The amount of energy going into the formation of elastic waves in the far field radiation zone was determined for the underground nuclear explosions Boxcar, Benham, Salmon, Schooner, Shoal and Sterling. The method is based on Sharpe's spherically symmetric model. The analytical solution of Sharpe's problem is developed into an energy equation and applied to the experimental results. The results obtained in this manner are in the same range found by other investigators. A basic conclusion is that seismic energy efficiency appears to be significantly lower for cratering events than for contained events.