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When the violent earthquake caused by an earthquake of magnitude 7.5 finally stopped, Selvi Susanti stepped up and realized that something was happening strange. First, she saw the ground suddenly begin to flow. Then the sidewalk cracked under his feet like a broken plate and began to rise.
Terrified, she clung to a small piece of asphalt and surfed a muddy river that was moving fast as she swallowed whole neighborhoods, carrying her higher than coconut palms over a quarter of a century. thousand.
"What I saw – oh my God, the houses were collapsing, they started rolling like waves, it's like a tsunami, but the difference was that they were Earth waves, "said Susanti, 38, crying in memory of seeing so many people just disappearing into the earth while they were screaming for help. "I had the impression of being in a boat moving in. But the difference was that I was not in the water, but in the mud."
Many, like Susanti in the devastated village of Petobo, did not know that they were already in an area already identified by the government as a high-risk area for the devastating geological phenomenon that causes liquefaction of loose soils during earthquake.
But Indonesian scientist Gegar Prasetya was not surprised by any of the events at the twilight of September 28, killing nearly 2,000 people and leaving thousands dead. For years, he had warned the population that the area around Palu Bay on Sulawesi Island had already been affected and that it was due to another combination of factors that could create a perfect storm capable of cause earthquakes, landslides, tsunami waves and liquefaction of soil.
"I knew that right away," said Prasetya, co-founder of Indonesia's Tsunami Research Center, who had met with government officials and local residents to try to raise awareness of the threat. "I posted in our group and I said:" It happened. "
Indonesia, part of the "Ring of Fire" of the Pacific Basin, is an archipelago of about 17,000 islands located over many fault lines that have produced some of the earthquakes, tsunamis and eruptions most important and deadly volcanic rocks in history.
Other scientists around the world have wondered how this type of earthquake – on a slipped fault, which usually does not produce dangerous tsunamis – could generate waves up to 6 meters high.
Once again, Prasetya knew.
He had published an article nearly twenty years ago highlighting six other tsunamis recorded in the Makassar Strait over the last century, predicting that a repetitive event could be expected about every 25 years. The last occurred in an area north of the city in 1996. Previously, Palu Bay had been hit in 1968 by a very similar magnitude 7.4 earthquake, which had generated waves of 10 meters. from above.
"This whole village went to sea," he said about the 1968 event. "You can still see the trees from the top of the water."
Some experts have speculated that the September 28 earthquake, in itself, would not have generated a big wave despite its shallow depth and proximity to the coast. They are convinced that it is rather the soft soils of the region that have been the real catalyst of the disaster. The violent long tremors of the temblor have probably triggered one or more submarine landslides due to unstable sediments deposited on the seabed by rivers. This disruptive movement may have created the great wall of water that crossed the ocean until it was drawn into the long, narrow bay that surrounds Palu, forcing the wave to rush.
"Imagine what happens if you drop a brick in a flat pond – the ripples are spreading in all directions," said Robert Hall, a geologist at Royal Holloway University in London, who studied the area. "Now drop an object of the same size into a bathtub, the waves can be reflected on the sides, amplify and become larger in the direction of the length of the bath."
But it's not just the low sediment in the ocean that has given way. A sandy and moist soil also separated and became alive thanks to liquefaction in some areas due to the radical vibrations of the earthquake. The soil simply lost its strength and turned into mush under the feet of people, creating a mud that acted like a moving sand. People, houses, cars and streets were swallowed and covered with a thick carpet of what, a few seconds earlier, was dry land. Rapid landslides were also launched above the ground, possibly causing even more localized tsunami waves.
"We did not necessarily expect all the worst possible factors to happen together," said Willem De Lange, a scientist from the University of Waikato in New Zealand, co-author of the research on this topic in 2001 with Prasetya. happen."
Many questions remain about exactly what happened in this complex disaster. Prasetya will begin field work with the Indonesian navy this week to try to better understand what happened under the water. A team of international experts is expected to arrive next month to conduct an assessment.
The Palu population has exploded in many high-risk areas since the 1968 event, which killed 200 people and also turned the soil into mud, leaving many newcomers vulnerable without a local history.
However, the central government produced a map in 2012 identifying large areas of Palu, a city of 380,000, where liquefaction could occur. The Petobo region, for example, has been classified as having high risk potential. The report also recommended that residential and industrial areas be better constructed in areas of low risk of liquefaction. He suggested mitigation efforts, including the construction of structures with deep foundations anchored in stronger soil layers.
Good urban planning is ultimately the key to saving lives, said Sri Hidayati, head of earthquake mitigation at the Ministry of Energy's Geology Agency, which produced the report that was shared with the provincial and district governments of Sulawesi. She said that it was incumbent on her agency to provide the mapping, but to the local authorities to "use it or not". Monday's calls to the vice mayor of Palu and the mayor's help remained unanswered.
"If everything is planned in the future on that basis, I think we may not make a lot of victims in case such a disaster recurs," she said. "Or probably no victim at all."
At a press conference in Jakarta on Monday, the head of the country's disaster agency also confirmed that the loose soil areas of Palu were not suitable for housing.
"It is impossible to rebuild in areas of high risk of liquefaction, such as Petobo and Balaroa," he said, adding that people who would still live there would be resettled.
Indonesia has been criticized for lifting the tsunami warning for the Sulawesi coast too early. The earthquake destroyed the power and telecommunications towers, preventing sirens from ringing and alerts from turning on mobile phones. The online video showed a disturbing scene as cars and motorcycles drove at a normal speed on a coastal road and unconscious people were moving on the beach while the big fast wave could be seen getting closer before it started. explode on the ground.
"It is almost impossible for a tsunami warning system to predict what we have seen the other day," said Adam Switzer, tsunami expert at the Observatory of the Earth in Singapore. "The earthquake is a warning, if you are somewhere in Southeast Asia and on the coast and you feel an earthquake, move inland, head to the heights and stay there. "
But it's not just about warning people right before something happens. Prasetya said that the history and geology of Palu Bay should be taken into account in reconstruction plans. He added that soil surveys should be done to determine if deep piles are needed to stabilize buildings. Local knowledge should also be considered as an inexpensive way to save lives, such as building wooden houses with thatched roofs, instead of concrete and tiles.
In Petobo, the tsunami warnings would not have helped because it was not time to react. It remains only the village, located about 30 minutes from the center of Palu, a muddy wasteland where only the ends of the roofs remain on the surface. The satellite images show a densely populated area extending over more than 100 hectares (24 hectares) devoured by what looks like a giant layer of chocolate milk. In cell phone videos, buildings are seen sliding like slabs on a plate of ice. Some people spent hours trying to find their home after the disaster and located them about 2 kilometers from their previous place of residence.
"It was like we were shooting in a blender," recalls Susanti, who survived by jumping on solid ground and running after the mud flow finally slowed down. "I've seen houses change position." The houses located to the east moved west and vice versa. "I saw the mud twisted into a dough."
The recovery of Petobo's bodies and other hard-hit areas, such as Balaroa, has proven difficult, as large equipment will sink into loose soil and be unusable there. The government plans to turn some of these sites into mass graves, according to Indonesian Minister of Security Wiranto, who uses a single name.
"It's already a ghost town, I will not go back there even if they pay me 1 billion rupees ($ 65,800)," said Erli Yati, 32, who said: also survived a "terrestrial tsunami" of some in Petobo.
She had no idea of previous disasters nor the fact that the soil could become alive.
"I will not go back to this place," she added. "It was the worst experience of my life."
Prasetya praised the idea of closing the liquefaction zones and said that mitigation measures – whether it's creating and enforcing appropriate building codes in risk areas or moving people to safer places – should be implemented before the next disaster.
"How many souls must be sacrificed until the government knows that mitigation is important?" he asked, adding that he was sounding the alarm since the tsunami caused by the 2004 earthquake in the island of Sumatra, which killed 230,000 people in a dozen countries , more than half of them in Indonesia. "Everything is back in the usual order."
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Mason reported from Jakarta. Associated Press reporter Tassanee Vejpongsa in Petobo, Indonesia and Eko Prasetyo of Jakarta, Indonesia.
