President Joko Widodo, who is running for re-election in April, told disaster agencies to install early warning systems, but experts said that, unlike with tsunami caused by earthquakes, little could have been done to alert people that waves were coming.

Memories of 2004

"Tsunamis from volcanic flank collapse are generated right at the coast and often close to populations," said Eddie Dempsey, lecturer in structural geology at Britain's University of Hull.

"The interval between the volcanic collapse and the arrival of the waves is minimal."