why don't nuclear tests start earthquakes?

When the nuclear bombs are tested underground, why don’t they trigger an earthquake?
Has this ever been a concern for those in charge of these tests? Could such a thing happen or am I just too worried?
I read that when a country test a bomb that other countries can tell that they did by reading the machines that measure earthquakes.
What would it take to start those fault lines to moving?

Most of those tests are done in geologicaly “stable” areas… ones that don’t have fault lines running through them. The possibility of a nuclear test setting off an earthquake is pretty low, but it has been theorized that maybe, just maybe, if you set off a large enough device at EXACTLY the right spot on a fault line, then a quake might be triggered. Another one for the conspiracy nits.

And, don’t forget, there’s a whole LOT of land to move.

In anything other than very a delicate or unstable geologic area there would not be nearly enough energy released. An atomic bomb’s energy would be pitifuly small on a geologic scale.

Astro has it right. As powerful as we like to think of a nuclear warhead being, they are pathetically small potatoes when compared to an earthquake or a volcano’s energy. And in any event, the nuclear weapons tested are typically very small yield (in the last few decades anyhow).

Nukes are just not that powerful. It’s like an ant compared to a bulldozer. Well, maybe a mouse. Hard to say, really. :wink:

I’ve experience an artificially stimulated earthquake, about a 4.0.

In the oil fields about 100 miles north of my hometown, they were trying to push the last of some oil out of one area by pumping water down several existing drilling sites and hoping that any oil would come out of a central drill site. But what happened was that the water lubricated the area between two rock layers, and they shifted, which is basically what an earthquake is. A 4.0 is noticable, even 100 miles away, but it didn’t cause any damage.

This was the basis for scientists thinking that they might be able to use this process to de-stress known fault lines before a major earthquake happened. But they’re all in fear that they’ll either overdo the water or the fault will shift more than calculated, and they’ll set off a much bigger quake.

I think distance to the nearest fault line has a lot to do w/ it. If not then any earthquake would trigger all others

he set off a nuke to prevent an earthquake andre the giant helped

For reference, here’s a mapping between Richter numbers and tons of TNT:

4.0 = 6.3 tons TNT
5.0 = 200 tons
6.0 = 6.3 kilotons
7.0 = 200 kilotons
8.0 = 6.3 Megatons
9.0 = 200 Megatons

For comparison, the biggest weapon the US ever made was the Mk-41, a 25 Megaton 3-stage thermonuclear weapon. It’s not currently deployed though. I guess they decided they didn’t really need anything that big.

I originally thought that even a large nuclear device would be something like the ant vs. bulldozer analogy above, but then I did some research, and what I found kind of shocked me:

According to this site, this is the Richter formula:

log[sub]10[/sub] E = 4.4+1.5M where E is the energy in Joules, and M is the richter number.

According to this one, the kiloton is defined to be exactly 10[sup]12[/sup] calories or 4.186x10[sup]12[/sup] joules.

So let’s compare a 25 megaton nuke:

25 x megatons
25 x 1e3 x kilotons
25 x 1e3 x 4.186e12 joules
105e15 joules
1.05e17 joules.

…with a magnitude 9 earthquake:

log[sub]10[/sub] E = 4.4 + 1.5 (8)
log[sub]10[/sub] E = 4.4 + 13.5
log[sub]10[/sub] E = 17.9
E = 10[sup]17.9[/sup]
or about 7.94e17 joules.

so a 9.0 earthquake is almost 8 times more powerful than a 25Mt bomb, but the bomb is almost 4 times more powerful than an 8.0 earthquake (2.51e16 joules). Which jibes almost exactly with SmackFu’s table.

Unless, of course, there’s something wrong with my arithmetic or my formulas. There is the fact that, in order to be equivalent to an earthquake, the weapon would have to be detonated a good distance underground. Otherwise it just makes a crater and does local damage, and the energy just disperses into the atmosphere.

Oh, a major difference is that the energy released by an earthquake is almost all kinetic energy, whereas energy released by a bomb is mostly radiation and heat. Even though the numbers compare, it’s still very different, since heat doesn’t translate easily or efficiently into motion.

Thanks, you have all been very helpful.
I did have a hard time with the formuals Joe Cool but I think I got the point.
Thank you.