Sunday, August 26, 2007

Fault mechanisms boring story - II

If you recall, I left you asleep at the point of wonder why we have earthquakes at all in Eastern North America (ENA). Really, if the continent is one big massive piece of tombstone granite, then any stress-relieving earthquake should just carve a 'stress-hole' like boring a mine.

Once this giant Tim Hortons donut gets formed, the remaining rock arches around it, and leaves a perfectly stable structure, with an extinct fault in the middle. This doesn't happen on the plate margins, with all those plates sliding past each other, like those sliding number puzzles we had as kids. For the equivalent, we would need mountain ranges or visible fault sliding to keep the similar ENA earthquake hotspots going for a few million years.

Take a typical fault zone trying to grow up in ENA. It starts with a fractured weak spot, perhaps reactivated by the modern stress field. Eventually (perhaps), the shear stress builds up along a fracture, and the bath-tub feet let go! The fractured rock behaves exactly the same as in California or Peru.

But that's it! Our poor little earthquake has shot its load. The feet have lost their stress, and the surrounding rock has clamped up on our poor little fellow. He can't grow up...

That is, if our earthquake follows conventional thinking for ENA earthquakes. But no, our earthquake fault zone is smarter than that, it is following Harold's original thinking, which doesn't get into the university cartel, because they all got tenure!

Once our earthquake has caused its bump (like a Utah coalmine!), it looks around and sees what it has done, which is to freshly fracture a lot more rock. Its seismic effort hasn't gone into mountain building, or plate sliding, it's gone into fracture surface energy! The academics never thought of that!

But this isn't enough for Quakey (like that name?), in its battle against the implacable rock. Luckily, he's (she's?) under a big hunk of water, and water slowly flows into the new fractures. Now things are cooking! The water does all sorts of wonderful things. First, the water pressure reduces the normal stress on the faults and makes them more likely to slide again. Second, the water hydrofractures and extends existing tension cracks. Finally, the fresh water is corrosive and starts chewing at the adhesion points.

Quakey has become a growing entity, like the Blob that ate New York. So, under Hamilton, New Madrid, countless other ENA locations, we have growing things, which although not actually alive, can give a darn good account of themselves!

As Quakey grows, he can tap into more stress from the surrounding rock, just like we were excavating a new Utah coal mine (or an underground nuclear waste thingie!). He gives many signs of his growth, lots of little earthquakes, just like his bigger plate margin brothers. All the bathtub feet are working, and eventually there is a large, scale-limited earthquake. The process begins again for a bigger earthquake!

Quakey may start as a simple thrust fault along a pre-existing fracture, but soon suffers growing pains. The old fracture provides the water, but a simple thrust fault isn't enough to suck out all the stress from the surround rock. In order to avoid another stress lockup, he must start developing shear wings, which produce strike-slip earthquakes. When he eventually grows into a monster, he resembles the New Madrid fault system.

All the time, these zones are sending the signals to prove Harold is right, and the others are wrong, but the scientists and politicians are too cheap to provide the detailed seismic monitoring that is required. Only the Southern Ontario Seismic Network probably has sufficient horsepower and density, but the Hamilton zone is just a baby!

Everyday, these zones try to show themselves. They send typical 'fluid injection' earthquakes, they mix thrust and strike-slip mechanisms, and they only show up under bodies of water. But without monitoring, the world goes on arguing with Harold....

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