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Indonesia is experiencing earthquakes every day, but the magnitude of the Palu Friday earthquake and tsunami surprised local residents and scientists.
More than 800 people have died, but the final figure could be thousands, say the officials.
As scientists explained to the BBC, a combination of geography, timing and inadequate warnings meant that what had happened in Palu was the worst case scenario.
A dangerous place
Earthquakes are caused by Earth's tectonic plates that slide against each other or overlap. This happens constantly, but sometimes the movement is large enough and close enough to populated areas to have devastating consequences.
Small tremors occurred throughout Friday in Palu, but early in the evening, the Palu-Koru Fault suddenly slipped, just a short distance from the shore and only 10 km from the surface, causing a magnitude earthquake 7.5.
Hamza Latief, of the Bandung Institute of Technology, has been studying the fault line since 1995 and says that the impact of the earthquake has been magnified by the thickness of the sediments on which the city rests.
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Buildings resting on thick sediments, as here in Palu, will be more easily destroyed than those on the rock.
As the rock trembles, the sediments move much more, behaving essentially like a liquid. Few poorly constructed properties can withstand such a movement.
"Any place where people have put buildings in ruins – where they dug the earth and moved it somewhere else – will not be as solid as the rock in place," says Jess Phoenix, an American geologist who has studied the land. 39; Indonesia.
With regard to earthquakes, she says: "These are the areas to consider."
An unexpected wave
"Normally, you do not pay much attention to the Palu-Koru fault line, with regard to tsunamis," said Professor Philip Liu Li-Fan of the National University of Singapore.
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Indeed, the two plates move horizontally and not with the vertical thrust usually required to trigger dangerous waves.
"We tried to understand what was really going on," says Professor Liu.
It may be that the earthquake triggered a landslide underwater, which disrupted the water, he says, or there may be inaccuracies in the description of the fault, but "we do not know again".
But once the wave began to move, Palu – at the end of a narrow bay 10 km long, was a sitting duck.
Tsunamis are not dangerous at sea. But when the waves get closer to the ground, their base drags on the seabed, which raises them.
In less than three minutes, Palu was hit by three waves.
Jess Phoenix said geography and timing meant it was "basically the worst case scenario".
"When water enters a horseshoe-shaped area of this type, instead of raising the wave simply because the ocean becomes shallow, you also get the bowl effect, which will reflect the waves on the shoreline around it.
Latief said the region had already been hit by tsunamis. Historical records show that in 1927, a wave reached 3 to 4 m at the mouth of the bay but reached 8 m when it reached the city.
An ineffective warning system?
After the 2004 tsunami in the Indian Ocean killed a quarter of a million people, huge sums of money were spent on an early warning system.
A complex system of sensors has been installed in the region. These are intended to help scientists quickly assess whether an earthquake is likely to trigger a tsunami and warn the public to move to a higher level.
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A system of buoys has been given to Indonesia to send tsunami threat alerts – but none currently works
The head of the Indonesian Disaster Mitigation Agency said that a key part of the country's warning system – a series of buoys connected to seabed sensors designed to detect tsunami – no longer functioning since 2012. He highlighted the lack of funding.
According to Professor Liu, the system works "in a way and does not work at all", and the southern regions of Indonesia have been mainly focused on the southern regions, which were hit so hard in 2004.
A tsunami warning was issued after the earthquake, but, says Jess Phoenix, "it will be interesting to see what will happen to know if the warning has been implemented properly, if people do not. ;have heard".
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Like Sri Lanka, many countries affected by the 2004 tsunami have since installed signs indicating the way to the heights.
It is not uncommon for people to be killed by a tsunami, as they stayed to watch the spectacle of the sea suddenly withdrawing as the wave unfolded. Since 2004, considerable efforts have been made to educate people on inland navigation should the water recede.
A man told Rebecca Henschke, of the BBC, that he had seen the sea disappear and that his family had run away while he was catching a close child and hanging on to a tree.
Despite the warning, it seems that there are still many people on or near Palu Beach, most of them preparing for a festival.
"The people of this region know very well what the tsunami can do, I hope that, if there were victims on the beach, they were trying to leave," Ms. Phoenix said.
"But a 10-foot wave is really important – to evacuate everyone in a very short time, evacuation routes must be very clear.
"From my experience working in Indonesia, I have not seen very clearly marked exit routes, I'm not sure about this particular city, but it's really hard to do." quickly get out a lot of people from one area. "
