Illusive tsunamis: The difficulty tracking mega waves
As the destructive surges only occur after a small percentage of underwater earthquakes, warning systems struggle to predict when they're likely to prove deadly
As the death toll from the tsunami that struck the Indonesian city of Palu mounts, local scientists are in the firing line for their apparent failure to give adequate warning.
Defunct equipment, lack of investment and inter-agency squabbling have all been blamed for the disaster that began last Friday when a 7.5 magnitude earthquake unleashed a wall of water that smashed into the city, reaching heights of 6 meters.
The region’s geophysics agency, BMKG, issued a tsunami alert moments after the earthquake struck near the island of Sulawesi around 6 p.m. local time.
Around 30 minutes later, it withdrew the warning, sparking allegations it had prematurely given the all-clear.
Yet this reckons without the nature of tsunamis, whose speed, power and unpredictability pushes even the most sophisticated prediction technology to its limits.
Take the charge that the BMKG cancelled its warning too early. The epicentre of the earthquake was around 70 kilometres north of Palu, about an hour’s drive away.
But the tsunami ripped across the sea at the speed of around 800 km/h – comparable to a jet aircraft – and took barely five minutes to reach the city.
The BKMG has also faced criticism after admitting that not even the most basic detection equipment was installed near Palu.
Yet it is also unclear if this would have made a dramatic difference to the scale of the devastation.
As well as its proximity to the site of the quake, the city is located at the end of a long and narrow bay. Its contours may have focused the tsunami’s energy, driving it to much greater heights than those detected off-shore.
Whatever the cause, the result was a giant wave over 5km wide packing the punch of a supertanker travelling at hundreds of kilometres an hour.
There is no construction on earth that can withstand such appalling violence.
And nowhere on Earth knows this better than Indonesia. In December 2004 the region experienced a colossal 9.2-magnitude quake, followed by tsunamis up to 30m high that struck northern Sumatra killing over 170,000.
That catastrophe – which led to another 60,000 deaths in over a dozen countries around the Indian Ocean – prompted an international effort to beef up tsunami detection systems.
Leading the charge was the US National Oceanic and Atmospheric Administration, which has pioneered the use of buoys linked to pressure sensors on the sea bed which can detect the emergence of tsunamis, with a satellite network issuing alerts at light-speed.
Indonesia was supposed to have a similar system in place, but – according to reports – the network is largely defunct through lack of maintenance and vandalism.
As a result, the region is guarded by an ageing network of tidal gauges and seismographs, with warnings issued by text messages and sirens.
Yet even the most sophisticated tsunami detection systems fall victim to the age-old problem of false alarms. Over the decades, around three-quarters of all alerts have proved inaccurate.
Part of the reason is that tsunamis are relatively rare, with around 90 per cent of submarine quakes failing to produce them.
So when even a highly reliable detection system finds hints of a tsunami, the chances are that it’s a false alarm.
Earlier this year, a strong earthquake off the coast of Alaska led to residents of Kodiak being told to immediately leave their homes and get at least 30m above sea-level. Later than night, the town’s harbour was lapped by waves barely 15cm high.
Such false alarms breed confusion and complacency – sometimes with deadly consequences.
Following a huge quake off the coast of Chile in May 1960, a tsunami alert was issued for the town of Hilo on Hawaii. Suspecting yet another false alarm, many inhabitants ignored the warning. Over 60 died when the tsunami arrived, four hours after the first alert.
While scientists wrestle with the challenge of tsunami predictions, there’s one bit of advice that has saved thousands of lives over the years.
If you are near the sea when an earthquake strikes and see the water pull away from the shore or hear a distant roar, get to higher ground as fast as you can.
And do not delay: you may have only minutes to make it.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK
Updated: October 4, 2018 01:19 PM