The Japan Meteorological Agency says it has not figured out the mechanism of the tsunami triggered by Saturday's volcanic eruption near Tonga because it lacks regular features.

Tsukada Shinya, the head of the Earthquake and Tsunami Observation Division, told reporters on Sunday that the agency's estimate of the arrival time of the tsunami was based on the assumption it was generated near the volcano.

But Tsukada said the tsunami hit Japanese coasts more than two-and-a-half hours earlier than predicted.

He explained that tsunami usually arrive at locations closer to a volcano before reaching more distant places, but only small changes in tidal levels were recorded at observation points between Tonga and Japan.

He said the agency doesn't have a way to inform the public about unusual changes in sea surface and tidal levels, so it issued tsunami warnings and advisories.

Tsukada added that tidal levels may continue to change after advisories are lifted. He urged people to stay on the alert when they work at sea or engage in marine leisure activities.