There was an earthquake in Afghanistan in March 2002. Coincidentally, there was a lot of bombing going on. Or was it coincidence? Many people ask, Do aerial bombardments cause quakes? Scientists immediately say No. First of all, they don't like the word "cause."
An earthquake is the sudden release of strain energy in the Earth's crust (see "Earthquakes in a Nutshell"), but strain builds up from several different causes: the movements of plate tectonics, the weight of sediments shifting from erosion or from deposition, changes in fluids underground, and more obscure factors like mineral phase changes in the mantle (see "Deep Earthquakes"). They all add up, and we can't single out one of them as the cause. Scientists prefer to talk about what might trigger or induce an earthquake.
Do Bombs Induce Quakes?
OK, then, let's ask again. Did the bombing induce the quake? From a century of experience, we can confidently answer No. Some human activities do induce earthquakes, but not bombing like that in Afghanistan.
The question is easy to test: just look at the seismic record and see if earthquakes increase after episodes of bombing. Unfortunately, there has been plenty of bombing in the last hundred years. Fortunately, seismologists have monitored earthquakes for that whole century. No increase in quakes—not even little ones—follows bombing campaigns.
Some authorities have tried to blame quakes on bombs. After the 31 May 1970 Peruvian earthquake (still remembered for the deadly landslide it triggered from the mountain Huascarán), the Peruvian government accused the French of setting it off with their atomic tests in Mururoa Atoll, all the way across the Pacific. Peruvian scientists at the multinational research agency CERESIS responded in the press that this was nonsense.
There are other short-period natural mechanisms that you could compare with bombing: stresses from the tides, storm winds blowing against mountain ranges, landslides, volcanic eruptions and the shaking of nearby earthquakes. Those mechanisms involve a great deal more energy than bombing, and they're more coherent—less random.
None of those correlate with earthquakes either, with one uncommon exception: sometimes a large earthquake can trigger smaller ones at a distance. The 1992 Landers quake was the first clear example, and that was a large event, magnitude 7.3. The 2002 Denali earthquake in Alaska, magnitude 7.9, triggered events thousands of kilometers away at Yellowstone in Wyoming.
So the forces that ordinary bombs exert don't cause earthquakes. The amount of energy in bomb explosions is microscopic compared to the energies of earthquakes. It's like the difference between playing on a piano and dropping the piano down a flight of stairs.
Can Humans Induce Earthquakes at All?
We do have some good examples of human activities causing triggered quakes. It takes years of concerted effort, but they happen.
One type of induced quake occurs where fluids are pumped out of or into the ground. Oil-producing districts, for example, or areas where toxic wastewater is pumped into deep disposal wells experience small shocks that are sometimes strong enough to feel.
Another type happens where new water reservoirs are created. The first famous example was after the Hoover Dam created Lake Mead in southern Nevada. Hundreds of local earthquakes, some up to magnitude 5, happened in the decade after 1934.
Underground mines disrupt the natural stress state of underground rocks. The wall of a mine passage has all the weight of the rock above it pressing down, but no weight on the open side. Sometimes the wall bursts from the strain, spraying the passage with rock bits and destroying anything nearby.
Seismograph records of rock bursts look much like earthquakes. The largest known rock burst happened in a German potash mine in 1989 and had a magnitude of 5.7. It demolished the mine openings and damaged most of the town of Volkershausen.
Another kind of mining-induced quake happens as the ground subsides into the mined-out space. These are smaller, gentler events than rock bursts. But when large open spaces are mined out, the energy involved in a ceiling collapse can be substantial. The "mountain bump" that destroyed the Crandall Canyon coal mine in Utah on 6 August 2007 reached magnitude 3.9.
All of these types of human-induced seismicity happen from long-term changes in the underground stress field, not the brief, tiny stresses from bomb explosions.
OK, what about nuclear explosions? Well, they indeed cause earthquakes—that is, they are earthquakes, releases of energy that are felt as shaking and recorded by seismographs around the world. But not even the largest bomb test has ever induced a natural earthquake. (The Berkeley Seismological Laboratory explains more.)
PS: Like natural earthquakes, underground nuclear tests have aftershocks. We know this because they act like aftershocks: they are smaller than the main event, cluster around its location, and decrease as time passes. Also, the largest bomb tests have the same magnitude as moderate earthquakes, and their aftershocks are smaller by a full unit of magnitude (that is, they obey Båth's law), just like natural aftershocks.