When an earthquake occurred on Sept. 24 in the waters near Torishima island, it set off a sense of urgency at the Japan Meteorological Agency office in Tokyo, nearly 600 kilometers north. 

That is because the JMA feared the phenomenon known as a “mysterious tsunami” that occurs once every 10 years in the Izu islands. 

The JMA for the first time issued a warning against such a giant wave in advance. 

This was due to technological advances as well as the resourcefulness and work of the JMA staff. 

“Artificial intelligence (AI) can’t do it,” a JMA official said. 

The magnitude of the quake that occurred at 8:14 a.m. on Sept. 24 was estimated to be 5.8 on the Japanese seismic intensity scale of 7. 

That set off alarm bells at the JMA's earthquake and volcano operations room on the fifth floor of the agency's office, located in Tokyo’s Minato Ward.

When earthquakes of a similar magnitude occurred in the waters around Torishima island in 1984, 1996, 2006, 2015 and 2018, tsunami were observed.

Some studies have pointed to a relationship with a nearby undersea volcano, but the JMA has been unable to make a clear connection.

Even when tsunami exceeding the 20-centimeter threshold were observed, the JMA could not send out warnings in advance, issuing them only after the occurrence of a larger tsunami. 

There are many factors to consider in the causes of a tsunami, such as the depth of the quake's focus.

However, with the exception of “mysterious tsunami,” there have been few cases of tsunami generated by earthquakes smaller than about a magnitude 6.5.

An increase of one in magnitude equals a 30-fold increase in energy.

A “medium earthquake” of about a magnitude 6, which is smaller in magnitude than a “major earthquake” of a magnitude 7 or greater, usually produces few tsunami.

Earthquakes of a magnitude 7 or greater are called “major” quakes. Earthquakes smaller than that and of about a magnitude 6 are called “moderate” quakes.

Moderate quakes usually do not generate tsunami.

However, Tatsuo Kuwayama, a research officer of the JMA’s earthquake and tsunami monitoring division, revealed that the JMA considers a moderate quake that generates tsunami a unique case and has been studying it.

Regarding the decision to issue warnings and advisories for tsunami, which occurs only a few times a year, including “mysterious tsunami,” the JMA staff have been sharing information and conducting daily drills for similar scenarios, Kuwayama said.

In addition, five days before the Sept. 24 earthquake, the JMA had issued an eruption warning for the surrounding sea area for a possible undersea eruption on Sumisujima island, which is about 100 km north of Torishima island.

Kuwayama said that there were signs of a tsunami, and that the staff “were prepared for one."

In fact, at 8:20 a.m. on Sept. 24, six minutes after the earthquake hit, the JMA issued a tsunami advisory for the Izu and Ogasawara islands.

Tsunami of 4 cm or more were observed over a wide area along the Pacific coast from Chiba Prefecture to Kagoshima Prefecture.

A tsunami of up to 70 cm was observed on Hachijojima island at 8:58 a.m.

In some cases, tsunami reach the coast quickly after an earthquake.

So, speed is of the upmost priority when issuing tsunami advisories.

The JMA issues advisories using a database that stores 100,000 simulations of when, where and with how much force a tsunami will strike.

These simulations are made based on the location, depth and scale of the focus, and the direction of the fault slip, among other factors.

In 2015, the database was expanded to include cases in the waters around Torishima island. Since then, preparations have been under way for occurrences. 

In the Sept. 24 quake, a tsunami warning was announced six minutes after the earthquake.

In the case of a normal earthquake and tsunami, the warning would have been issued within three minutes.

Shigeki Aoki, the JMA’s earthquake and tsunami response planning officer, said that the difference in time “was due to the judgment of skilled staff.”

In offshore areas such as around Torishima island, there are fewer seismic observation networks than on land, making it difficult to determine the focus, which is one of the most important factors in predicting tsunami.

On Sept. 24, the day of the earthquake, the JMA staff of the operation room, who had specialized knowledge and experience, and the “national team leaders” in charge of the operation room, among others, carefully compared the waveform of the earthquake with similar quakes that had generated tsunami in the past.

They then decided to issue a warning.

In the past, there have been earthquakes that killed more than 20,000 people in tsunami even when the quake intensity was only a magnitude 2 to 3—for example, the 1896 Meiji Sanriku Tsunami.

Also, in January 2022, an undersea volcano erupted in the South Pacific island nation of Tonga, causing a tsunami.

The “mysterious tsunami” in the waters near Torishima island is not the only case of a tsunami occurring even when no major tremor is felt.

Aoki said about the Sept. 24 operation: “Because this is a rare phenomenon, it was necessary for experienced officials to make the decision to issue the warning. I don’t think machines or AI can do that.”

Nov. 5 is World Tsunami Awareness Day, which is important for increasing the public's vigilence. 

“The ability to issue accurate warnings is important to increase people’s trust," Aoki said. "Even if you don’t feel any tremors, if the JMA issues a warning, I hope people will take action, such as moving away from the coast.”

Osamu Sandanbata, an assistant professor of geophysics at the University of Tokyo’s Earthquake Research Institute, who is an expert on earthquakes and tsunami in the sea around Torishima island, said, “It is very impressive that the warning was issued based on past experience and knowledge of the waters around Torishima island.”

There are more than 30 known offshore volcanoes in the waters surrounding Japan.

However, the mechanism of volcanic tsunami generation is not well understood in many areas, and predicting tsunami caused by volcanic activity is difficult.

“It is a difficult field to link to disaster prevention,” Sandanbata said.

The focus of the Sept. 24 earthquake was in an area of the ocean where tsunami have repeatedly occurred about once every 10 years.

“In a way, (the Sept. 24 quake) had predictable characteristics," Sandanbata said. "Other volcanoes can cause such tsunami, so unexpected tsunami are possible.”

He added, “In addition to academic research on the mechanism of tsunami, it is important to improve the accuracy of forecasts by expanding tsunami observation and volcano monitoring systems.”

(This article was written by Fumi Yada and Shoko Rikimaru.)