Rule of thumb to avoid drowning if near active volcano surrounded by water

The Guardian, with help from reports at Agence France-Presse and Associated Press, summarizes what scientists learned about the tsunami that struck Indonesia along the shore of the Sunda Strait (see the Guardian site for a graphic illustrating what happened and photos and video of the volcano eruptions):

Saturday’s tsunami in Indonesia was caused by a chunk of the volcanic Anak Krakatau island slipping into the ocean, it was confirmed on Monday, as officials at the country’s natural disaster agency said it must develop a new tsunami early warning system. ...

Well yes they certainly must develop a new system. But while the Guardian doesn't mention that the tsunami waves were higher than they would've been at other times because water displacement from the volcano wall splashdown happened during a full moon high tide, I think it's possible to devise a rule of thumb for survival without having to wait on technology:

If you live on the shore near a spewing island volcano, stay the hell away from the shore on a full-moon night.

Here is more from the Guardian report:

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.

Anak Krakatau had been spewing ash and lava for months before a 64-hectare section of its southwest side collapsed, an Indonesian official said. “This caused an underwater landslide and eventually caused the tsunami,” Dwikorita Karnawati, the head of the meteorological agency, said.

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

The fact the tsunami was triggered by a volcano rather than an earthquake meant no tsunami warning was triggered, scientists said. Coastal residents reported not seeing or feeling any warning signs before waves of up to three metres high surged in.

Hundreds of military personnel and volunteers spent Monday scouring beaches strewn with debris in search of survivors. At least 1,016 people were injured and more than 600 homes, 60 shops and 420 vessels were damaged when the tsunami struck.

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.

“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.”

He also 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.”

The death toll is expected to rise as 57 people were still missing on Monday. At least 1,600 people have also been displaced. Dody Ruswandi, a senior official at the disaster agency, added that the rescue effort was likely to last a week.

Sutopo warned locals to stay away from the coast. “People should not carry out activities on the beach and stay away from the coast for a while,” he told reporters.

The University of Queensland volcanologist Teresa Ubide said Anak Krakatau had been erupting for the past few months, which was not unusual. “It seems like the volcano is active at the moment and it may happen again,” Ubide said.

“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.

Richard Teeuw of the University of Portsmouth said sonar surveys were needed to map the seafloor around the volcano, but that work usually took months.

"The likelihood of further tsunamis in the Sunda Strait will remain high while Anak Krakatau volcano is going through its current active phase because that might trigger further submarine landslides,” he said.

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