Relative source time functions (RSTF) have been estimated for four underground nuclear explosions and seven earthquakes in Central Asia using broadband P waveforms of nearby smaller events as empirical Green's functions (EGF). RSTFs of the four explosions (mb=5.3 to 6.5) are each dominated by a simple pulse with a source duration of 0.4 to 0.8 s. RSTFs for two of the explosions show a significant secondary pulse with a pulse width similar to that of the first pulse. We conclude that the secondary phases are most likely associated with the spall slapdown phenomenon. Seismic moment releases by the spall phases are less than one third of those by the first explosion pulses. Elastic radii of the explosions are estimated to be about 0.25 to 0.5 km, and stress drops of the explosions range from 13 to 52 MPa. In contrast, RSTFs on earthquakes studied (mb=5.5 to 6.6) typically comprise multiple source pulses with a total source duration from a few to several tens of seconds, indicating that the complex source process involves a fault dimension of several tens of kilometers. Stress drops of the earthquakes are much smaller than those of the nuclear explosions, ranging from 0.5 to 3.5 MPa. Our study demonstrates the power of the EGF method for retrieving RSTFs and reveals that differences in RSTFs and source parameters can be used to distinguish large nuclear explosions from moderate to large earthquakes (mb≥5.5). ¿ American Geophysical Union 1995