More than 400 people are confirmed dead with 20 or so still missing following last week’s devastating tsunami in the Sunda Strait, Indonesia’s disaster agency announces.
Researchers have examined satellite images of Anak Krakatau to calculate the amount of rock and ash that sheared off into the sea.
They say the volcano has lost more than two-thirds of its height and volume during the past week.
Much of this missing mass could have slid into the sea in one movement.
It would certainly explain the displacement of water and the generation of waves up to 5m high that then inundated the nearby coastlines of Java and Sumatra.
The Center of Volcanology and Geological Hazard Mitigation (PVMBG) has been studying pictures from a number of radar satellites, including the EU’s Sentinel-1 constellation and the German TerraSAR-X platform.
Radar has the advantage of being able to see the ground day or night, and to be able to pierce cloud.
The capability has allowed some initial measurements to be made of Anak Krakatau’s lost stature, in particular on its western side.
What was once a volcanic cone standing some 340m high is now just 110m tall, says the PVMBG.
In terms of volume, 150-170 million cubic meters of material has gone, leaving only 40-70 million cubic meters still in place.
Quite how much mass was lost on December 22 itself and how much in the following days is unknown. Scientists may have a better idea once they have had a chance to visit the volcano and conduct more extensive surveys.
However, with the eruptions still ongoing and a safety exclusion zone in force – no-one is going near Anak Krakatau.
Cone collapse with tsunami generation was considered a potential hazard before December 22.
In 2012, scientists had modeled the possibility six years ago, even identifying the western flank of Anak Krakatau as the section of the volcano most likely to fail.
The study, although simulating a larger event, predicted wave heights and coastal inundation times that were remarkably similar to what actually happened.
A deadly tsunami triggered by Anak Krakatau volcano’s eruption struck Indonesia on December 22, at 21:30 local time, during a local holiday.
Giant waves crashed into coastal towns on the islands of Sumatra and Java, killing at least 281 people and injuring 1,016.
Sea water did not recede as it would with an earthquake tsunami and experts say that even if there had been warning buoys near the volcano, there would have been minimal alert time.
On December 23, coastal residents near Indonesia’s Anak Krakatau volcano have been warned to keep away from beaches amid fears it could trigger a new tsunami.
It is thought that volcanic activity set off undersea landslides which in turn generated the killer waves.
Anak Krakatau erupted again on December 23, spewing ash and smoke.
Video shot from a charter plane captured the magnitude of the volcanic event in the Sunda Strait, between Sumatra and Java.
Rescue efforts are being hampered by blocked roads but heavy lifting equipment is being transported to badly hit areas to help search for victims.
The spokesman for the National Disaster Management Agency, Sutopo Purwo Nugroho, told a news conference that another tsunami is a possibility because of the continued volcanic eruptions of Anak Krakatau.
He said: “Recommendations from [the] Meteorology, Climatology and Geophysical Agency are that people should not carry out activities on the beach and stay away from the coast for a while.”
Anak Krakatau, which emerged in 1927 from the caldera that was formed during the eruption of Krakatoa in 1883, has seen increased activity in recent months with people asked to avoid the area around its crater.
On December, Sutopo Purwo Nugroho put out a series of tweets explaining why there was no early warning for this tsunami. He said that Indonesia’s early warning system is set up to monitor earthquakes but not undersea landslides and volcanic eruptions, which can also generate deadly waves.
With 13% of the world’s volcanoes in Indonesia alone, it was crucial for the country to develop such system.
Sutopo Purwo Nugroho confirmed there was no tsunami advance warning system the night of the disaster, adding that because of lack of funds, vandalism to the buoys and technical faults there had been no operational tsunami warning system since 2012.