How do underground nuclear tests work? originally appeared on Quora: the place to gain and share knowledge, empowering people to learn from others and better understand the world.
Answer by Richard Muller, Professor of Physics at UC Berkeley, author of Now, The Physics of Time, on Quora:
In the 1950s, there was great concern that atmospheric tests were responsible for climate change. Tornadoes, hurricanes, floods and droughts were being attributed to the testing program.
In addition, there was an issue with more substance: the growing level of radiocarbon in the atmosphere starting in the early 1960s. There was a big jump in that and in several other radioactivities. Were they a real danger? Probably not, but they were real.
I show the increase in C-14 in the attached figure. The big rise in C-14 was a result of huge thermonuclear bombs (H-bombs) being exploded in the atmosphere. The left-hand scale is in parts per thousand; the jump almost doubled the radioactive carbon in the atmosphere. The subsequent drop comes from the exchange of atmospheric carbon dioxide with the mostly non-radioactive carbon in the oceans; as they mix, the amount in the atmosphere decreases.
In 1962, Linus Pauling won the Nobel Peace Prize for his campaign to stop atmospheric testing. The US government signed a treaty with Russia outlawing atmospheric testing (and testing in space and under water) in part because they believed that they could learn what they needed about new designs with underground testing.
Since climate is still changing, and most of the change (except for global warming) is not clearly attributable to greenhouse gases, I sometimes jokingly remind people of the explanation for climate change of the 1950s. Since the variability of the weather was attributed to atmospheric testing, my joke is that the current variability must be due to the atmospheric test ban.
In an underground test, a hole is dug at the end of a tunnel, big enough so that the initial expansion of the fireball can be measured and studied before it hits the walls. After the explosion, there is typically a large spherical cavity underground with a layer of melted rock on its inner surface. The explosion creates an earthquake that can be measured around the world. It can be distinguished as a nuclear explosion by several indicators, including the fact that it is mostly P-wave (compressional) with very little shear (S) wave, and the fact that the epicenter is typically shallow.
I believe that the North Koreans have an incentive to make their underground tests appear larger than they are. To do this, all they have to do is to make the initial cavity smaller than we would, so that more of the energy goes into shaking the rock. That makes it more difficult for them to study the explosion, but it can give them a substantial propaganda value, especially by analysts who feel obliged to interpret the large seismic pulse using the “worst case scenario”. These analysts overestimate the yield of the weapon. A similar “worst case” interpretation of missile numbers led to the missile arms race of the 1960s.
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