Shaken by a moment of great magnitude

What’s all this talk about the “magnitude” of an earthquake? Whatever happened to the Richter scale?

I’m glad you asked. The moment magnitude scale has gradually taken the place of the Richter scale since 1979, when it was developed by Hiroo Kanamori and Tom Hanks (no, not that Tom Hanks). It’s designed to be more reliable for larger events, being based on the energy released by the quake rather than how much the ground has moved.

Actually, it’s based on the logarithm of the energy, meaning that the scale goes up by one when the energy goes up by a factor of 10. There are many measurement scales based on logarithms, from pH to decibels, stellar magnitude to information, even the octave scale. In fact there is evidence to suggest that human beings find it more natural to estimate quantities based on logarithmic scales.

In the 19th century, Ernst Heinrich Weber and Gustave Theodor Fechner performed experiments like gradually increasing weights held by blindfolded people. Small increases were barely perceptible, but when the weight was increased by an amount comparable to what it started with, it could easily be detected – regardless of the starting weight.

As is often the case, the maths may sound complicated, but we’re awfully good at doing it subconsciously.

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