Tsunami a threat to many Pacific Islands [1]
Saturday, October 3, 2009 - 10:30. Updated on Sunday, April 20, 2014 - 17:29.
Editor,
I take issue with the letter from Firitia about small islands not being particularly vulnerable to tsunamis. While it is true that in the deep ocean a tsunami's wavelength is so broad that the wave's height is almost imperceptible, we all know that many small Pacific islands are barely above sea level, and many do not have the sheer drop-offs to the seafloor that would prevent a tsunami from slowing down and rearing up into a very perceptible and very dangerous wave. Both of these traits mean that tsunamis can be dangerous to many Pacific islands, whether the tsunami begins nearby or far away.
Even if the wave height of a tsunami is just a few feet above mean high tide at the shore, the volume of water behind the face of the wave can be tremendous. It is this volume - not so much the height, but the volume - that becomes problematic for anything in its path. (Firitia should keep in mind that boats in the mid ocean don't notice tsunamis in part because tsunamis have a low amplitude in deep water but also because boats are buoyant. Islands, on the other hand, do not float, so any sudden and substantial rise in sea level is going to be an issue.) A regular wind-driven wave breaks on the shore without causing much damage because its wavelength has become too short relative to its height: there simply isn't much volume or energy behind it. Tsunamis aren't giant breakers; they're more like sudden, massive floods that roll onto and across any landmass that is lower than the crest. Like a storm surge during a cyclone, a tsunami does not need to be tall in order to be destructive. I'm sure we all recall the amateur video footage of the tsunami flooding into Banda Aceh in Indonesia in December of 2004. It wasn't that the wave was so tall that it wreaked so much havoc; it was that it was so broad that the flood kept coming and coming and coming, churning the entire city into a cascade of bodies and debris, and sending seawater more than a kilometer inland.
And just a few hours later, the Maldives were flooded by the very same tsunami. The waves were not tall but they were broad enough to flood many of those islands...and severe enough to displace 15,000 people and kill nearly 100. If small islands in the middle of the ocean are not affected by tsunamis, what happened to the Maldives?
By the way, the Pacific warning system is not really meant to warn islands about tsunamis generated nearby; it's meant to warn islands about tsunamis generated in the North and East Pacific - ten to twelve hours away. Not all tsunamis can be mitigated by using a warning system to trigger evacuations, but some can.
Dan Clem
Science Editor and
Former Peace Corps, Ministry of Fisheries, Ha'apai, Tonga