Hampered by bad weather, Indonesian rescue workers continue to search for survivors of the tsunami which hit the Mentawai Islands west of Sumatra on Monday.
According to the sparse news trickling in from the remote archipelago, a tsunami that advanced several hundred metres inland hit the islands just minutes after a 7.5-magnitude undersea quake had struck at 9:42 pm local time on Monday night off the west coast of Sumatra.
More than 100 fishermen and beachgoers are reported dead, and around 500 missing.
The waves smashed several chartered boats anchored at Macaronis, a local tourist resort popular with Australians surfers.
A preliminary model based on assumed tsunami source and seafloor dislocation suggests the waves were of the order of two metres high.
“This means that the water level where the boats were sitting changed by 2 metres in about 12 minutes from the peak to the trough of the first wave,” says Jose Borrero, a consultant with ASR Limited in Raglan New Zealand and associate of the University of Southern California Tsunami Research Center in Los Angeles, who did the modelling.
“That plus a 2 to 4 knot current, and you have the recipe for disaster with two boats anchored near each other.”
Borrero is looking into chartering a boat from Macaronis to conduct a post-tsunami survey. “I have surfed in this area a few times over the years and been there for tsunami research purposes as well,” he says. “Kind of an interesting juxtaposition.”
The Mentawai Islands are covered by the Indonesian tsunami early warning system which issued a nation-wide alert few minutes after the quake. Along threatened Sumatran coastlines, thousands fled to higher ground when radios and loud speakers on mosques blared out the warning. But the Mentawai islands were too close to the quake epicentre for the alert to reach local villagers and surfer tourists in time.
“In such cases the quake itself is the only effective warning sign,” says Harald Spahn, a Jakarta, Indonesia-based tsunami early warning expert with the German Technical Cooperation.
Monday’s earthquake was probably a large aftershock of the magnitude-8.4 Sumatran earthquake of September 12, 2007.
Several sections of the Sunda megathrust have ruptured over the past decade, causing a series of earthquakes along the western coast of Sumatra. The devastating 9.1-magnitude quake and mega-tsunami on December 26, 2004 claimed the lives of 230,000 people around the Indian Ocean.
Monday’s local tsunami, generated by a far weaker quake, did not propagate westwards. But puzzlingly, tide gauge records at Cocos Island in the middle of the South Indian Ocean still showed a wave one fifth of the size of the 2004 wave, says Costas Synolakis, a tsunami researcher at the University of Southern California in Los Angeles.
“It’s probably a fluke, but it shows us how much more we need to understand about tsunamis,” he says.
Scientists warn that the current earthquake ‘super-cycle’ in the region is likely to culminate in another quake, and possibly tsunami, similar in size to the 2004 Boxing Day disaster.
Monday’s quake did nothing to alleviate the possibility of another big one. Indeed, it might bring the Sunda fault closer to failure, says Kerry Sieh, director of the Earth Observatory of Singapore.
When that might happen isn’t clear, though. “It could occur in 30 minutes or in 30 years,” he says
graphs: Geopgraphic location, wave heights and current speed. Jose Borrero.