A nuclear explosion can cause an earthquake and even an aftershock sequence. However, earthquakes induced by explosions have been much smaller than the explosion, and the aftershock sequence produces fewer and smaller aftershocks than a similar size earthquake. Not all explosions have caused earthquakes. The range of a possible earthquake triggered by an explosion is limited to a few tens of kilometers from the shot point.
The possibility of large Nevada Test Site nuclear explosions triggering damaging earthquakes in California was publicly raised in 1969. As a test of this possibility, the rate of earthquake occurrence in northern California (magnitude 3.5 and larger) and the known times of the six largest thermonuclear tests (1965-1969) were plotted and it was obvious that no peaks in the seismicity occur at the times of the explosions. The largest underground thermonuclear tests conducted by the U.S. were detonated at the western end of the Aleutian Islands in Amchitka. The largest of these was a five megaton test (codename Cannikin) that occurred on November 6, 1971 with an energy release equivalent to a magnitude 6.9 earthquake. It did not trigger any earthquakes in the seismically active Aleutian Islands.