The energy flux F at the rupture tip has been previously computed only for 2D steady‐state singular cracks. In this paper, I compute F for fully dynamic 3D ruptures, propagating both with constant and variable rupture speed (vr) over finite faults directed by a governing law with a cohesive zone (and thus nonsingular ruptures). The results presented here indicate that F is positive and increasing over the whole range of vr from zero up to P‐wave speed. This is in contrast with 2D steady‐state singular cracks, which predict the existence of a forbidden zone in the range of rupture speeds because in that interval F would be negative. Moreover, I found that in 3D ruptures with cohesive force, F is proportional to vr, again in contrast to 2D steady‐state singular cracks, in which F is not a unique function of vr and also exhibits an inverse dependence on vr. More specifically, it emerges that fast earthquakes tend to have a higher energy flux at the crack tip compared with slow ruptures. Finally, I show that the magnitude of F is basically due to its component aligned in the direction of the initial shear stress.

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