Seismic Boring Story

Now that the Japanese nuclear plants are shut down, and everything is quiet again, I veer away from controversy (once again), and do another long boring story. I have a lot of these, since over the years, I've had to put a lot of rowdy cousins to sleep at the cottage. Now, nobody comes anymore, because they are all working, and I'm left alone to fish.

This is the story of the humble seismic wave; it is created, travels a lot, and then dies. There are lots of different seismic waves, and I've done a lot of demonstrations of Slinky Seismology, but for now we can think of it as being a plain old seismic wave.

The seismic wave first appears when the earth is disturbed in some manner. It can be created simply by whacking the earth with a hammer, or the use of an explosion, but most seismic waves are created by earthquakes.

Seismic waves can be extremely useful. When artificially created, they can show oil reservoirs, or deep underground geology. When a big seismic wave is created by a large earthquake, it starts on a magical journey, which helps the Science guys. They need a lot of help against a hostile world.

Seismic waves travel in all directions from an earthquake. Every time they hit a layer in the earth, they split and reflect a little. Scientists record when the seismic waves finally emerge, with the use of an extensive network of seismometers. The other month, I talked extensively with a young lady scientist, on how the reflections of seismic waves were telling them about a very subtle feature of the earth's core. Seems there might be a thin orange rind around it, of lighter elements. For some reason, some people get very excited about that!

In fact, almost everything we know about the interior of the earth comes from the assistance of good old seismic waves. They are most helpful when used to locate earthquakes for us. Around the world, it became pretty obvious that something was going on with plate tectonics, when it appeared that earthquakes were clustered along plate margins. The ring of fire. In Ontario, they locate big nasty faults. SOSN

Of course, Nature doesn't know about good and evil, and really doesn't give a poop about us puny humans, so seismic waves have a dark side. An earthquake can really jack up the magnitude (size) of the wave, and this can knock down buildings, when combined with other things. It's a little known fact that seismic waves can get only so powerful before they can't travel anymore. A bit like cranking up your amp until the speaker blows! In this case, an extremely powerful seismic wave starts cracking the rock, (or moving water), and all its energy gets sucked up. A good, travelling seismic wave can only carry stress at a very low level, well in the linear range for fractured rock (and that isn't much!).

So, the average seismic wave in solid rock is the same as a tsunami in the deep ocean: not much, and is defined by a low PGV. What makes it nuke-crunching is when it runs along a fault (blind thrust is the worst), or it activates low velocity sediments. Much like jelly in a bowl, the whole thing shimmers and shakes like Santa Claus. We can then expect velocity amplifications of around 100 times. Sometimes there are standing waves, which crunch houses, and people can actually see them zooming along!

People only have a chance to play with the nice seismic waves if they are on solid rock. There is no chance for damage, since the PGV is perhaps only below 10 cm/sec. Of course, there isn't that much solid rock in the world, so most people make the best of it. Sometimes they do really silly things and ignore seismic waves, so they get whacked. Others are more careful, and respect the humble seismic wave.