The reduced displacement potential obtained from close-in observation of seismic displacement during an underground explosion usually takes the form of a step function with or without a small overshoot. Theoretical prediction by shock-wave calculation appears to agree with the close-in data. The step-function source has also been supported by the observations on Rayleigh waves at periods longer than 10 sec. We found, however, some inconsistency between the published data on residual potentials obtained from close-in data and those on seismic moments obtained from long-period Rayleigh waves. It appears that only about 13 of the residual potential is transmitted to the far-field at long periods. This discrepancy is, however, consistent with several observations made on teleseismic signals suggesting an impulse rather than a step as the primary form of the potential function. New observations of the two NTS events at distances 2.6 to 7.8 km using wide dynamic range, wide-band accelerometers, combined with data from the far-field, support a large overshoot 4 to 5 times the residual value. This result accounts for the efficiency of the Ms - mb discriminant between earthquakes and explosions with mb around 4 and greater. The compaction of the source volume by spalling was suggested as a possible mechanism for the large overshoot.

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