Rescuers searched for more survivors and victims on debris-strewn beaches on Monday as Indonesia reeled from the second deadly tsunami to hit it this year.
The waves that swept terrified people into the sea on Saturday night along the Sunda Strait followed an eruption and possible landslide on Anak Krakatau, whose name means "Child of Krakatoa and is one of the world’s most infamous volcanic islands.
Scientists said on Monday that a large chunk of the southern flank of the volcanic island may have slipped into the ocean just minutes before the tsunami hit. Such a scenario could strike anytime without any warning.
The death toll has risen to 373, with more than 1,000 people injured along the coastlines of western Java and southern Sumatra islands, the national disaster agency said on Monday. "The number of victims and damage will continue to rise," said agency spokesman Sutopo Purwo Nugroho.
Aid agencies were quick to deploy to the area, located just a few hours drive from the Indonesian capital, Jakarta. The roads were challenging but passable, Aulia Arriani, a spokesperson for the Indonesian Red Cross told the Telegraph.
“Our challenge is to find the missing and to evacuate the people who are still trapped. In the first phase of emergency response we will be evacuating people for the next two to three days,” she said.
Ms Arriani added that local hospitals and clinics had been flooded with casualties and dead bodies. “People in the disaster are need food, drinking water, tarpaulins and medical help,” she said.
The Indonesian Medical Association says it is sending more doctors and medical equipment and that many of the injured are in need of orthopedic and neurosurgery expertise. It says most patients are domestic tourists who were visiting the beach during the long holiday weekend.
The tsunami that struck the island of Sulawesi on September 28 was accompanied by a powerful earthquake that gave residents a brief warning before the waves struck.
On Saturday night, the ground did not shake beforehand to alert people to the oncoming wave that ripped buildings from their foundations in seconds and swept terrified concertgoers on a resort beach into the sea.
Dramatic video posted on social media showed the Indonesian pop band Seventeen performing under a tent on Tanjung Lesung beach at a concert for employees of a state-owned electricity company.
At least four band members and support crew were killed, lead singer Riefian "Ifan" Fajarsyah told followers in a tearful Instagram account. The band's drummer was among the missing, as was his wife, Dylan Sahara, whose 26th birthday it was on Sunday.
The worst-affected area was the Pandeglang region of Java’s Banten province, which encompasses Ujung Kulon National Park and popular beaches, the agency said.
Joko Widodo, the Indonesian president, who is scheduled to visit the stricken area on Monday, expressed his sympathy and ordered government agencies to respond quickly to the disaster.
"My deep condolences to the victims in Banten and Lumpung provinces," he said. "Hopefully, those who are left have patience."
In the city of Bandar Lampung on Sumatra, hundreds of residents took refuge at the governor’s office, while at the popular resort area of Anyer beach on Java, some survivors wandered in the debris.
Many of those affected were domestic tourists enjoying the long holiday weekend, but foreigners were visiting the area ahead of Christmas as well.
Some roads remained blocked by debris and traffic, with families streaming out of the area for fear of further tsunamis.
Why a landslide was the likely trigger
Scientists said on Monday that the consensus, based on satellite images and the information available, was that the collapse of a portion of the volcano triggered the killer waves.
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.
"Underwater landside is the leading theory," said Sam Taylor-Offord, a seismologist at GNS Science in Wellington.
"So when that land pushes into the ocean ... it displaces the ocean surface causing the vertical displacement that causes the tsunami," he said, adding however that the lack of data and access made it impossible to ascertain this theory.
Anak Krakatau, halfway between Java and Sumatra, has been spewing ash and lava for months. It erupted about 24 minutes before the tsunami struck, and that could have triggered the landslide.
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 signalled, 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 10 feet high surged in.
Jose Borrero, coastal engineering expert specialising in tsunami hazards at eCoast Marine Consulting, said landslide-generated volcanic tsunami behave idiosyncratically, compared with tsunami generated by earthquakes, which are better studied.
This is 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.
"In Indonesia, we've all been waiting for another big earthquake tsunami and then boom, here we have a volcanic landslide one," Mr Borrero said.
"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."
Anak Krakatau or "child of Krakatau" emerged from the Krakatau volcano, which in 1888 erupted with such force the blast was heard all the way in Perth, said Mika McKinnon, a geophysicist based in Vancouver, Canada.
Further eruptions have continued from the massive crater left behind.
Mr McKinnon said volcanoes were weak, sloppy heaps of loosely bound rocks all slanted downhill and they slip off all the time.
There are no early warnings systems that can detect such landslide-driven tsunami and Anak Krakatau is so close to shore there would never have been enough time to react and clear out the population.
"It's hard to identify landslide-triggered tsunami, especially quickly enough to issue useful warnings," said Mr McKinnon.
"A similar event at Anak Krakatau might trigger another one, or it might not. Maybe a month later, or year from now. We will never know," she said.