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Gonna do a quick thing on earthquake magnitude here on #SeismogramSaturday. There's a lot to say, so this is just a small part. I'll chime in more about magnitude in later weeks, so keep watching this space.
Meantime, here's today's lesson:
Meantime, here's today's lesson:
Magnitude is a measure of the raw size of the earthquake. It doesn't relate to how strong it felt or how much damage it did, because those things depend on factors like distance from the quake and the quality of infrastructure.
(those things feed in to a parameter called "intensity" which is different from magnitude...we'll discuss that later too).
Magnitude is just plain ol' HOW BIG WAS THAT QUAKE? It relates to the amount of energy the quake released.
When you see magnitudes listed by earthquake agencies, they may have label like Mw or Ms or mb or Ml. This is because there are different ways of measuring magnitude.
To measure quake size, we have to use the data we're given, which is largely how the ground shook (records of which are seismograms). Here's a seismogram from the December 2004 M9 Sumatra quake, as recorded in Talaya, Russia: 
Wave energy drops off with distance, so we also have to correct for how far away the seismometer was from the quake. In this case the station is about 5100 km from the earthquake.
The size of the wiggles are related to the size of the quake. But which part of the seismogram should we use? It varies a ton!
Let's start by measuring the P wave, the first arriving wave. If we do that, then we know exactly what we're getting--it's the initial punch, so the shaking isn't contaminated by previously arriving phases.
For what it's worth, Charles Richter, inventor of the first magnitude scale, used the amplitudes of P and S waves to measure earthquake size (after correcting for distance). So we're in good company here.
Magnitude values that are calculated with the P or S wave amplitude are called "body-wave magnitude" and abbreviated "mb".
Once we've measured the amplitude of the wave, we correct for distance traveled, as well as the depth of the quake, and voila, a magnitude value! Note that we have to take the log of the amplitude because these values have a HUGE range.
This figure shows P waves from two earthquakes, both of which were aftershocks of the Sumatra M9.1. Both were measured at the same station (in Russia). These wiggles show the first 10 seconds of the quake, so it's clearly the P wave.
These look pretty similar, yes? The red seems to move up a little higher at the beginning, but overall, there's not a ton of difference in how strongly the waves shook.
But hold on...here are the full waveforms.
When we look at it this way, the blue event is *clearly* much bigger. In fact, it was a 7.2, whereas the red was a 6.1. So despite the P waves looking similar the quakes were *clearly* not the same size.
It turns out that at some point (mostly around magnitude 6), P waves just don't get much bigger, even when the earthquake releases more energy. So using P waves to determine earthquake magnitude can be problematic.
mb turns out to be really good for deep earthquakes, as long as they're not too big. So we still use it, but we have to think of alternatives for larger or shallower quakes.
Next week we'll discuss surface wave magnitude, which is based on (you guessed it), the size of the surface waves. If you can't remember what those are, see my earlier #SeismogramSaturday post:
Questions about seismograms or earthquake magnitude? Ask away! But be advised that I might ask you to wait until next week or the week after, as we tackle other ways of calculating magnitude.
Happy #SeismogramSaturday, my people!
