The Flores earthquake of December 23, 1978, represents the first seismological evidence for active back arc thrusting behind the eastern Sunda Arc. This mb=5.8 earthquake occurred north of Flores Island, where seismic reflection profiling has revealed back arc thrusting, interpreted to be a reaction to compression of the arc following collision with the Australian continent. We investigate the source mechanism and depth of the Flores earthquake by inversion of long-period P waveforms and relocate the earthquake's epicenter incorporating arrival time data from local stations. We find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11.5 km (7 km below the seafloor). The best fitting fault plane solution is consistent with active southward thrusting of the floor of the Flores Basin beneath the volcanic arc along a 30¿ dipping fault plane. The morphology of the thrust zone and the free-air graity profile over the Flores Basin in the epicentral region resemble those of oceanic trenches, and the observed gravity field is best interpreted if the crust of the Flores Basin dips smoothly below the accreted wedge at an average angle of about 6¿. The position of the earthquake hypocenter south of the Flores Thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates. The Flores Thrust is thus the surface expression of a deep-seated thrust zone and may represent the initial state of polarity reversal of the eastern Sunda Arc.