JMA prepares for ‘mysterious tsunami’ to hit Izu Islands

When an earthquake struck the waters off Torishima Island on September 24, it created a sense of urgency among residents. The Japan Meteorological Agency office is in Tokyo, almost 600 kilometers to the north.

This is because the JMA was afraid a phenomenon known as the “mystery tsunami” that occurs once every 10 years in the Izu Islands.

This is the first time the JMA has given advance warning of such a giant wave.

This was due to technological advances as well as the ingenuity and work of JMA personnel.

“Artificial intelligence (AI) will not be able to do this,” a JMA spokesman said.

The magnitude of the earthquake, which occurred at 8:14 a.m. on September 24, was estimated at 5.8 on the Japanese seismic intensity scale of 7.

This set off alarms in the JMA’s earthquake and volcano operations room on the fifth floor of the agency’s office in Tokyo’s Minato Ward.

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

Some studies indicate a connection to a nearby undersea volcano, but JMA has been unable to establish a clear connection.

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

When determining the causes of a tsunami, many factors must be taken into account, such as the depth of the earthquake source.

However, with the exception of the “mystery tsunami”, there have been several cases of tsunamis caused by earthquakes of magnitude less than 6.5.

An increase in value by one is equal to a 30-fold increase in energy.

A “moderate earthquake” of about magnitude 6, which is smaller in magnitude than a “large earthquake” of magnitude 7 or greater, will usually produce a small number of tsunamis.

Earthquakes of magnitude 7 or greater are called “major” earthquakes. Earthquakes smaller than this value and magnitude around 6 are called “moderate” earthquakes.

Moderate earthquakes usually do not cause tsunamis.

However, Tatsuo Kuwayama, a researcher at the JMA’s Earthquake and Tsunami Monitoring Division, said the JMA considers the moderate tsunami earthquake to be a unique event and is studying it.

Regarding the decision to issue warnings and advisories for tsunamis that occur only a few times a year, including the “mystery tsunami,” JMA staff share information and conduct daily drills on similar scenarios, Kuwayama said.

In addition, five days before the September 24 earthquake, the JMA issued a warning about a possible underwater eruption on Sumisujima Island, which is about 100 km north of Torishima Island.

Kuwayama said there were signs of a tsunami and that staff were “prepared for it.”

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

A tsunami of 4 cm or more in height was observed over a wide area along the Pacific coast from Chiba Prefecture to Kagoshima Prefecture.

A tsunami up to 70 cm high was observed on Hachijojima Island at 8:58 am.

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

Therefore, speed is of utmost importance when issuing tsunami warnings.

The JMA issues recommendations using a database of 100,000 models of when, where and with what force a tsunami will hit.

This modeling is performed based on the location, depth and scale of the source, and the direction of 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 underway for what is happening.

During the September 24 earthquake, a tsunami warning was issued six minutes after the earthquake.

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

Shigeki Aoki, JMA earthquake and tsunami response planning officer, said the time difference was “due to the decision of qualified personnel.”

Offshore areas, such as around Torishima Island, have fewer seismic observation networks than on land, making it difficult to determine the source, which is one of the most important factors in tsunami forecasting.

On September 24, the day of the earthquake, JMA operating room staff with specialized knowledge and experience and “national team leaders” in charge of the operating room, among others, carefully compared the earthquake waveform. with similar earthquakes that have caused tsunamis in the past.

Then they decided to issue a warning.

There have been earthquakes in the past that have killed more than 20,000 people in tsunamis, even when the magnitude of the earthquake was only 2-3 – such as the 1896 Meiji Sanriku tsunami.

In addition, in January 2022, an underwater volcano erupted on the island of Tonga in the South Pacific, causing a tsunami.

The “mysterious tsunami” in the waters near Torishima Island is not the only case of a tsunami occurring even when no strong shock is felt.

Aoki said of the Sept. 24 operation, “Since this is a rare occurrence, the decision to issue a warning should have been made by experienced officials. I don’t think machines or AI can do it.”

November 5 is World Tsunami Awareness Day, which is important to increase public vigilance.

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

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

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

However, the generation mechanism of volcanic tsunamis is not well understood in many regions, and tsunamis caused by volcanic activity are difficult to predict.

“This is an area that is difficult to relate to disaster prevention,” Sandanbata said.

The source of the September 24 earthquake was in an area of ​​the ocean where tsunamis have repeatedly occurred approximately once every 10 years.

“In some ways, the Sept. 24 earthquake had predictable characteristics,” Sandanbata said. “Other volcanoes can produce the same tsunami, so unexpected tsunamis are possible.”

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

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