The March 3, 1985 central Chile earthquake (Ms=7.8) ruptured a well studied seismic gap along the Chilean subduction zone. The epicenter of this event is located near the center of an approximately 300 km long region which ruptured in a great event in 1906 (Mw=8.2). The northern portion of the 1906 zone has since ruptured in 1971 (Ms=7.5) and 1973 (Ms=6.7). We have determined the rupture history of the 1985 central Chile earthquake from the deconvolved source functions of body waves (both P and PP). The source functions show one major pulse of moment release, the onset of which occurs about 16 seconds after an initial small pulse. The azimuthal directivity in the P waves indicates that the rupture front propagated from the epicenter southward and that the moment release of the major pulse is concentrated in a region between the epicenter and approximately 75 km south of the epicenter. We conclude that the region of high moment release is the dominant asperity and was loaded by slip in the relatively weaker northern region.