Tuesday, February 5, 2008

Rock Mechanics - 5

Rock Mechanics
Rock Mechanics - 2
Rock Mechanics - 3
Rock Mechanics - 4

Having determined that there were generally low PGV's for solid rock, I was prepared to slide to home plate, by going back to Rock Mechanics! This subject is mostly about the behaviour of cracked, wet rock, which is exactly what have in earthquakes.

I was running a finite-differences wave propagation computer code, which I had written myself. In it, I could view traveling seismic waves, look at the generated stresses, and bounce waves off tunnels! All in all, better than Nintendo! Best of all, I could make the rock properties non-linear, and I found that if I introduced the tiniest non-linearity, the wave propagation would fail.

What's the source of true non-linearity in rock? It's when the stress disturbance of the wave causes some energy absorption in the rock. This can come from water flow in and out of pores, or sliding along a fracture. I was most interested in what could induce rock to slide.

That was fairly easy to look at, since you can't really disturb rock without making a ton of micro-seismic noise, and the South African gold mines had been wired up for sound, a long time ago. They had the great ability to actually generate earthquakes, and you could see the effect by mining through it afterward! In general, I found that the micro-crack damage zone was very confined to the actually slipped fault, and the seismic waves only induced rock failure, at a distance, if things were extremely unstable.

My next step was to estimate how close my target rock (Eastern North America, ENA) was to failure. This would also help determine the maximum induced stress, for if you knew what could cause micro-slippage, you would have another confirmation, along with the low PGV. For that, I ventured into Grand Geology, and Induced Seimicity!

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