<p>The impact of lithospheric rheology and subduction dynamics on intraslab stress and earthquake characteristics, as well as the cause of the scarcity of continental mantle earthquakes remain debated. We investigate the 2006 Pingtung offshore earthquake doublet (Mw 6.9) in the northern Manila subduction zone, where thinned continental lithosphere is subducting. These events exhibit complex source processes at mantle depths, making their fault geometry poorly constrained. Using potency density tensor inversion on teleseismic waveforms, we resolve the source process without predefined fault geometry. The first event exhibits normal faulting with minor reverse faulting at a deeper depth, while the second shows deeper strike-slip faulting. These diverse focal mechanisms reveal scattered mantle faulting and intraslab stress heterogeneity. Geodynamic modeling indicates mantle earthquakes require both strong subducting mantle lithosphere and substantial bending-induced differential stress. The global scarcity of such events stems from insufficient differential stress in most continental lithospheres.</p>