What are the differences between explosions and earthquakes?

Both earthquakes and nuclear tests can rapidly release a large amount of energy. The energy source for small yield (typically less than 50 kilotons) thermonuclear devices is the splitting of heavy radioactive isotopes. This process produces about 20 million times the energy of each reacting atom in a chemical explosive. The energy source for an earthquake is tectonic strain accumulated by the relative motion of Earth's tectonic plates which is driven by mantle heat flow in the presence of the earth's gravitational field. In a nuclear test, all of the energy is suddenly (within milliseconds) released in the form of heat from a relatively small volume surrounding the thermo- nuclear device. The tremendous heat causes rapid expansion of a spherical cavity, which in turn generates seismic waves. The heat gradually conducts away from the cavity into the surrounding rock. However, rock is a poor conductor of heat so it can take many years for the thermal signature of the thermonuclear explosion to subside and the increase in the surface temperature above the explosion is insignificant.

Nuclear tests are also very shallow sources with the depth of burial generally less than a few hundred meters (the depth of burial is typically proportional to the cube root of the expected yield). The estimated yields of the larger Indian and Pakistani tests are approximately 2-40 kilotons. In a large earthquake, the elastic strain energy stored in the Earth's crust is released, within a few seconds to a few tens of seconds, by rupture along a fault and the strain energy is released from a relatively large volume of rock surrounding the fault rupture. For example, the magnitude 6.5 earthquake in Afghanistan (5/30/98 at 06:22:28 UTC 37.4 N, 70.0 E) had a source duration of about 5 seconds and an estimated source volume of order 4000 cubic kilometers. This earthquake also had a focal depth of 18 km. The energy release is equivalent to a 2000 kiloton nuclear explosion.