Offshore, tsunamis can go unseen and by the time they are noticed, it is too late, writes Dick Ahlstrom
Where there are earthquakes, so too will there be tsunamis. Anything that can cause large-scale displacement of water can also trigger a tsunami. NUI Galway's Prof Mike Williams has been studying tsunamis for years, something that came almost as a by-product of his own research into storm surges. Tsunamis are typically blamed for the scattering of boulders far inland, but powerful storms are more than enough to accomplish this, he says.
If storms are enough to throw boulders about, imagine then the force behind the tsunami that smashed into communities around the Indian Ocean on December 26th. The wave resulted in the deaths of at least 156,000, with the search for victims continuing.
"Tsunamis can have a variety of origins with earthquakes a common cause," says Williams. Underwater landslides and the collapse of volcanic islands can also trigger these great waves.
All tsunamis have one common feature: their origins in the displacement of large volumes of water. An earthquake shifting the seabed or a mass of sediment sliding down the continental slope delivers a similar result.
A huge megathrust earthquake off the west coast of northern Sumatra caused the tsunami on December 26th. "The size of the event is related to the size of the area affected," explains Chris Bean of the Department of Geology at UCD. In this case the area affected was huge.
The earthquake caused a huge shift in the relative positions of the Indian and Burma tectonic plates. Up to 1,200km of the plate boundary slipped an average of 15 metres, and perhaps 1,000km of seabed lifted by several metres. This megathrust caused an instantaneous displacement of cubic kilometres of water, surging away in all directions from the epicentre, says Williams.
There would have been little or no warning that the wave was on its way.The Pacific Tsunami Warning Centre watches for tsunamis in the Pacific basin, but no such system exists for the Bay of Bengal where the recent disaster occurred. The epicentre was 30km underground and about 250km from Banda Aceh in Indonesia. It took just 15 minutes for the tsunami to slam into the Indonesian coastline.
"The people in Indonesia felt the ground shake and knew there was an earthquake but didn't realise the potential effect of an earthquake in the Indian Ocean," says Prof Alan Jones, head of geophysics at the Dublin Institute for Advanced Studies. Seabed displacement caused by an earthquake typically produces a series of waves, he says. These can travel at up to 700km per hour, given the immediate displacement of the water when the undersea crust heaves.
Tsunami waves are nothing like ordinary waves, which do not actually move the water along, he says. "They differ in their wavelengths." They have wavelengths of between five and 15 minutes, which means they are pushing a huge amount of water along with them. A typical wave laps the shore but is thrown back, but a tsunami has too much water coming behind it to be so easily turned. "In the case of a tsunami the wave turns into a turbulent bore with the water moving in one direction only. Then they go out and going out is as dangerous as coming in," he says.
Even a few miles offshore the wave might pass unnoticed, looking more like a surface swell of as little as half a metre. "It is only when it starts to impinge on the seabed that it begins to build up," says Williams.
"The height of the wave is important but it is not the height but the bore that moves forward inexorably. It does this because of the mass of water behind it." The volume of water has nowhere to go but up as it rolls onto a beach, but the wave typically does not crest, it simply pours onto the shoreline running forward for as long as its volume of water allows. This water then flows back to the sea and equilibrium returns.