Data derived from studies by other workers of prominent, well-defined aftershock sequences are employed to obtain evidence for the source dimensions of earthquakes in the magnitude range m = 412 to 6. The characteristic length of the aftershock zone is defined as the longest dimension of the map view of the aftershock epicenters. On the basis of this aftershock data, the following rough limits may be placed on the likely size of small earthquakes in the western United States: for magnitude 5 events, 5 to 25 km; for magnitude 4 events, 2 to 15 km. These limits are about an order of magnitude larger than those proposed by Press for earthquakes and underground explosions and represent an independent confirmation of the results for earthquakes of Wyss and Brune. These results have an important bearing on recent studies of the excitation of surface waves by earthquakes and underground explosions.

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