A Japanese research team says shock waves running through the air were observed across Japan about seven hours after Saturday's volcanic eruption off Tonga.

The team at Kochi University of Technology has observation equipment at 25 locations across Japan. The devices are designed to detect shock waves in frequencies too low to be felt by humans, and which are produced by earthquakes or volcanic eruptions.

The team says shock waves with the same characteristics were observed at all of the locations after the eruption.

The team says its analysis so far shows that the equipment began detecting shock waves with long wavelengths about seven hours after the eruption.

It also says that from about 25 minutes later, the equipment repeatedly logged strong shock waves with shorter wavelengths for about an hour.

Yamamoto Masayuki, a professor at the university, said it is extremely rare for shock waves with the same characteristics to be observed across the country simultaneously. He said the finding underlines the scale of the eruption.

The team will share the data with other experts in the country for further analyses.

Yamamoto said that by analyzing changes in the waves' characteristics, he hopes to uncover the mechanism of the tsunami that reached Japan, about 8,000 kilometers away from the volcano.