We study the mechanism of low frequency earthquakes (LFEs) in the Nankai Trough in western Shikoku, Japan. Precise locations have previously suggested that they represent shear slip on the plate boundary. In this paper we examine the mechanism of these events. Due to the low signal-to-noise ratio, we analyze stacked LFE waveforms and compare them with the waveforms of nearby earthquakes of known mechanism within the subducting Philippine Sea Plate. Analysis of both the focal mechanism using P-wave first-motions and the moment tensor using S waveforms indicates that LFEs represent shear slip on a low-angle thrust fault dipping to the northwest, namely the plate interface. Together with reports that deep tremor consists of a swarm of LFEs, our results suggest that deep tremor is generated directly by shear slip on the plate interface, and as such represents a seismic signature of the accompanying slow slip events.