Sunday, August 12, 2007

Fault rupture boring story

It's fun on the dock, fishing. All my intellectual energy is totally drained from me, especially if I'm cut off from civilization for a long time. I'm back for a couple of days, and thought it's time for another boring story. This one is about fault mechanics, which I think is the most misunderstood subject of all time (it helps to have a rock mechanics background!).

The whole reason we have earthquakes is because of a simple little physical phenomenon that we observe when we have a shower in a cheap hotel, while forgetting to put in the rubber bathmat.

One minute our feet are firmly on the tub, stuck like glue. The next second, we do one little thing, and swoosh! If we are lucky, the clingy plastic shower curtain has saved us.

This is the most dramatic demonstration that I know, showing the difference between static and dynamic friction. I've done some more details in Fault Friction.

Wet, fractured rock behaves almost the same throughout the world, and this is the stuff of earthquakes! Without a difference, in wet rock, between static and dynamic friction, we would be without most earthquakes (those extremely deep earthquakes are a bizarre exception, but who cares about them?)

So all earthquakes start with a single crystal (grain) of rock (mineral), rubbing against another. There is shear stress, which is a force trying to slide the grains past each other, and there is the normal stress, which is the force jamming them together. At the very tiny point of contact, the minerals (quartz, most likely), are cold-welded, making a very strong adhesion bond.

The shear stress attempts to break these bonds, by inducing tiny molecular earthquakes. Studies show that if we bake these things so that there is no water, then new bonds are formed as quickly as old ones are broken. Thus, the resistance to slipping remains a constant value.

Thus, good old water is need for some action! If we sprinkle in some water, then two very interesting things happen. Firstly, we do not have stability at the near-failure point, because the water acts to eat away at the adhesion points. This little thing, called stress corrosion, is one of the best, totally unappreciated discoveries from the giant money-sucking pit, called the AECL Underground Research Laboratory (URL) (RIP). (Thanktheloard for massive gov't pork!)

Secondly, once the adhesion bonds start to break, they are covered in little water molecules, just like the other potential contact points. When a new adhesion contact wants to form, there's that nasty water gunking things up!

So now it takes some time to form some new bonds, time that the slippery feet (or mineral grains) don't have if there is a big following force (like gravity, or the San Andreas!). The sliding surface starts to zoom, and we have dynamic friction.

As you fall asleep, you may wonder what bathtub feet, and mineral grains have to do with a big old fault. The answer lies in self-similarity and Power Law. Much like a ratty old US bridge, everything can be represented as tiny little feet slipping, over and over again, combining into larger and larger feet, until the whole shebang blows! But those details are in the next boring story!

4 comments:

monado said...

Nice to hear from you again! That was a very clear explanation.

I've been looking for Ontario geology information; so far I haven't found much between the tourist board level ("We have lots of pretty scenery here") and the academic papers and big government maps showing all of Canada in coloured patches. I did find one set of lecture slides.

Harold Asmis said...

I'm also starting to look for material for my future course. Everything has some sort of restriction, or is behind the paywall. Very difficult to break the University Cartel!

monado said...

That's what Continuing Education courses are for. Suddenly, you're a student and a member of the university. And if I recall correctly, courses are free if you're older than 65. My aunt took delight in studying the history of some of the things she'd lived through (such as World War II) and explaining to the hippie love-children that we didn't really fight Hitler out of sheer blood-thirst.

Harold Asmis said...

Hey, I'm not that old! I retired from Hydro, remember?