Saturday’s tsunami that struck Indonesia without warning along the rim of the Sunda Strait, between Java and Sumatra islands, was caused by a large chunk of the volcanic Anak Krakatau island sliding into the ocean.
Anak Krakatau had been spewing ash and lava for months before a 64-hectare section of its south-west side collapsed, said Dwikorita Karnawati, the head of the meteorological agency. “This caused an underwater landslide and eventually caused the tsunami,”
“Landslide-generated volcanic tsunami behave idiosyncratically,” said Jose Borrero, coastal engineering expert specializing in tsunami hazards, “compared with tsunami generated by earthquakes, because there are so many different variables and there is a ‘sweet spot’ of exactly the right speed and volume of rocks slipping into and sea and deeper to generate a wave.”
Sutopo Purwo Nugroho, the chief spokesman for the Indonesian disaster agency, said the country had no early warning system for landslides or volcanic eruptions. “The current early warning system is for earthquake activity,” he wrote on Twitter according the report in today’s Guardian. “Indonesia must build an early warning system for tsunamis that are generated by underwater landslides & volcanic eruptions … Landslides triggered the 1992 Maumere tsunami and the Palu 2018 tsunami.”
Nugroho, said Indonesia’s tsunami buoy network had “not been operational since 2012”. “Vandalism, a limited budget, and technical damage mean there were no tsunami buoys at this time,” he said. “They need to be rebuilt to strengthen the Indonesian tsunami early warning system.”
Sutopo said on Twitter: “Anak Krakatau has been erupting since June 2018 until now,” he said. “Yesterday’s eruption was not the biggest. The October-November 2018 period had a larger eruption.”
“It seems like the volcano is active at the moment and it may happen again,” said University of Queensland volcanologist Teresa Ubide. Anak Krakatau had been erupting for the past few months, which was not unusual.told the Guardian. “The volcano is very close to the shoreline so … there wouldn’t be much time to warn because it’s close and the tsunamis can travel very fast,” she said. The lack of seismic activity that would accompany an earthquake was also significant, she said.
Image credit top of page: photograph: Nurul Hidayat/AP