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What caused the tsunami in Indonesia and why was there no warning?

The Guardian logo The Guardian 24/12/2018 Lisa Martin

What happened?

Search-and-rescue efforts were continuing in Indonesia following a deadly tsunami in the Sunda Strait which claimed more than 280 lives. More than 1,000 people were injured and 11,600 people displaced. The district of Pandeglang, on the western tip of the island of Java was worst hit, with 207 killed and 755 injured.

smoke coming out of the water: An aerial view of the volcano Anak Krakatau in the Sunda Strait off the Indonesian island of Java where a tsunami killed more than 280 people. © Reuters An aerial view of the volcano Anak Krakatau in the Sunda Strait off the Indonesian island of Java where a tsunami killed more than 280 people.

Anak Krakatau volcano is thought to have erupted underwater which may have caused an undersea landslide, triggering the killer waves.

Experts won’t know the exact cause of the tsunami until sonar monitoring can be undertaken, but it’s too dangerous to do that at the moment.

The University of Melbourne associate professor David Kennedy said the work can be done by relatively small, 10-metre vessels using multi-beam sonars which are basically a more powerful version of a “fish finder”.

Why wasn’t there a warning system?

In the case of tsunamis caused by earthquakes, the shaking earth can act as a warning but it’s much trickier to anticipate tsunamis from volcanic eruptions.

“There’s usually a big draw down in water below low tide levels, so you if you’re on the coast you’ve got a matter of minutes to get to high ground,” Kennedy said.

A local resident affected by the tsunami stands next to debris in Carita beach in Pandeglang © Getty A local resident affected by the tsunami stands next to debris in Carita beach in Pandeglang

Kennedy said if a buoy network had been in place around Anak Krakatau, a one-to-two minute warning of a pending wave was the most anyone could expect.

“The expense of doing that everywhere is just impossible,” he said.

People inspect the damage at a tsunami-ravaged neighborhood in Carita, Indonesia © Getty People inspect the damage at a tsunami-ravaged neighborhood in Carita, Indonesia Indonesia has 147 volcanoes and 76 are considered active.

Kennedy said population growth on coastlines and popularity of beach resorts exposed more people to the risk of tsunamis.

Krakatau’s history

Krakatau is part of the Pacific Rim of Fire.

Related: Indonesia tsunami (provided by Photo Services)

The Australian-Indonesian plate is going back down into the earth underneath the Eurasian plate.

“As the plate goes down into the mantle it actually starts melting, so what you get are volcanoes that sit on top of that,” Kennedy said adding they have a high level of silica which traps the gases as well as water.

A car is seen among ruins after a tsunami hit Carita beach in Pandeglang, Banten province © Getty A car is seen among ruins after a tsunami hit Carita beach in Pandeglang, Banten province “They tend to be really viscous, really sticky volcanoes and they produce massive eruptions. That’s because it’s remixing all the old ocean floor,” he said.

In 1883, eruptions at Krakatau caused tsunami waves that reached 36.6 meters and wiped out an estimated 36,000 people.

The entire island of Krakatau was vaporised and volcanic gas, ash and rocks spewed 80km high.

The eruptions, turbo charged by steam, were one of the loudest noises heard by human beings in modern in history, Kennedy said.

Tsunami waves hit Banten and Lampung areas © Getty Tsunami waves hit Banten and Lampung areas “There were reports they heard the sound in Darwin and some reports said they heard it as far south as Perth,” he said.

He said the ash clouds from Krakatau cooled the global temperature by over a degree for many years.

Krakatau was quiet until late 1927, when a new eruption began on the seafloor. The following year a rising cone burst through the ocean. Two years later it became an island called Anak Krakatau “Child of Krakatau”.

Kennedy said minor eruptions over the years had been slowly building up the edifice of Anak Krakatau. “You’ve got more magma and lava coming up underneath,” he said.

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