Krakatau volcano partial collapse triggers Indonesia tsunami

WELLINGTON - A large chunk of a flank of the volcanic Anak Krakatau island slipped into the ocean and triggered a tsunami that hit Indonesian shores, killing hundreds of people, officials and scientists said on Monday.

At least 281 people were killed, hundreds injured and many buildings were heavily damaged when the tsunami struck, almost without warning, along the rim of the Sunda Strait, between Java and Sumatra islands, late on Saturday.

Anak Krakatau had been spewing ash and lava for months before a 64-hectare (0.64 square km) section of the southwest side of the volcano collapsed, an Indonesian official said.

"This caused an underwater landslide and eventually caused the tsunami," said Dwikorita Karnawati, head of the meteorological agency, adding that the waves hit shorelines 24 minutes later.

Images captured by the European Space Agency's Sentinel-1 satellite showed that a large portion on the southern flank of the volcano slid off into the ocean, scientists said.

"When that land pushes into the ocean ... it displaces the ocean surface causing the vertical displacement that causes the tsunami," Sam Taylor-Offord, a seismologist at GNS Science in Wellington, said of underwater landslides.

Taylor-Offord said the eruption and "high noise environment" may be why the landslide was not recorded seismically.

The fact the tsunami was triggered by a volcano, and not by an earthquake, may be the reason why no tsunami warning was triggered, scientists said.

Coastal residents reported not seeing or feeling any warning signs, such as an earthquake or receding water along the shore, before waves up to 3 metres (10 feet) high surged in.

Jose Borrero, coastal engineering expert specializing in tsunami hazards at eCoast Marine Consulting, said landslide-generated volcanic tsunami were more of a mystery than tsunami generated by earthquakes, which are better studied.

There are so many different variables involved with landslide-generated tsunami and a "sweet spot" of exactly the right speed and volume of rocks slipping into and sea and down submerged slopes to generate a wave.

"In Indonesia, we've all been waiting for another big earthquake tsunami and then boom, here we have a volcanic landslide one," said Borrero.

"I've seen a few bits of imagery that suggest there's some sort of slant collapse that may extend underwater but none of this will be confirmed until there can be an offshore survey where they go and map the sea floor."