specially appointed Professor Hashimoto (seismology) will give a presentation at the Japan Society for Natural Disaster Science to be held in Shiga Prefecture on the 19th. Some have long questioned the credibility of the 30-year probability of the Nankai Trough, this which is likely to affect future discussions.

The 70-80% probability was calculated using the “temporal prediction model” proposed in 1980 by Kunihiko Shimazaki, professor emeritus of seismology at the University of Tokyo, and others, predicting the next earthquake in the mid-2030s. This model hypothesizes that the greater the ground uplift caused by an earthquake, the more energy it releases and the longer it takes to store energy for the next earthquake . Based on the survey records of seabed depth changes due to the uplift of Murotsu Port recorded in historical documents from the Edo period to the beginning of the Showa period, we predict when the next earthquake will occur.

However, this article and a bibliographic study by the specially named professor Hashimoto revealed that construction was repeated almost every year during the Edo period at the port of Murotsu. There is a strong possibility that the depth of the harbor has been artificially changed, and the possibility that the survey values ​​of Murotsu Harbor are inappropriate as the data used as the basis for the prediction has come to light.

◆ The probability is about 50% when calculating taking into account the survey error

Moreover, it is clear from the literature that survey errors do occur. There are no detailed records in the literature, such as the time and place of the survey. According to modern observations, there is a difference of about 50 centimeters in the tide level at Muroto between high tide and neap tide, but this difference is not taken into account in the model. Project professor Hashimoto made a trial calculation with the current model taking into account the error, and the 30-year probability dropped to about 50%.

The Hashimoto project professor points out: “It has become clear that credibility is considerably questionable. It is essential to re-examine the probability. Nao Hirata, professor emeritus at the University of Tokyo and chairman of the earthquake research committee, said: “We have to wait for experts to develop an opinion against the model, so there are no plans to review it. for the moment.”

Nankai Trough earthquake 30-year probability problemEarthquake forecasts along the ocean trenches have been released for six locations across the country, including along the Sagami Trench. The Nankai Trough is the only one of them to use a method called “temporal prediction model”, and it is 70-80% calculated from uplift data from Murotsu Port in Kochi Prefecture. If you use a simple averaging model that averages the intervals between seismic events, as in other regions, it drops to about 20%. When the probability of the Nankai Trough was determined by the Earthquake Research Committee in 2013, seismologists questioned the credibility of the temporal prediction model. There was also a proposal that “at least the 20% figure should be published with both arguments”, but disaster risk reduction experts fiercely opposed this, saying: “The disaster risk reduction budget will not decrease”, and the temporal forecasting model was adopted. . It became 70-80%.

The need to prepare for the Nankai Trough earthquake forecast remains unchanged

The Earthquake Investigation Committee adopted the temporal prediction model when formulating the 30-year probability in 2013, saying that “there are doubts about its credibility”, but that “it does not There is no theory that can be completely denied”. Since then, there have been no discussions on the suitability of the model, and this article seems to cause a stir on the Nankai Trough earthquake prediction based on the temporal prediction model.

Without using a temporal prediction model, the probability drops to 20%, but this value is still a high probability that an earthquake could strike at any time, and one should always be prepared.

Although there were doubts based on the model, it was drowned out by voices aiming to secure a budget for disaster prevention by showing high probability. Promoting disaster prevention by intentionally making the probability of the Nankai pit appear high does not give the public a real sense of crisis. Other regions are also affected.

Kumamoto Prefecture, where the 2016 Kumamoto earthquake occurred, announced that the probability of occurrence was lower than that of a Nankai Trough earthquake and attracted business. It is possible that the area where the probability seems to be low was born careless and extended the damage. Some people in the affected areas of Hokkaido cried, saying, “I thought the Nankai Trough would be next, but it was a surprise attack.”

Japan is an earthquake-prone country, so it’s no surprise that an earthquake can happen anywhere and anytime. Unless the government and experts are swayed by probabilities, discuss how far they can scientifically tell, and continue to provide detailed explanations after demonstrating the current capabilities of seismology, we must be prepared for “unfortunate events.” .

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