On August 31, 2012, a large (Mw 7.6) thrust earthquake occurred within the subducting Philippine Sea plate seaward of a low seismicity region of the plate boundary (9.5°N–11.5°N), possibly as a result of horizontal compressional stress accumulation offshore of a locked megathrust. The mainshock ruptured from ∼30–50 km depth, with high radiated-energy/seismic-moment ratio and enriched short-period P-wave radiation. The nine largest aftershocks with global centroid moment tensor solutions (Mw ∼5.2–5.6) were shallow (10–13 km) normal-faulting outer-rise events, and a waveform template analysis using regional broadband data indicates many (48/110) similar normal faulting events (mb 4.0–5.5) and a few (8/110) likely shallow thrust faulting events on the megathrust with additional very small unidentified events. Coulomb stress perturbations may contribute to the mix of intraplate and interplate faulting. Geodetic assessment of any slip deficit on the megathrust is essential for quantifying the potential for a future large interplate rupture in this region.