Updated, 12.19

WE ARE ALL aware that the severity of an earthquake is measured by the Richter Scale – but it’s less common to understand exactly what a figure on the Richter Scale suggests.

Given that earthquakes measuring 9 or higher are exceptionally rare – and because, since records began, no quake has ever measured a magnitude of 10 – it’s understandable to think of the scale as essentially being a scale from 0 to 10. This isn’t the case, however – there is no upper limit to the scale.

When comparing seismic measurements, however, it’s important to realise that the Richter Scale isn’t a straightforward measurement of activity. It’s easy to assume, for example, that a quake measuring 5 on the scale is exactly twice as severe as one measuring 2.5, and so on.

The scale isn’t quite so simplistic, however: rather than assigning some sort of unit of measurement to severity, the scale is instead a logarithmic one.

What this means, in a nutshell, is that quakes must get progressively more and more severe in order to move upward on the scale. Put simply, a quake measuring 4 on the scale is precisely a thousand times more destructive than one measuring 2. Likewise, a quake measuring 6 is a thousand times more destructive than one measuring 4, and so on.

Similarly, an increase of 0.1 in a quake’s magnitude means its effects are around 1.42 times more severe.

By such calculations, the earthquake experienced in Japan yesterday – measuring 8.9 on the Richter Scale – was a barely credible 700 million times more severe than the 3-magnitude quake that those in Japan had been warned to expect.

Comparing yesterday’s Japanese quake to that of Christchurch in New Zealand last month – which registered 6.3 – shows that the Japanese quake was 7,943 times more powerful.

Update: Thanks to the readers who have pointed out that the Richter Scale, in recent years, has been superseded by the Moment Magnitude Scale – though we’ve checked it out, and the ratios between comparative magnitudes on that scale is identical; an increase of 2 in magnitude means a quake 1,000 times more severe, and so on.

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