Tsunami caused by Tonga volcano eruption stumps Japan weather experts

Kyodo -- Jan 18
The eruption of an underwater volcano off Tonga on Saturday that caused tsunami waves of up to around 1 meter to hit parts of Japan's coast stumped weather experts, as volcanic eruptions are said to rarely cause a rise in tide levels.

Hunga Tonga-Hunga Ha'apai, an underwater volcano located about 8,000 kilometers from Japan, erupted around 1:10 p.m. Saturday Japan time, resulting in a plume of smoke reaching a height of 16,000 meters and blanketing the area in a thick film of volcanic dust, according to satellite imagery.

Japan's weather agency released information at 7:03 p.m. saying slight changes in sea levels were expected but initially denied there was cause for alarm.

But it changed its tune and decided to issue a tsunami warning and advisory after tide levels continued to rise along Japan's Pacific coast from around 8 p.m.

The tidal changes, which came two and a half hours earlier than expected, did not match the characteristics of a tsunami, the agency said, adding at a press conference later that it "did not know the mechanism."

Tsunami caused by earthquakes occur when the violent movement of tectonic plates that contain the seafloor creates massive displacement in a body of water.

When a tsunami is generated in distant waters, it passes through several observation points before reaching Japan, enabling its size and arrival time to be estimated.