A weak seismic event that occurred on May 4, 1983, close to the Lop Nor underground test site in Southern Sinkiang province, China, has been studied for source discrimination. Digital waveforms, obtained at Gauribidanur array, India, of this event, of six known shallow-focus earthquakes from Southern Sinkiang and of two known underground nuclear explosions in Lop Nor, were processed for further analysis. The estimates of signal complexity and TMF (third moment of frequency) parameters of the May 4 event were found to be quite different from those of the known explosions whereas they were consistent with those of the known earthquakes. On this basis the event under investigation appeared to be a crustal-focus earthquake.