Why the Richter Scale is no more

Last Modified: 27 Feb 2008
By: Julian Rush

The Moment Magnitude scale is much more useful than the Richter.

Charles Richter and Beno Gutenberg developed their scale in 1935 to measure shallow Californian earthquakes. Why Charles got his stab at immortality and Beno didn't is one of those mysteries of science we'll never know.

But that's why most geologists now call it the Local Magnitude scale. So last night's shaking in Lincolnshire should correctly be described as Magnitude 5.2 and written as M_L5.2.

Trouble is, the Richter scale doesn't work very well. The way it's defined means it can't measure quakes above about 6.8. What's more, when it comes to very faint quakes - which coudn't be detected back in the Thirties - it gives a negative number.

That's because it's a logarithmic scale - an increase of one point on the Richter Scale is a 10-fold change in the amplitude of the seismic wave being measured.

Nowadays geologists have a new scale, the Moment Magnitude scale, or M_W, developed in 1979 by Thomas C Hanks and Hiroo Kanamori.

This is much more useful because it measures the amount of energy released in the quake rather than the size of the seismic wave, which is affected by the depth of the quake and the geology of the rocks it passes through.

The formula for M_W is adjusted, though, so the numbers of the two scales roughly correspond.

And the energy released last night was quite considerable: about the same as the atomic bomb dropped on Nagasaki.