Predicted and postshot geological and geophysical factors associated with the large underground nuclear detonation CANNIKIN are presented. An average mb magnitude of 7.02 and an Ms magnitude of 5.74 were determined from 56 stations of the WWSSN and Canadian seismograph networks. No change was detected in the naturally occurring pattern of earthquakes recorded at regional and teleseismic distances to indicate that the shot affected natural seismicity other than at close distances. A complex group of waves with mixed Love- and Rayleigh-type motion was observed in latter parts of the surface-wave train recorded at stations across Canada and the United States, which could possibly be associated with an earthquake occurring several minutes after the shot. Surface-wave magnitudes computed for the Love-type motion were smaller than the magnitudes computed from maximum surface-wave amplitudes.