Seismic array based analysis of the major Kokoxili earthquake (Tibet, 14 November 2001) yields an unambiguous reconstruction of the seismic rupture history and relates it to the generated seismic radiation. We demonstrate that after a classical sub-Rayleigh velocity stage, the rupture speed has jumped to supershear values close to compressional wave velocity over a 175-km-long fault segment, before abruptly slowing down in the late part of the earthquake. The transition locations between these three phases are correlated with the fault geometry and are associated with the most energetic radiation. This observation proves that the rupture velocity changes, as theoretically predicted, are a primary source of high-frequency seismic radiation. This result requires reconsidering the origins of seismic damage, generally attributed to slip variations.