Odds of a disaster, predict when tsunami will come next<\/p>\n
John Gribbin, Guardian News Service, London<\/p>\n
The fact that such a disaster happened last month in no way
\nalters the odds of a similar event occurring next month, or even
\ntomorrow. Probably not in exactly the same location, as far as
\nthe epicentre is concerned, but quite possibly in the same part
\nof the world.<\/p>\n
The statistics that apply to such events are essentially the
\nsame as the statistics that apply to tossing a perfectly balanced
\ncoin -- if it comes up heads on one toss, the odds are still one
\nin two that it will come up heads next time as well. The only
\ngood news is that the odds of a disaster on the scale of much
\nsmaller than one in two.<\/p>\n
There are lots of ways in which you might guess that
\nearthquakes (the cause of tsunamis like the recent one) are
\ndistributed in time. At one extreme, most earthquakes might be
\nvery large, releasing lots of energy which then takes a lot of
\ntime to accumulate again.<\/p>\n
At the other, most earthquakes might be very small, repeatedly
\nreleasing tiny amounts of energy and never doing much harm. Or
\nthere could be some typical size for an earthquake, with both
\nlarger and smaller events relatively rare.<\/p>\n
Clearly, guessing is futile, and we need a proper scientific
\nassessment of the statistics. Indeed, when the records of
\nearthquakes are investigated to see how many of each size have
\noccurred around the world over a long period, they show none of
\nthese patterns.<\/p>\n
The first person to analyse the records in this way was
\nCharles Richter, who introduced the eponymous scale used to
\nmeasure the intensity of earthquakes.<\/p>\n
This scale is another source of confusion to the uninitiated.
\nHow come a magnitude nine event is so much worse than a magnitude
\nseven event? The answer is that the scale is logarithmic. A
\nmagnitude two event is not twice as powerful as a magnitude one
\nevent but 30 times as powerful. A magnitude three event is 30
\ntimes as powerful as a magnitude two event, and so on. So a
\nmagnitude nine event is 900 times as powerful as a magnitude
\nseven event.<\/p>\n
Almost fifty years ago, Richter and his colleague Beno
\nGutenberg used this scale as a basis for investigating the
\nfrequency of earthquakes of different sizes. They combined quakes
\ninto \"bins\" of half a magnitude on the scale, so all the
\nearthquakes between five and 5.5 went in to one bin, all those
\nwith magnitudes between 5.5 and six into the next bin, and so on.<\/p>\n
Since the Richter scale is logarithmic, they took the
\nlogarithm of the number of earthquakes in each bin, and plotted
\nthe results on a graph known as a \"log-log\" plot. The results lay
\non a straight line. Every subsequent study has shown the same
\nthing.<\/p>\n
This is one of the simplest (and most common) patterns of
\nbehaviour found in nature. It is called a power law, and in this
\ncase it means that for every 1,000 earthquakes of magnitude five
\nthere are roughly 100 earthquakes of magnitude six, 10 of
\nmagnitude seven, and so on. This law applies across a huge range
\n-- a magnitude one event is about the same as the rumble you feel
\nwhen a heavy lorry passes your house, while a magnitude nine
\nevent is some 500bn times more energetic.<\/p>\n
The power law tells us that although large earthquakes occur
\nmore rarely than small earthquakes, they are produced by the same
\nphysical process. You do not need to invoke a special cause for
\nwhy large earthquakes happen -- they just do. The power law also
\ntells us that for any magnitude you choose, there is a particular
\nprobability of an earthquake that size occurring during, say, any
\none year. Small earthquakes are relatively common, but they occur
\nin the same random way as the way the numbers come up on a set of
\ndice.<\/p>\n
Large earthquakes are rare, but they also occur at random. An
\nearthquake of any size could happen at any time in an earthquake
\nzone, even if one the same size happened last month.<\/p>\n
The moral is plain. We should be no less, and no more,
\nconcerned about a tsunami disaster affecting the Indian Ocean
\ntoday than we were on Christmas Day. If hindsight suggests that
\nit would have been a good idea to have a tsunami warning system
\nthen, that need is exactly the same now. All this will have been
\nobvious to anyone with high school physics; such a pity, then,
\nthat only about 30,000 students in the UK the subject in 2003,
\nand that so few politicians have any background in science at
\nall.<\/p>",
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