Long and narrow bay likely made tsunami more devastating in Palu
Experts said narrow bay running into Palu, a city of 380,000, squeezed the tsunami into a tight space
The sun had just slipped behind the mountains, leaving a soft pink glow as the blue sea melted into the darkening horizon.
It could have been a video postcard from a tropical paradise, except for the long white wave stretching the width of the bay – getting larger and closer with each passing second.
By the time the fast-moving wall of frothing water slammed into the city of Palu off Indonesia’s Sulawesi island on Friday, it was between 3 to 6 metres high.
The tsunami, triggered by a magnitude 7.5 earthquake, destroyed the idyllic scene in seconds, leaving hundreds dead and injured. Many more were believed to be missing.
The video clip, shot on a smartphone and widely broadcast on Indonesian TV, showed water swallowing an entire row of buildings and gushing into streets and a damaged mosque as onlookers ran in terror.
Indonesia is all too familiar with deadly earthquakes and tsunami.
In 2004, a quake off Sumatra island triggered a tsunami across the Indian Ocean, killing 226,000 people in 13 countries, including more than 120,000 in Indonesia.
Questions are sure to be asked why warning systems set up around the country after that disaster appear to have failed on Friday when the latest quake struck.
The meteorological and geophysics agency BMKG issued a tsunami warning after the Friday quake but lifted it 34 minutes later, drawing criticism it had withdrawn it too quickly.
But officials said they estimated the waves had hit while the warning was in force.
Palu is at the head of a narrow bay, about 10km long and 2km wide, which had “amplified” the force of the wave as it was funnelled towards the city.
“Because of the bay, all the water comes there and collects together. And then it makes it higher,” said Nazli Ismail, a geophysicist at the University of Syiah Kuala in Banda Aceh on Indonesia’s Sumatra island.
Ismail said he was surprised that a tsunami was generated off the coast of central Sulawesi, which sits on a strike-slip fault, producing earthquakes that typically move in a horizontal motion and do not usually displace large amounts of water.
In contrast, temblors occurring where one tectonic plate is lodged beneath another – called subduction zones – can move large amounts of water vertically when the strain forces one plate to pop up or dive down.
The force can create devastating tsunami like the one in Sumatra and off Japan’s northeast coast in 2011.
But Danny Hilman Natawidjaja, a geologist with the Indonesian Institute of Sciences, said the Sulawesi event was more complicated.
While it occurred on a strike-slip fault, he said the part that ruptured was on a small segment that can move in a vertical motion.
He said that could have triggered the tsunami, which also could have been created by an underwater landslide.
“It may be that the shock of the quake triggered a landslide underwater, but we don’t have any proof yet,” Abdul Muhari, who heads a tsunami research team that advises the Indonesian government, said.
Large quakes are the main drivers of tsunami, but the phenomenon can also be sparked by other cataclysmic events, such as volcanic eruptions and landslides.
In 1883, a volcano shattered the Pacific island of Krakatoa, causing a blast so loud that it could be heard 4,500 kilometres away, followed by a tsunami that killed some 30,000 people.
The tsunami of December 2004 in the Indian Ocean was caused by a monstrous 9.1 magnitude earthquake off the Indonesian island of Sumatra.
It released energy equivalent to 23,000 of the atomic bombs dropped on Hiroshima, according to the US Geological Survey.
Several factors determine the height and destructiveness of a tsunami.
They include the size of the quake, the volume of displaced water, the topography of the sea floor as the waves race to the coast and whether there are natural obstacles that dampen the shock.
Destruction of protective mangroves and coral reefs and the building of homes or hotels on exposed beaches are the leading causes of high death tolls from tsunami.
The Pacific Ocean is particularly prone to earthquakes and therefore to tsunami.
But research has found that, over the millennia, tsunami have occurred in many parts of the world, including the Atlantic and Mediterranean.
A global monitoring network, overseen by the UN, has been set in place to alert areas at risk.
Agence France-Presse, Associated Press, Reuters