The seismic profile of a nuclear detonation is easily distinguished from an earthquake.
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EVASION SCENARIOS
If a nuclear explosion is conducted deep underground (depth around 1 km) at the center of a large spherical cavity in hard rock, with radius greater than or equal to about 25 meters times the cube root of the yield in kilotons, then the seismic signals can be reduced by a so-called decoupling factor that may reach up to about 70. It is easier to build cavities of the same volume that are elongated rather than spherical, and such aspherical cavities can also achieve high decoupling factors, but they also increase the concentration of stress on the cavity and can make it much more likely that radionuclides will be released into the atmosphere and be detected by the radionuclide monitoring network of the IMS. An overall evaluation of the cavity decoupling scenario therefore raises a number of different technical issues:
http://www.ldeo.columbia.edu/~richards/SeismoandCTBTVerif.html