Why the science of tsunami tracking is far from perfect

As the death toll mounts from the tsunami that struck the Indonesian city of Palu, local scientists find themselves under fire 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 up to six metres tall that smashed into the city.

The region’s geophysics agency issued a tsunami alert moments after the earthquake struck near the island of Sulawesi at about 6pm local time.

About 30 minutes later it withdrew the warning, sparking claims that it had prematurely given the all-clear.

But the speed, power and unpredictability of tsunamis push even the most sophisticated prediction technology to its limits.

Take the charge that the agency cancelled its warning too early. The epicentre of the earthquake was about 70 kilometres north of Palu, about an hour’s drive away.

But the tsunami ripped across the sea at a speed of about 800 kilometres an hour – comparable to a jet aircraft – and took barely five minutes to reach the city.

The agency has also faced criticism after admitting that not even the most basic detection equipment was installed near Palu, but it is also unclear if this would have made much of a difference to the scale of the devastation.

As well as its proximity to the site of the quake, the city is 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 offshore.

Whatever the cause, the result was a giant wave more than 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 30 metres high that struck northern Sumatra, killing almost 170,000.

That catastrophe, which led to another 50,000 deaths in more than a dozen countries around the Indian Ocean, prompted an international effort to strengthen tsunami detection systems.

Leading the charge was the US National Oceanic and Atmospheric Administration. It 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 the speed of light.

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, about three quarters of all alerts have proved inaccurate.

Part of the reason is that tsunamis are relatively rare, with about 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 is a false alarm.

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 30 metres above sea level.

Later that night, the town’s harbour was lapped by waves barely 15 centimetres tall. Such false alarms breed confusion and complacency – sometimes with deadly consequences.

After a strong quake off the coast of Chile in May 1960, a tsunami alert was issued for the town of Hilo, Hawaii. Suspecting yet another false alarm, many residents ignored it. More than 60 died when the tsunami arrived, four hours after the first alert.

While scientists wrestle with the challenge of tsunami predictions, there is 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.