Photo/Illutration A tsunami advisory, in yellow, was issued by the Japan Meteorological Agency on Oct. 9 after a quake struck at 5:25 a.m. near Torishima island. (Captured from Japan Meteorological Agency website)

Scientists remain puzzled over what caused the tsunami that was observed over a wide part of Japan on Oct. 9.

Normally, the Japan Meteorological Agency will issue a tsunami advisory within three minutes of an earthquake if a wave of at least 20 centimeters is expected.

But the advisory issued on Oct. 9 came after the tsunami was spotted in various areas.

By 9:30 a.m. on Oct. 9, tsunami ranging between 60 and 30 cm had reached the Izu island chain as well as Kochi Prefecture on the main island of Shikoku and Kagoshima Prefecture on the main southern island of Kyushu, according to the JMA.

Agency officials held a news conference at 8:40 a.m. on Oct. 9 and admitted they could not pinpoint the magnitude of the earthquake that likely triggered the tsunami.

The officials said an earthquake that struck at 5:25 a.m. near Torishima island in the Izu chain was the likely cause, but they added that the tsunami might have been generated by other factors not normally observed.

While there are fewer observation points in the ocean compared to land, underwater earthquakes can be measured for magnitude if they are of a certain size.

However, the inability of JMA officials to determine a magnitude for the Oct. 9 quake indicates it was likely smaller than usual.

The tsunami could have been caused by an underwater volcano eruption or a quake-triggered seabed landslide.

But JMA officials said they had not uncovered enough details to determine the cause.

Osamu Sandanbata, an assistant professor of geophysics at the Earthquake Research Institute at the University of Tokyo who has studied earthquakes and tsunami near Torishima island, called the latest tsunami “an unusual phenomenon.”

He said it will be difficult to immediately specify the cause, despite the interest among many scientists over what had occurred.

“There is a high possibility that the tsunami was linked to volcanic activity on the seabed,” Sandanbata said.

Seismic activity has occurred since about Oct. 2 about 100 kilometers southwest of Torishima island, including a magnitude-6.5 earthquake at 11 a.m. on Oct. 5.

But an analysis of the seismic waves of those quakes produced a pattern different from normal tremors in the area. This indicates that underground magma might have had an effect.

The active volcano Sofugan is located near the epicenters of the recent series of quakes. An eruption of the underwater volcano or magma movement might be behind the Oct. 9 tsunami.

“There is the possibility that volcanic activity has moved to the next stage with the eruption of magma,” Sandanbata said. “Because we do not completely understand the mechanism that caused the tsunami, caution should continue to be observed because we cannot deny the possibility that an even larger tsunami might strike.”

Underwater volcanic activity has led to major damage in other areas.

In 2018, a volcanic island in Indonesia collapsed, triggering a huge tsunami that killed about 400 people along the coast.

In Japan, a collapse of a mountain in Nagasaki Prefecture in 1792 resulted in a tsunami that inundated Kumamoto Prefecture on the opposite shore. About 15,000 people died in the disaster.

“Tsunami can be caused by volcanic eruptions and sector collapse of volcanoes,” said Yasuhiro Ishimine of the Yamanashi prefectural government’s Mount Fuji Research Institute. “There is also the possibility the cause is an underwater volcano we are unaware of.”

He called for preparations to evacuate immediately because the lack of observation equipment in the outlying islands means there is a shorter window between the issuing of a tsunami advisory and the actual evacuation.

(This article was written by Ryo Sasaki and Takahiro Takenouchi.)

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