Did the earth move for you? Well that was a seismic wave. They are complex buggers so we'll lump them all together here in one definition to make it even more confusing. Give you something to work out on your own, so you don't get bored with life.
OK then.
First, you got yer body waves and then yer surface waves. The body waves travel through the earth's body like lunch travels through yours. Surface waves on the other hand are more like creepy caterpillars inching along your skin.
Body waves not only travel through the inside of the earth but they move in every possible direction, all at once. They get called a lot of names. You have yer p-waves and yer s-waves to start. And maybe some others, unofficially.
"P" is a primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave.
"S" is a shear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave.
Now for yer P think of a slinky toy on a table, with you pushing it back and forth, away from you and toward you. Compression and rebound. Fine and dandy.
For yer S, think of a wave on water, only real s-waves can't travel through liquids or gases (or the liquid core of the earth), but they would look like water waves if you could see them. Hot waves. Real hot waves. Prolly spicy too, as far as rock goes.
Now then that leaves surface waves. They are different from body waves. They don't travel through the earth but stay on top where it's more fun and they can watch stuff happen if they get bored.
Oddly enough, surface waves and the s-wave type of body waves are the strongest, at least at the surface, and cause the most damage as they vibrate around and do what-all. This is how buildings fall down. Good time to be somewhere else, as so often happens in life.
And if you're wondering what the heck this has to do with backpacking, well it doesn't, unless you get (un)lucky and wake up on top of an earthquake. This is so you know. You get to decide if it's fun or not, your ownself.
So if you wake up and it feels like the earth is one big snake wiggling around underneath you, that's a p-wave down there, coming up for air. On the other hand if it feels like you're getting booted in the gut or the butt (depending which side you sleep on), and you seem to be bouncing all over, and you actually are all alone, and it's the earth what's doing stuff with you, well that's an s-wave or an uppity surface wave dropping by to say hello and give you a few kicks just for fun. I can still do that part.
Enjoy if possible.
As for me, all my references have gone dead, so I can't prove any of this. Time to sit back, have another beer and do some kitty tickling, I guess. I can still do that part.
See tabs at the top for definitions and books.
Have anything worth adding? Then try
sosayseff+eff@gmail.com
Me? Recently bumped by something that went by real fast. Think it said "Wheeeee!" But maybe not.