Our ability to predict ground motions from future earthquakes hinges on the accurate modeling of the radiation from the earthquake source. A successful approach to modeling earthquake radiation has been to represent faults as a series of discrete, independently-rupturing subfaults, although their physical interpretation has remained largely unclear. Our simulation of ground motions from twenty-six well-recorded moderate-to-large earthquakes substantiates the hypothesis that large ruptures are made up of a sequence of smaller subevents. The size of a subevent follows a simple linear relationship with the size of the earthquake fault in an apparently deterministic manner. The strength of the high-frequency radiation is controlled by the maximum slip velocity, which varies stochastically over a small range. Both the characteristic subevent size and the slip velocity appear to be region-independent, indicating a remarkable uniformity in earthquake source properties. This observation helps to reduce the uncertainty in ground-motion prediction by constraining its essential and previously ambiguous parameters. ¿ 2001 American Geophysical Union