What caused the tsunami?

Scientists are still trying to determine the exact cause. The earthquake that occurred on Friday morning was not a thrust earthquake, the kind that is responsible for most tsunamis, where tectonic plates move vertically up and down and displace water.

Instead it was caused by what is known as a strike-slip fault, where tectonic plates move horizontally. According to Phil Cummins, a professor of natural hazards at the Australian National University, these earthquakes usually lead only to very weak tsunamis.

It has been suggested that Friday’s earthquake could have caused a large underwater landslide that displaced the water. This submarine landslide could have occurred either in Palu bay, close to the shore, or further out to sea.

Usually tsunamis are caused by earthquakes hundreds of miles from shore, and the shaking is rarely felt on land. As Cummins noted: “It is unusual to see a double disaster like this.” It will take months of field research and underwater exploration to determine the cause.

What early warning systems were in place and did they fail?

There have been claims that Indonesia’s meteorological and geophysics agency, BMKG, may have removed a tsunami warning too early, before the waves hit the coast of Palu, and was thus responsible for some of the loss of life. There have also been suggestions that buoys out to sea that detect earthquakes and tsunamis as part of the early warning system had not been serviced for six years and were faulty.

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However, Cummins and Adam Switzer, the chairman of the Asian school of environment at Nanyang Technological University in Singapore, were in agreement that the disaster was a failure not of technology but of education. Unlike the 2004 tsunami that devastated south Asia, this wave was not was prompted by an earthquake hundreds of miles out to sea. Instead it was a localised tsunami resulting from an earthquake close to the coast. It has been estimated that the tsunami waves hit Palu only 30 minutes after the quake. “For the people on the beach and in the city, the earthquake should have been the early warning,” Switzer said.

Cummins said: “The focus on the technological points of failure here is misguided because this was a localised tsunami. In that case you can’t rely on a warning system; people should seek high ground immediately. They cannot afford to wait for a siren or a warning, they need to move instantly. The problem is, from what I’ve seen from the footage, many people appear not to have done that.”

He added: “Either they didn’t know they needed to do that or they didn’t believe anything would happen, and in either case that says the people in Sulawesi were not educated about what they need to do in this situation. And that’s what killed people.”

What made the tsunami so destructive?

Questions remain about how far out to sea the tsunami originated, and therefore how much speed it gathered – some estimates have suggested it was moving at 500mph in Palu bay but slowed down substantially before it hit the shore. The waves were six metres high in some places and reached up to a kilometre inland.

It has been suggested the narrow shape of Palu bay concentrated and amplified the force of the wave. “The shape of the bay in Palu could play a role,” said Cummins. “It can funnel the energy and concentrate it at the tip and that will focus the tsunami. It is also a very a deep bay, which means the tsunami could move at high speed.”

Switzer said the initial impact of a tsunami caused the most destruction, though the movement of debris when the wave pulled back could also prove deadly. “The most devastation from the tsunami is generally the force of the water hitting objects as it strikes the coast. The tsunami water going between buildings also accelerates the velocity,” he said.

Were the earthquake and tsunami unexpected in the area?

Switzer said: “There’s a large and well-documented fault system that runs through Palu, which is about 200km long. There was an event like this in 1937 and other events in the early 1900s, though it’s not clear if they caused tsunamis. And there was a paper published in 2013 in which it was suggested that the Palu fault, which is very straight and very long, had the potential for causing a very destructive earthquake and tsunami. So it’s not like this is unexpected. But the question is, did we learn anything from past incidents? It doesn’t seem so.”

Dr Kerry Sieh, of the Earth Observatory of Singapore, said: “It’s well known that that fault in Palu has been storing strain and accumulating strain at a few centimetres per year, so it’s a been very rapid slipping fault for years.”