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Can nuclear war cause geological impact like earthquakes and could it say break the USA into four large quadrants NE, SE, NW, and SW.

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    $\begingroup$ it can cause earthquakes but not break the USA into pieces. $\endgroup$ – A. C. A. C. Nov 20 '17 at 20:12
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    $\begingroup$ Are you committed to nukes or might any plausible weapon be used? Redirecting asteroids might break up part of the crust, and really highlights why it's not really a viable plan. $\endgroup$ – user25818 Nov 20 '17 at 23:20
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    $\begingroup$ @notstoreboughtdirt The required asteroid would be something like the Chicxulub crater event. These would be very rare (fortunately) with perhaps millions to tens of millions of years between such events on average. $\endgroup$ – StephenG Nov 21 '17 at 5:23
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    $\begingroup$ I can't imagine it being relevant--any event powerful enough to cut two lines down to sea level (the minimum needed to accomplish what you're after) would wipe out the human race. Maybe bacteria would survive. $\endgroup$ – Loren Pechtel Nov 21 '17 at 6:17
  • $\begingroup$ Practically, no. You probably could set off an earthquake with a nuclear explosive, but you would need to set it off in a fairly deep hole on the fault line. But explosives used as weapons would almost always (except trying to destroy things like en.wikipedia.org/wiki/Cheyenne_Mountain_Complex ) be detonated above the surface (airburst) to cause maximum damage. Of course physically splitting continents would be impossible. $\endgroup$ – jamesqf Nov 21 '17 at 18:44
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It all depends on the kind and size of the H-bombs used.

Current versions are way to feeble to do what you ask.

For reference:

  • $1\times 10^{17}$ J Energy released on the Earth's surface by the magnitude 9.1–9.3 2004 Indian Ocean earthquake
  • $2\times 10^{17}$ J Yield of the Tsar Bomba, the largest nuclear weapon ever tested (50 megatons)
  • $4\times 10^{22}$ J Estimated total energy released by the magnitude 9.1–9.3 2004 Indian Ocean earthquake

This means the largest tested H-bomb is more than 5 orders of magnitude less powerful than the largest earthquake registered... which did not split tectonic plates.

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    $\begingroup$ Note for readers not familiar with scientific notation: 4E22 is a 4 with 22 zeros after it, and 2.1E17 is 21 followed by 16 zeros. The quoted earthquake energy (4E22 J) is 20000 times larger than the Tsar Bomba bomb. $\endgroup$ – Nathaniel Nov 21 '17 at 2:20
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    $\begingroup$ @Nathaniel: You are actually missing one zero. Earthquake beats Tsar by a factor about 200000 (five zeros). $\endgroup$ – ZioByte Nov 21 '17 at 7:56
  • $\begingroup$ You are correct of course. (I did actually check it with a calculator, but must have then mistyped the answer.) $\endgroup$ – Nathaniel Nov 21 '17 at 8:22
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Geologically, no.

Physically moving the masses involved to split the USA into essentially four new tectonic plates with oceans between them is far beyond the range of even the strongest nuclear weapons. Even if nukes could move that much mass in a short time, it would inject enough energy into the biosphere as to wipe earth clean. Consider that man's larger nuclear weapons leave big holes equivalent to Mount St. Helen's (25 Megatons) which is only a bit of one mountain in one mountain range. Displacing the Rockies would take absurd numbers of nukes.

Fun note: If you want to know how big a crater, a given amount of energy will leave behind, have a look at the NASA "Craters are a blast!" PDF.

Further, the earth's crust is not a pane of glass that can be neatly shattered into four pieces.

Geopolitically, though...

Yeah, a few nukes in the right places might fracture the potentially fragile US Federal government. If the center of the country was turned into a wasteland that you can't even fly over, I could see how the East Coast and West Coast decide to go their different ways.

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  • $\begingroup$ energy sufficient to move large continents any distance quickly would just liquify them instead. +1 for finding a solution however. $\endgroup$ – John Nov 21 '17 at 14:29
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Nuclear explosions can indeed trigger earthquakes. But they can only release the energy already stored in Earth's crust, they can not create any new geological faults. Magnitude 8 earthquake is comparable to 15 megaton bomb in the amount of energy released, and magnitude 9 earthquake - to 500 megaton bomb. We need much bigger bombs to split tectonic plates, so US is safe for now.

What is the energy of an earthquake?

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  • $\begingroup$ If you created an absurdly powerful device (say a gigaton) and lowered it into the sea bed to a deep ocean fault zone you could trigger massive earthquakes and tsunamis capable of extreme damage to the US west coast. A similar sized device detonated in Yellowstone park might be sufficient to crack the crust enough to unleash the Yellowstone super volcano. XKCD did some calculation with a 100 foot diameter meteor and needed to reach .99 c to blast through the crust, which gives you an idea of the energy levels you are dealing with. $\endgroup$ – Thucydides Nov 20 '17 at 23:58
  • $\begingroup$ @Thucydides Massive damage - yes, plate fracturing - still no. Yellowstone hotspot had been exploding a number of times during the last several million years, but North American tectonic plate is still intact. $\endgroup$ – Alexander Nov 21 '17 at 0:05
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Geologically impossible.

First of all, landmasses like the US are made of continental crust which is between 30 and 45 km thick. You would have to bomb through that over horizontal distances of thousands of kilometers creating chasms (the amount of energy needed has already been dealt with in another answer). Those chasms would immediatly collapse due to the pressure from the sides.

Every plate the North American Plate moves as one solid object. The movement of the plates is governed by deep mantle convection and can't be influenced from the surface. Changes in mantle convection can of course break continents apart, but that's a process on geological timescales.

So even if you managed to create deep enough fissures through the continent it won't make the separated parts move differently towards each other, it's more likely that after the walls collapsed they will just fuse again and all you end up with is some deep valleys filled with water.

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