We re-examine data from a deep earthquake beneath NE Taiwan as recorded by the Formosa Array to explain mechanisms of a significant later P phase. For 2D simulations, we set up an initial velocity structure with (A) a high V_p anomaly in the mantle wedge above Ryukyu subduction, and (B) a slightly high V_p Eurasian lithosphere to the west with a sub-vertical boundary – based on results of tomographic studies. We conclude that (1) the small-amplitude first P phase is attributed to the energies radiated near the nodal point of the focal mechanism and propagated through (A), (2) those of the significant later P phase are analogous to a head wave that propagates and generates spherical waves along (B) that are received at the surface. Accordingly, stations of zero delay time between the first and second P provide a first-ever portrayal of the Eurasian lithosphere boundary by waveform constraints.