Tuesday, August 21, 2007

Fault stress boring story

We've seen that some very simple physical concepts lead to big earthquakes: dynamic friction, and fractal self-similarity. Finally, we have to deal with the fact that those darn earthquakes keep happening at the same spots, over and over again.

When you slip in the bathtub, you go head-over-heels and dent your posterior. It's over and done with. You don't get up and do it again! We can say that all the energy has been dissipated, the stresses relieved, and this particular mechanism (of laughter for others!) is dead.

But, along all the great faults in the world, those feet just keep slipping, and along the same lines. We have continuing fault mechanics. Most people just take this for granted without looking into the actual beauty of it.

An earthquake always starts with stressed rock. This activates the shear stress on the feet, that is resisted by static friction. People sometimes think that those feet act on their own and cause the big ruckus, but they don't. The entire mass of rock surrounding the feet can be thought of as a giant spring, compressed and fixed at distant points.

If we just cut all the rock around the fault with a giant rigid cookie cutter, then we have an isolated system. Somebody tickles the feet, and they let go. Big earthquake that knocks down stone houses! But something has happened to our system: it's out of gas, and can't generate another fun earthquake. The fault is locked up and extinct, and the people can build more stone houses with impunity.

Nobody really thinks of this on the plate margins, because the plates move, and soon our system is charged up again. If the feet have moved many times in the past million years, they have buffed up a nice bathtub, and the action always happens at the same spots. It's rare that things move to a new bathtub (it's happening in California!).

And now for something completely different! In Ontario (which nobody cares about!) we have mostly rigid rock, which acts as the giant stiff cookie-cutter frames of our previous thought experiment. One would think if we had an earthquake, all the stress would be relieved, and it would never happen again at the same spot. In fact, this is the religion amongst all our emergency measures organizations! :)

The harsh fact is that our earthquakes happen at the same spots again and again, and have done so for millions of years. Many scientists have been baffled by the lack of giant mountain ranges at these locations, since all that seismic energy has to be going into building something! If you look at all seismic hotspots of Eastern North America (ENA), you will see the same boring thing: flat lowlands with lots of water. If you go down to the New Madrid location (hottest of hot spots!), you have to place your head on the ground to see what they call a 'ridge'.

Good night, and we'll continue with ENA mechanisms another time (maybe it'll be on TV! ha, ha!).

No comments: