Earthquake Magnitude Scales

Did you know that scientists no longer use the Richter Scale to measure earthquake magnitude? Now, we tend to use the “Moment magnitude” scale, which was built off of the tenants of the Richter scale.

Charles Richter & Beno Gutenberg developed the first magnitude scale in the 1930’s to quantify earthquakes by comparing the size (amplitude) of seismic waves plotted against distance (as calculated from S-P arrival times).

But there was a problem. They weren’t able to record all of the frequencies from large earthquakes. The Richter scale did a great job with smaller, local earthquakes but it underestimated large earthquakes. Since then, seismologists and engineers have developed more sensitive seismometers that, along with faster computers, have enabled us to record & interpret a broader range of seismic signals. These improvements allow us to better determine the energy released by large earthquakes.

So now seismologists no longer look at just the amplitudes of seismic waves but instead use much more information contained in the seismogram to calculate what is called the “seismic moment” The seismic moment, which defines how much force is needed to generate the recorded waves, is defined by factors like the rigidity of the rocks, the area of the fault that slipped, and how far the fault moved. Again, this is a big deal because seismologists can use the Moment Magnitude Scale to measure and describe the size of earthquakes in terms of the energy released, not just the amplitude of the recorded waves.

(This was originally a Twitter thread I wrote for IRIS)

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