Science Lesson #4: MAGNITUDES!

Like most things sciency, there is more to reporting an earthquake’s magnitude than meets the eye. When an earthquake happens, all you hear on the news is “Magnitude 6.7 on the Richter scale. Epicenter 2.3 miles from Buttfuck, California”. What you don’t hear is the squabbling between geophysics tribes about the earthquake’s Moment Magnitude, Duration Magnitude, Richter Magnitude, Body Wave Magnitude, and Surface Wave Magnitude. I shit you not: there are that many magnitude scales.

Let’s sort them out together. To make this easier to understand, I will compare each magnitude scale to an actress with the first name of Jessica.

Richter Magnitude = Jessica Lang: The stalwart scale we all know and love. Charles Richter and Beno Guttenberg were fancy-pants geophysicists at CalTech in the early part of the 20th century. Together they built a Wood-Anderson seismograph – a very specific kind of torsion seismograph. Reporting an earthquake’s magnitude on the Richter Scale means that you’ve recorded the earthquake on one of these special seismographs at exactly 100km (about 60 miles) from the earthquake.

The sucky part about a Wood-Anderson seismometer is that it can only record earthquakes up to about a 6.8. Bigger than that and the mechanisms inside go all screwy and you don’t get a good record of the event. You can make all kind of mathematical corrections, but it basically blows your seismogram off scale.

Note: SEISMOGRAPH = THE INSTRUMENT
SEISMOGRAM = THE PIECE OF PAPER WITH THE SQUIGGLES ON IT

Even though Richter magnitude may not be the hot young thing it once was, we all should give props to Chuck and Beno for deciding that magnitudes should be recorded on logarithmic scales. You see, graphically a log scale looks like this

So as numbers get bigger and bigger on the x-axis, they don’t get much bigger on the y-axis. This is great because you can describe an infinite range of magnitudes on a relatively small scale. Theoretically you could have an earthquake with a magnitude of 20 or 100 or infinity, but an earthquake that size would mean that the entire Earth had been torn into two pieces. If that happens I wouldn’t really care if it was a 20.1 or a 22.7, I would be worried about my head exploding a la “Total Recall” in the vacuum of space.

Duration Magnitude = Jessica Alba: Fails to deliver consistent results. This scale deals with how long an earthquake lasts. This can get you into trouble because the duration of an earthquake in any one location depends on what’s going on with the rocks you’re standing on. If you’re in a basin filled with sand (I’m looking at you, Los Angeles) the earthquake is gonna last a lot longer than if you are standing on good, solid bedrock.

Moment Magnitude = Jessica Biel: These days this is the popular magnitude scale. The Moment Magnitude scale relies on something called the seismic moment of an earthquake which incorporates the area of the fault that slipped and how much the fault was displaced. No special kind of seismograph, no instructions on where to record the earthquake. Just quantities that are physically measurable. Nice.

Body/Surface Wave Magnitude = Jessica Simpson: I'm gonna sweep these two scales under the rug for now. It's not that they aren't useful, but they entail some explanation of how earthquake waves travel through the earth. You can live without them.