How a Tsunami Wave Works

Tsunamis are long, powerful waves that are created by sub-sea earth movements – earthquakes, land and ice slips, meteor strikes. But not all earth movements create these waves, many give little or no effect.

A Tsunami occurs because the water mass of the ocean is displaced and, much like throwing a stone into a pond, waves are generated.The displacement in a Tsunami creates a wave which is very long – up to 200 km (125 miles).These waves travel in a very predictable way that is determined by the water depth. Anyone with a calculator can predict its speed (square root of 9.81 times the water depth) so in the deep abyssal plain of the Indian Ocean at 5,000 m this is 221m per second, about 800 km an hour (500 mph) – the speed of a jumbo jet. In spite of this speed and power, out at sea the wave is usually less than one metre high. It would take about 15 minutes to pass a boat and would be barely noticeable.

As the wave hits the shoreline it rapidly slows to about 50 km an hour (30 mph) but the back of the wave is still travelling faster in deeper water and so is catching up with the front – a water traffic jam is taking place.The water has to go somewhere, so it goes up, building a wave of 10, 20, even 30 m in height (some 30 to 100 ft).This wall of water finally arrives at the coast travelling at 30 to 50 km/hr (30 mph) causing massive destruction.

Dr Jon Copley is a marine biologist who specialises in deep sea animals. He joined an international team of scientists and film makers on an expedition looking for visual evidence of the earthquake that caused the devastating Boxing Day 2004 Tsunami. In this article he explains how biology can help geologists date the aftermath of an earthquake.

The steep sides of an underwater cliff materialise in the lights of the remotely operated vehicle. Aboard the ship, two and a half kilometres above, geologists crowd around the monitor, anxious to know if this is what we are searching for. But as we turn to survey the cliff face, an innocuous-looking bamboo coral dashes those hopes. It is firmly cemented to the cliff wall and certainly decades old.

We came here, off the coast of Sumatra, to look for signs of seabed upheaval from the earthquake that triggered the devastating Asian Tsunami. Led by Kate Moran of the University of Rhode Island, David Tappin of the British Geological Survey and David Mearns of Bluewater Recoveries Ltd, our expedition was one of several trying to piece together the events of 26 December 2004. How did the movement of the plates of the Earth's crust, deep beneath the ocean floor, translate into movements of the seabed and the ocean surface?

In the aftermath of the waves, researchers rushed to investigate the site of the earthquake, which was one of the largest ever recorded. In March 2005, Tim Henstock and Lisa McNeill of the National Oceanography Centre, Southampton, joined Dave Tappin aboard HMS Scott, the Royal Navy's premier deep water survey vessel. Their high-resolution maps of the earthquake zone identified the targets for our investigation: signs of scarps, slumps and possible underwater landslides that might have formed during the latest or previous earthquakes.

Two months later, our team of geologists, geophysicists, wave modellers and biologists from the UK, US, Canada, France and India assembled for a three week expedition, filmed for documentaries on Discovery Channel and the BBC. The geologists and geophysicists made seismic surveys of the seabed features to reveal the structures beneath them, looking for faults that might link them to the deeper movement of the plates. Meanwhile, other members of the team directed dives using a remotely operated vehicle (ROV) to examine the same features close-up. Perhaps surprisingly, this is where biologists like me could contribute to the investigation.

In many cases, deceptively fresh looking cliffs and cracks turned out to be home to sessile marine life too old to have grown since the earthquake. But at one site, nicknamed ‘the ditch’, there were no obvious signs of life, consistent with recent disturbance of the seafloor during an earthquake. Watching the video monitors in the darkened and air-conditioned ROV control room, it was often easy to believe that we were down there on the ocean floor, rather than sailing in hot equatorial sunshine. Paul Tyler and I from NOCS were funded on the expedition by the international Census of Marine Life, as the ROV dives provided a glimpse of the denizens of this little explored corner of the Indian Ocean. Sediment samples collected by the ROV may well contain new species of nematode worms and other small animals, thereby contributing to the Census goal of uncovering a million new species of marine life by 2010.

Since the Tsunami, other expeditions by Japanese and European researchers have surveyed the earthquake zone and wave modellers have refined their simulations to incorporate the information gleaned from the seafloor. In December 2006 a special session at the American Geophysical Union meeting in San Francisco brought together the results so far and highlighted areas for future collaboration. Although it may take years to fully interpret all the data from the earthquake, geologists and oceanographers should be better able to predict the impact of such events and help authorities plan for when the next Tsunami occurs. And on our dynamic planet, with 60 per cent of the population living on or near a coast, it is ‘when’ and not ‘if ’.

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