Interesting article on the precise causes of the earthquake and tsunami, with good graphics which I can't download so you'll have to follow the link for them:
https://www.theguardian.com/world/2...nesia-tsunami-and-could-lives-have-been-saved
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.
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.”
And it really has been a perfect shitstorm in Palu - as if an earthquake and tsunami weren't bad enough, in one area the earth liquefied and the buildings and the people inside them just sunk into the ground, and in another area a huge horizontal mudslide simply swept everything away - the BBC today showed footage of it happening with houses travelling at maybe 20 mph, and the whole village ended up half a mile (c 1 km) from where it started and of course full of mud and most inhabitants presumably dead.
https://www.theguardian.com/world/2...il-liquefaction-blamed-for-building-collapses
Dramatic footage has emerged from the city of Palu on the Indonesian island of Sulawesi, with buildings collapsing as the ground slides beneath them.
The phenomenon, known as soil liquefaction, is thought to have occurred as a result of the recent 7.5 magnitude earthquake, which also triggered a devastating tsunami. The death toll has already passed 1,200 and is expected to rise further.
Soil liquefaction can occur in saturated loose soil that is suddenly shaken – for example by an earthquake.
“When soil is saturated, the space between individual particles is completely filled with water,” said Dr Stavroula Kontoe of Imperial College London. “Seismic shaking increases the water pressure between the soil particles; the particles can lose contact with each other which in turn leads to an overall loss of soil strength and stiffness.”
The upshot is that, with the particles no longer bound tightly together, the soil loses its structure and behaves like a liquid.
“In very simple words, when liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced,” said Dr Carmine Galasso from University College London.
Kontoe said that certain areas are at higher risk than others from the phenomenon.
“Reclaimed land and river banks which typically consist of loose deposits are prime locations for liquefaction in case of strong shaking,” she said, adding that it is unlikely that the tsunami was involved in the soil liquefaction seen in Indonesia, but rather that the earthquake was responsible.
Kontoe added that it is not only building and above-ground structures that can be affected by the phenomenon, noting there can also be damages to pipelines such as those for clean water, wastewater and gas.
The scenes from Indonesia are not the first time soil liquefaction has caused damage following an earthquake. The phenomenon is thought to have caused severe and widespread devastation in Japan after an earthquake struck in 2011. It was also blamed for contributing to the damage done by the 2011 Christchurch earthquake in New Zealand.
But Kontoe says disasters are not inevitable. “There are several mitigation techniques which can limit or even eliminate [soil liquefaction’s] consequences. These techniques usually involve strengthening the soil deposits in areas where liquefaction has been identified as a major hazard and/or adopting drainage measures to prevent the increase of the water pressure during the strong shaking.”
Galasso agrees there are ways to reduce risk. “Building codes and standards in many countries require engineers to consider the effects of soil liquefaction in the design of new buildings and infrastructure such as bridges, embankment dams and retaining structures,” he said.
https://www.theguardian.com/global-...ried-alive-the-mud-swamped-indonesian-village
“It’s like that movie, 2012: doomsday,” says 24-year-old Joshua Michael, as he struggles for a fitting reference to the fate of Petobo, his village on the Indonesian island of Sulawesi where more than a thousand are feared buried in the mud that swept over it after Friday’s earthquake.
Standing next to the remains of his house, the silver roof flattened to the ground, clothes strewn all around it, he is trying to make out where the homes of his neighbours once were.
“The houses just got sucked into the earth and then the mud came over and sealed them over,” he says. “I saw my neighbours get buried alive.”
When the 7.5-magnitude tsunami-triggering quake hit Palu it virtually annihilated Petobo.
Residents describe the road moving up and down, like a wave, and then side to side like a snake. They called it a land tsunami.
The powerful tremor ripped up the road leaving huge jagged chasms running through it. In some parts, the height of the road is meters apart. It swallowed up houses whole, as it rolled through the village.
The US Geological Survey explains it as a process that liquifies the earth, which occurs when soil, saturated with water, and shaken by an earthquake, acts in liquid form.
“It’s doesn’t make sense,” says local resident, Dicky Christian, “but it’s real.”
Joshua Micheal was driving his motorbike home through the rice fields after playing a game of football when it all happened.
He says he has no words for the apocalyptic scene that unfolded before him. People were running and screaming, others were trapped, as the earthquake split open the land and houses tumbled in. He heard people yelling as they went down, then came the fast flows of liquid earth, and mudflows, that sealed them in.
Joshua looks over what was once the village, now covered with some haphazard rice plants, as one of the sites where his neighbours’ houses lie beneath.
Led the back way to the village by the Petobo resident, with the front entrance sealed off under military guard as excavators worked through piles of dirt metres high, the Guardian saw what was left of a village battered by multiple forces of nature.
The search and rescue team in Petobo had pulled out 19 dead from the village as of Wednesday morning, but the death toll is expected to soar. Teams are only 400 metres in.
“It could be hundreds. Or it could be thousands. We don’t know yet,” said search and rescue worker Chandra.
Each victim has their own story: some were killed after they were covered in mud, others trapped in the debris, their cars, or in the mosque because it was Magrib prayer. And not all bodies, he adds, were found in one piece.
Petobo residents estimate the final death toll will be far higher.
“There were about 11,000 people in Petobo and I think maybe half are gone,” reckons Sudiryo Djalano, a political campaigner, who collected population data on the village during a recent political drive.
“I mean Petobo just sank into the earth, and it was a densely populated village,” he says.
Muhammad Mansur, a national park officer and Petobo resident guesses it could reach 2,000.
“Some houses were moved about a kilometre away,” he says in disbelief.
In other parts of Palu, where long lines of cars continue to spill out of every petrol station, search and rescue missions are also slow going. From the ground there is the sense the current death toll of 1,374, as of Wednesday morning, does not yet reflect the true extent of the tragedy.
cameras instead, in attempt to identify victims. They fear the building might crash on top of them at any minute which, from its state, looks possible. The first floor is no longer visible after it sank into the ground, the rest of the foundations cut the skyline at unnatural angles. Somehow a folded white towel atop a chair remains unmoved.
Martinius Hamaele, a carpenter and civil servant has been at the hotel since Friday night, when he rushed down to rescue his daughter, who worked in the hotel’s sales department.
He says he crawled through the rubble on his hands and knees calling out her name, Mauren. She did not answer but others called out for help. “Don’t waste your energy using your voice,” he told them, “find something to bang on the wall and I will trace the sound to you.”
Martinius pulled out seven people that night, one of whom later died, none of them his daughter.
A hotel engineer on the scene said the hotel was almost fully occupied on Friday, which means there would have been at least 100 guests inside. Martinius’ daughter may be among many the victims rescue teams retrieve in the coming days or weeks.
“I believe today is the last chance to find her,” said Martinius, from the chair where he was anxiously watching the proceedings on Wednesday morning, “After that she will be added to the list of the dead.”
