Extreme Earth - Tsunami

Tsunami waves can devastate coastal towns in a matter of minutes; we look at the science behind the destructive force

THE RECENT earthquakes in Haiti and Chile both triggered tsunamis – massive waves of fast moving water capable of wiping out coastal towns in one hit – but how does one event lead to the other, and what can be done to protect vulnerable towns on the coastline?

Tsunamis are caused by the displacement of large bodies of water. Underwater earthquakes in oceanic subduction zones are responsible for the vast majority, but other submarine occurrences like volcanic eruptions, explosions, landslides – both those underwater and on land – and even meteors can all cause tsunami waves.

“The granddaddy of tsunamis is caused by meteors,” said Dr Eddie Bernard, Director of the Pacific Marine Environmental Laboratory, “It’s really quite simple; anything which displaces or moves water causes a tsunami.”

The largest tsunami wave in recent history was caused by an earthquake off the west coast of Sumatra, Indonesia on Boxing Day 2004 – waves as high as 30m were recorded in some areas and the quake and subsequent tsunami affected many countries in Southeast Asia.

A rupture in the Northern section of the Sunda megathrust fault, where the Indian and Burma plates collided, caused the 9.1 magnitude quake. The Indian plate slipped under the Burma plate which led to a sudden vertical rise of the seabed, displacing a huge volume of water.

Any earthquake located off shore and measuring over 7.5 on the moment magnitude scale has the potential to cause a tsunami: the recent quake in Chile provided optimal conditions –the 8.8 magnitude quake occurred off the coast. In contrast, the quake in Haiti took place inland and measured 7.0 on the moment magnitude scale; it’s believed costal landslides were responsible for the waves, which reached up to 3m high.

Imminent tsunami waves offer very few warning signs. In the deep ocean, waves have a small amplitude, a wavelength of about 200km and can travel up to speeds of 800km per hour but as they reach shallower waters, the waves are compressed and slow to about 80km per hour. The wavelength decreases to less than 20km, but their amplitude grows, producing a distinctly visible wave. Every tsunami is different: each tsunami has unique wavelengths, wave heights, and directionality, which can make predicting tsunamis and issuing tsunami warnings in real time difficult.

Different types of waves occur on either side of the fault. Positive or crest waves – the more dangerous of the two because of their vague warning signs – are caused by the plate on one side of the fault moving upwards. The crest of the wave arrives first, and the sea level rises rapidly, overwhelming everything in its path. When this wave passes, the water recedes quickly as the second wave arrives. In the 2004 tsunami-causing quake, the Burma plate was pushed up, and the waves struck places like Sri Lanka.

Negative or trough waves are characterised by a rapidly receding sea which is followed by a sudden rush of water at high speed. The drawback occurs because the tectonic plate on one side of the fault sinks suddenly which causes the overlaying water to propagate outwards with the trough at the front. It was a negative wave which hit the island of Phuket in Thailand in 2004.

Scientists from University College London and EPICENTRE (the Earthquake and People Interaction Centre) are beginning to understand how tsunami waves behave after building a tsunami generator. Built with funding from the Engineering and Physical Research Council and engineers from HR Wallingford, the generator uses a pneumatic system to suck water into a tank – mimicking a tsunami drawback. It then releases the water in a controlled way over a model coastal slope, enabling tsunami behaviour and effects to be studied in detail1.

“Although the basic concept is actually quite simple, this is the only facility that has ever been able to replicate the draw-down phenomenon in the laboratory,” said Dr Tiziana Rossetto, EPICENTRE’s director, “We’ve already used the generator to mimic the 2004 Indian Ocean tsunami at 1:75 scale.”

The generator can be used to enhance understanding of water flows and forces unleashed by tsunamis, and will aid in the design and building of structures able to withstand a tsunami wave.

Tsunamis cannot accurately be predicted, even if the location or magnitude of a submarine quake or eruption is known. And not all seismic events will result in a tsunami wave, but automated systems such as bottom pressure sensors can be used to provide warnings immediately after a quake in time to save lives. The sensors can be attached to buoys to monitor the pressure of the overlying water column in real time and used create models which can estimate the amplitude and surge height of the approaching tsunami.

The National Oceanic and Atmospheric Administration (NOAA) has developed a warning system that detects signals of tsunamis directly from the sea floor. DART – or Deep-ocean Assessment and Reporting of Tsunamis – determines what happens to sea levels during a quake and whether a tsunami has been triggered. As the tsunami waves propagates across the ocean, the DART system report sea level changes back to Tsunami Warning Centres, where the information is processed to produce a more accurate estimate of the source2.

“DART constantly measures the oceans’ waves. After an underwater event, information collected by buoys is used to infer what happened to water at the source,” said Dr Bernard, “We can then project into the future what will happen to coastlines.”

There are 50 DART buoys all over the world strategically placed in areas previously struck by a tsunami. Dr Bernard believes that tsunami warnings and education can both be improved to prevent loss of life.

“The buoys give an accurate forecast of what will happen. If the public know how to respond, then the appropriate action can be taken,” he said, “If the waves detected are particularly strong then they can be alerted to move to higher ground. If we know the waves are quite small, we can advise people that they may experience strong currents and suggest that they move fishing boats into deeper waters, away from the coast.”

Tsunamis cannot be prevented, so the key is to be prepared. Earthquake-prone areas are biased to tsunamis, and researchers from Stanford University believe reinforcing structures will help to reduce the damage caused.

Greg Deirlein, professor of civil and environmental engineering, and colleagues studied the Indonesian city of Padang, which could have been struck by a tsunami following a 7.6 magnitude quake offshore in September 2009. They studied how the city’s buildings fared during the quake and were surprised by how many modern buildings had collapsed.

“It was like living in a big laboratory,” Deirlein said, “We were able to see how buildings performed and how the city reacted to the threat of a tsunami.”
The team concluded that vertical evacuation – evacuating to higher ground – could save thousands of lives – but only if the buildings were reinforced to withstand earthquakes and tsunamis.

“Existing buildings can be strengthened to perform better under future earthquakes and tsunamis,” he said. Deirlein said horizontal evacuation – by car or foot – is clearly not adequate. Retrofitting old buildings and ensuring new builds are able to withstand both quakes and waves could save thousands of lives.

Other places have built floodgates and channels to redirect water, although the effectiveness of this has been questioned as waves can easily scale the walls. Natural factors such as shoreline tree cover can negate the effects of a tsunami – coconut palms and mangroves are able to absorb tsunami energy for example. When the 2004 tsunami hit India’s Tamil Nadu region, the village of Naluvedapathy suffered only minimal damage because the wave broke against a forest planted along the shoreline only two years prior.

So they can’t be prevented, and it’s difficult to predict if another extreme event will have a knock-on effect and trigger a tsunami, but better preparation could be the key to saving thousands of lives.