Please explain in a way that is understandable. thank you!
Chosen Answer:
Every time that any earthquake goes off, no matter big or how small, it releases two types of waves, body waves, and surface waves. Surface waves travel… on the surface, and they do a lot of damage, but otherwise are of no help here. Body waves travel through the crust, into the mantle, all the way through with the exception of the seismic shadow (which I’ll get to in a minute).
The whole process used to be seismic reflection, then it was seismic refraction, and it still is, but those represent only 2-dimensional paths. Recent seismic tomography gives a whole area picture. But to continue.
We all know that it takes about 5 minutes for the local news to tell you that a magnitude X earthquake just hit location Y according to the USGS, and that’s fine, and it’s based on the fact that you have two types of body waves. The P-wave, and the S-wave. P-waves are longitudinal waves, meaning that if you stretched out a slinky and just pushed it forward once, the energy would travel straight down the slinky. They’re very fast. 10 kilometers per second is a reasonable number. S-waves are transverse or “shear” waves, which if you went back to your slinky and shook it side to side once, you’d get that side to side wave travel all the way to the other side. These are slower than the P-waves, because they’re expending energy to the sides instead of just going straight. The quick USGS calculations are based on a fairly straightforward equation similar to the 6th grade distance = rate * time, except taking into account the “lag” between P- and S-wave arrival time at a monitoring station. With this and at least three seismographs, they can tell you where the epicenter is.
But if you want to get *down* into the earth, you can still do it. Body waves really are body waves. They basically expand out like a sphere from the focus/hypocenter beneath the surface. Now, you’re just going to have to trust me on this next part, because unless you’ve studied geophysics, it’s rather complicated. First is Snell’s law. That’s the easy part. Waves will refract when they hit a boundary between two different mediums. Like light making your spoon appear to “bend” when it’s in a glass. If you look at the little graphic below, you’ll see how these waves spread. They get bent, they travel, and because we’re dealing with curves, not a flat surface, they eventually come out somewhere.
Now look at the core, you’ll see that S-waves stop, and P-waves concentrate, and you have the “shadow” on either side. The reason is that the outer core is a liquid, and transverse waves can’t travel through liquids (which is not 100% true, but close enough for this explanation unless you want to see a lot of partial derivatives), so the S-waves that hit the outer core are just absorbed and die and we never hear from them again. P-waves on the other hand, can travel right through, and because of the way that the refraction occurs, they get concentrated through the core. What you end up with is the area of seismic shadow. There are no S-waves left to hit there, and all of the P-waves are busy elsewhere.
So why does this help us at all? Because any given wave, knowing where it started, can travel only *one* specific path to get to the location that it’s recorded. Again, partial differential equations. Let’s say that you have earthquake focus X, and seismograph Y. In this theoretical situation a wave that reaches the seismograph 40 seconds after the earthquake (as measured by the P-/S- lag) can only have taken one path through the Earth. 7 km down here, then 30 degrees to the north and relative to horizontal for 100 km, then up and out another 7 km to point Y. It can only go that ONE path.
Since we can calculate these *one* paths, and we have tens of thousands of earthquakes recorded for the past number of decades, we have just built a better and better picture of what’s beneath our feet.
It’s very cool. Geophysics is an incredibly fascinating subject, though I wouldn’t recommend specializing in it as a profession.
by: Earth Man
on: 24th May 10