Large earthquakes are caused by the release of accumulated stress in the crust along faults. The stress accumulates over hundreds to thousands of years in response to ongoing motions among the Earth's tectonic plates at rates of 1-15 cm/year. Theoretically, it is possible that a very large blast could yield enough energy to trigger an earthquake. However, the earthquake would have to be on the verge of happening "on its own," and the shock from the blast could act only as a catalyst. The blast itself would not in any way cause the build-up of stress along the fault which ruptured to produce the earthquake. Most geophysicists consider the influence of nuclear blasts, and even other earthquakes, to have a negligible effect on new earthquake activity.
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