Dynamic rupture of reverse and normal fault intersecting the surface are investigated. In the case of a normal fault nucleating at depth and propagating upwards, coupling of rupture-radiated stress and free boundary conditions at the surface may induce a shallow secondary nucleation anticipating up to a few seconds the arrival of the main rupture front. Indeed, the free surface induces normal stress fluctuations modifying the fault frictional strength. No significant effect on rupture velocity is observed in the case of reverse faulting. These incidences are explained by a stress analysis and illustrated by some numerical simulations in the case of dynamic normal faulting in a homogeneous half-space. The described effects could explain some observations of high frequency radiation close to the surface in documented shallow earthquakes like in Kalamata, 1986 [Bouin, 1994>. ¿ 1998 American Geophysical Union