For months, experts were at a loss as to what caused the tsunami observed over a wide stretch of the Pacific Ocean in October last year.

No major earthquakes were detected that day.

But the Japan Coast Guard said it may have cracked the mystery.

The JCG announced that it found the most likely culprit as it located an underwater volcanic crater believed to have erupted within a year near Torishima island in the Izu island chain south of Tokyo.

The tsunami occurred on the morning of Oct. 9.

According to the Japan Meteorological Agency, the tsunami was observed in wide areas stretching from the Kanto region to Okinawa Prefecture, as well as the Izu and Ogasawara island chains.

It measured up to 70 centimeters on Tokyo's Hachijojima island.

In a sea area southwest of Torishima island in the Izu island chain, which is believed to be the source of the tsunami, four quakes in the class of a magnitude 6 to 7 were observed between Oct. 3 and 6.

Several small quakes also occurred on Oct. 9.

But because no major earthquakes that could trigger tsunami were observed, the JMA issued a tsunami advisory only after the ocean waves reached Hachijojima.

Although it was pointed out that the tsunami could have been triggered by an underwater volcanic eruption or a seabed landslide, the cause remained a mystery.

The government’s Earthquake Research Committee also said during a news conference on Oct. 11 that it had yet to determine the cause of the tsunami at that point.

In January, a JCG survey vessel conducted acoustic surveys of the seabed topography near Torishima.

The data that was obtained was compared with that taken in December 2022 to analyze whether there were any significant changes to the seafloor in the past year or so.

It was learned that a crater about 1.6 kilometers in diameter is forming inside a caldera of Sofukaizan, a submarine volcano located south of Torishima.

A portion of the volcano had disappeared, losing about 430 million cubic meters in volume, equivalent to about 350 Tokyo Domes.

One section of the crater is now 451 meters deeper than before.

The slope of the crater had also collapsed, shrinking about 140 million cubic meters in volume.

About 10 days after the tsunami was detected, pumice believed to have come from a volcano was found in the sea area surrounding Sofukaizan.

An analysis showed that the pumice formed recently because there were no living organisms attached to it.

The JCG concluded that it was highly likely that the traces of this eruption are associated with the mysterious tsunami, citing that the seabed topography has changed significantly within just about a year and that volcanic pumice was found in the surrounding sea area.

"It is rare for the seabed topography to change considerably in one year," said an official from the JCG's Hydrographic and Oceanographic Department. "It is possible that the upper part of the volcano was blown away by the eruption, causing the tsunami."

A total of 14 minor quakes occurred between around 5 a.m. and 6:30 a.m. on Oct. 9, triggering a series of 14 tsunami.

That was according to an analysis conducted by a research team led by Osamu Sandanbata, an assistant professor of geophysics at the University of Tokyo’s Earthquake Research Institute, who is well-versed in earthquakes and tsunami occurring near Torishima.

When the waveforms of the earthquakes and those of the tsunami were compared with each other, it was learned that the interval between the tsunami gradually shortened.

In particular, for the six that occurred in the latter half of the series, the interval between the tsunami overlapped the cycyle of the next tsunami, causing a rare phenomenon of doubling their height.

"It is an achievement that we found a crater formed within about a year, and we want to continue our research further to find out how each earthquake and tsunami was triggered during the eruption process," Sandanbata said.

Compared to regular tsunami resulting from earthquakes caused by fault slip and other factors, it is difficult to forecast tsunami generated by volcanic eruptions and other changes to the seafloor.

Whether a volcano is active can be determined based on plumes on the sea surface and other signs.

But no plumes or changes in color of the sea surface were confirmed in the sea areas near Torishima.

"Tsunami are usually associated with seismic activities, but just like the recent tsunami generated by underwater volcanic activities, they can occur when there is no significant shaking," Sandanbata said. "It is important to be aware of such conditions and be prepared."