Elastodynamic considerations suggest that the acceleration of ruptures to supershear velocities is accompanied by the release of Rayleigh waves along the fault from the stress breakdown zone. These waves generate a secondary slip pulse trailing the rupture front, but manifest almost entirely in ground motion perpendicular to the fault in the near-source region. We construct a spontaneously propagating rupture model exhibiting these features and use it to explain ground motions recorded during the 2002 Denali fault earthquake at pump station 10, located 3 km from the fault. We show that the initial pulses on both the fault normal and fault parallel components are due to the supershear stress release on the fault, whereas the later-arriving fault normal pulses result from the trailing subshear slip pulse on the fault.