Abstract Megathrust earthquakes often break the slab‐crust interface and radiate high‐frequency seismic waves from the down dip of their coseismic slip. In contrast, we show that the 2021 Mw8.1 Kermadec earthquake deviates from this mode. We resolve the kinematic rupture process of the event by leveraging finite fault inversions and a calibrated tele‐seismic back‐projection imaging method. The finite fault inversions reveal a compact slip patch with a ∼7‐m peak slip of the earthquake located on the slab‐mantle interface at the depth of 20–50 km, while the back‐projection shows the high‐frequency energy was radiated from ∼20 km depth along the updip edge of the slip patch, and coinciding with the slab‐forearc Moho intersection. The rougher frictional patches responsible for high‐frequency radiation are likely caused by strong material contrast transitioning from slab‐crust to slab‐forearc mantle contacts. Significant slip on the slab‐mantle contact necessitates a re‐evaluation of earthquake hazards in conventionally considered aseismic regions.