Tsunami warnings for small islands [1]
Friday, October 2, 2009 - 11:45. Updated on Sunday, April 20, 2014 - 17:22.
Editor,
The recent red wave (or tidal wave) in Tonga will of course rake up the discussion again whether Tonga should spend money on an early warning system. My answer is clearly and definitely: no; as is very obvious if one considers the physical properties of red waves, which apparently are not very well known.
A red wave is a wave with the huge wavelength of perhaps 200 km, traveling at a speed of 800 km/h, so that it has a period of 20 to 30 minutes, but the amplitude is quite tiny, perhaps only a few decimeters. Ships in the middle of the ocean even do not notice one coming along. Observers on tiny islands in its path would see the water rising and falling a little bit over the timescale mentioned above, this usually repeated a few times, with the rise and fall towards the middle of the cycles being the highest. It is just as whether the tide comes in and out on the accelerated timescale of minutes rather than hours. At no instance will there be breaking waves.
This picture of course changes when the red wave comes in over tidal flats and people have their houses built along the beach practically on sea level. A huge wave rolling over the flat, moving faster than you can run, dragging along anything on its path, that may cause a lot of damage. Also close to the source of the red wave, usually the epicentre of an earthquake, the energy of the wave has not spread out yet, and there the amplitude can be several metres easily. The red wave of last Wednesday, already called the Samoan tsunami, while the epicentre was in reality far south of it, and not so much more farther away from Niuatoputapu as it was from Upolu, clearly shows that. Samoa and Niuatoputapu were hit. Ha'apai had some minor flooding.
Tongatapu, anybody noticed anything there, or was the wave by then already too much diluted by its spreading?
Anyway, according to the news messages the earthquake itself happened at 6:48 Tonga time. A couple of minutes later the front of the wave arrived at Niuatoputapu, some minutes after that at Vava'u and Ha'apai, and shortly after 7 o'clock at Tongatapu; recall the speed by which a red wave travels. Depending on how many oscillations there actually were, before 8 in the morning it was already over, and it was only by then that the radio started to announce it and brought people to panic.
Briefly if you are in the middle of the Pacific and a red wave generating earthquake occurs, then either you are close and engulfed before any alarm can be risen, or if you are far away enough to get the news on time, then there is nothing to worry about. Tonga would do better to use any special funds to educate its people about red waves and how to recognise them and then what to do next (not to descend onto the reef to collect stranded fish, for example), than to waste money on an alarm system that, with the typical Pacific quality, would probably not work when it is needed in the first place. Or will no longer be trusted because of repeated false alarms as happened in Samoa.
But then why do they have tsunami warning systems in Japan? Why, in fact, has the Japanese word, tsunami, for this phenomenon become so well known? Because all what is written above is nice and well, but only for the middle of the ocean. For small islands, which are less than 100 km or so in size. Because it is a well known physical property of waves that they are not affected by anything much smaller than their wavelength, and vice versa will not affect it themselves.
But when a red wave nears the end of the ocean and approaches a huge obstacle in the form of a continent, the story gets different. The shallow seas can reduce the speed of the wave by a factor 10, and that props up it amplitude enormously. Then countries such as Japan may be inundated by gigantic waves of many meters high. And that all caused by an earthquake perhaps as far away as the coast of Chile in South America. The red wave caused by such an earthquake may travel through the Pacific ocean, no one noticing it, but a half day later it reaches Asia. Yes, for a tsunami with such devastating waves but with such a lead time, a warning system may be useful.
'Ofa atu,
Firitia (Kik Velt)