Nankai Trough megaquake: Two occurrence probabilities listed side by side - Earthquake Research Committee states "could occur at any time"

The government's Earthquake Research Committee has revised the 30-year probability of a Nankai Trough megaquake occurring from the previous "approximately 80%" to "approximately 60-90% or higher." This was computed using a new calculation method that accounts for errors in historical ground uplift data and other factors. The committee also listed the figure of "20-50%" alongside this, calculated using a different model employed for earthquakes in other regions. Both probabilities represent "the highest rank in the three-tier probability classification system."

This is the first time two occurrence probabilities have been listed for a single earthquake, demonstrating the limitations of earthquake probability science. While disaster prevention authorities of the designated area for countermeasure promotion have expressed confusion, the danger level of Nankai Trough megaquakes have not changed, and the committee emphasizes that "the situation remains one where it could occur at any time" (Committee Chair Naoshi Hirata). Rather than being swayed by the range of figures, vigilance and preparedness to minimize damage from the megaquake are essential.

Nankai Trough megaquakes are an earthquake of approximately magnitude 8-9 that occur along the trough (a trench-like undersea topography) extending from Suruga Bay in Shizuoka Prefecture to the Hyuganada Sea off the eastern coast of Kyushu. It is known that at least nine such earthquakes have occurred since 684 CE. The interval between earthquakes is generally 100-150 years, and approximately 80 years have passed since the previous occurrence.

The committee has annually calculated the occurrence probabilities of earthquakes anticipated at submarine trenches around Japan and active faults nationwide as of January 1 each year, updating and publishing them as necessary. Probabilities for a Nankai Trough megaquake have been published since 2013, but the meaning of the two listed probabilities is admittedly difficult to understand, requiring an understanding of how each figure was derived.

Calculation method revised: "approximately 60-90% or higher"

The first is "approximately 60-90% or higher," derived by incorporating new research findings.

In March 2011, the Tohoku-Oki earthquake that caused the unprecedented Great East Japan Earthquake Disaster occurred, intensifying social demand to "know the occurrence probabilities of submarine trench-type megaquakes and major earthquakes that cause severe damage." In response, the committee's 2013 long-term evaluation used a calculation method based on past earthquake occurrences in the Nankai Trough and crustal deformation (uplift amounts) at Murotsu Port (Muroto City, Kochi Prefecture) from three earthquakes since 1700, setting the 30-year probability at "60-70%."

The calculation method used here is called the "time-predictable model." After a major earthquake occurs, a calm period continues for a while, but eventually the landward plate is dragged along as the oceanic plate subducts. Strain gradually accumulates at the boundary between the two plates, and when it reaches its limit, the landward plate rebounds, causing another major earthquake. This seismological theory forms the premise of the model.

The occurrence probability based on the time-predictable model increases over time. It was raised to "approximately 80%" in January of this year.

This time, in addition to reflecting research findings indicating errors in the uplift data at Murotsu Port, data uncertainty was also considered. Recalculations were performed based on a "slip-size dependent BPT model" that combines the conventional time-predictable model with the BPT model used for earthquakes in other regions. The result of the recalculations is the figure "approximately 60-90% or higher."

"20-50%" still represents highest danger rank

The other figure is "20-50%," listed for the first time.

According to materials published by the committee, there is no uplift data at Murotsu Port for the six earthquakes from the 684 Hakuho (Tenmu) earthquake through to the 1605 Keicho earthquake used in probability calculations for Nankai Trough earthquakes. However, uplift data exists for the three subsequent earthquakes: the 1707 Hoei earthquake, the 1854 Ansei earthquakes (Tokai and Nankai earthquakes) that likely occurred in tandem, and the 1946 Showa earthquake (Showa Nankai earthquake). Therefore, conventional probability calculations for the Nankai Trough megaquake specifically incorporated data from these three past earthquakes.

In long-term evaluations of earthquakes other than the Nankai Trough, uplift data from Murotsu Port naturally cannot be used, and since similar data does not exist, the BPT model is employed rather than the time-predictable model. This time, the figure "20-50%" was calculated for the probability of a Nankai Trough megaquake using only the BPT model, just as for other earthquakes.

Not a choice Between "which is more appropriate"

The background to listing these two figures includes criticisms and questions about the conventional probability, such as "errors in uplift data at Murotsu Port are not being considered" and "isn't the occurrence probability inflated because a special model is used only for the Nankai Trough megaquake?" In response to these concerns, the committee conducted detailed reexamination before deciding to list both figures.

To communicate the danger level of submarine trench-type earthquakes in an understandable way, the committee divides the danger level based on occurrence probability into three ranks. The highest danger level is "Rank III," which applies when the probability value is 26% or higher. Even the lower of the two current probabilities, "20-50%," falls into Rank III.

The committee explains that "earthquake probabilities are calculated using two models, and since scientific superiority cannot be determined, both are listed." Furthermore, it states that "based on the principle of 'act when in doubt,' it is preferable to emphasize the higher figure of 'approximately 60-90% or higher.'" This can be described as a recommendation from the disaster prevention perspective.

It is incorrect to combine the two probabilities derived from separate calculation methods and interpret them as "approximately 20-90% or higher." While the debate over which model or calculation method is more appropriate may be meaningful seismologically (that is, scientifically), from the disaster prevention and mitigation perspective, it should not be treated as a binary choice. What matters is that a Nankai Trough megaquake "will definitely occur at some point."

Share a Sense of Urgency and Advance Disaster Prevention Measures

On July 1, the government revised the "Basic Plan for Disaster Risk Reduction" at the Central Disaster Management Council meeting (chaired by then-Prime Minister Shigeru Ishiba). For a Nankai Trough megaquake, it set a goal of reducing the anticipated worst-case death toll of 298,000 by 80% over the next 10 years. The revised basic plan also aims to significantly reduce the economic damage estimated at up to 292 trillion yen, although no numerical target was set. The designated areas for countermeasure promotion were expanded to 723 municipalities by adding 16 municipalities across six prefectures. The main points of the plan include developing tsunami evacuation facilities, promoting seismic retrofitting of buildings, strengthening evacuation drills, and supporting regional countermeasure planning at the municipal level.

Regional disaster prevention and mitigation measures are also advancing based on these plans. Consequently, voices of confusion have reportedly been heard from designated countermeasure areas regarding the revision of the probability calculation method and the listing of two probabilities. While the confusion in the designated area is understandable, it must not be forgotten that whatever the figures may be, they are merely one guideline for crisis management.

No new phenomena such as crustal deformation that would increase the danger level, or conversely, new findings that would decrease the danger level, have been identified for this revision. In other words, it simply amounts to revising the conventional calculation method based on the latest knowledge while simultaneously calculating using an alternative method (model).

Hirata also stated at a press conference that "earthquakes are uncertain phenomena, and we cannot specify when they will occur," urging that "(precisely for this reason) we want disaster prevention measures to continue." When it comes to not only megaquakes like the Nankai Trough but also active fault-type earthquakes, which are described as making "everywhere in the country dangerous" (Hirata), it is important for each individual in each region to share the sense of urgency that they "could occur tomorrow—or even today."