Why earthquake numbers had to compress a huge range
Earthquakes vary so widely that a simple linear scale would be awkward: a quake producing ten times the ground-wave amplitude would need a number ten times as large, and the strongest events would push the scale into unwieldy territory. Seismologists instead used a logarithmic scale, where each step represents a multiplication rather than an addition.
In 1935, Charles Richter developed the scale that bears his name to compare earthquakes recorded in Southern California. A seismograph traces ground motion onto a record; Richter’s method used the height of the recorded waves, adjusted for how far the instrument was from the quake. The logarithmic numbering let scientists describe very different events with manageable values.
One number, two different comparisons
The scale made it easier to compare the size of earthquakes, but magnitude is not the same as the shaking people experience at a particular place. Shaking depends on distance, local ground conditions, and the quake’s depth, among other factors. A single earthquake has one magnitude, while its effects can vary sharply from neighborhood to neighborhood.
The original Richter method also had limits, especially for very large earthquakes. Seismologists now commonly use moment magnitude, which estimates the size of the rupture and the fault movement rather than relying only on the height of waves on a particular seismograph. It retains the logarithmic logic, so the scale can compare small and enormous events without pretending their differences are small.
That is the useful trick behind the familiar numbers: they are compact labels for changes in the physical size of an earthquake that are anything but compact.
