Laboratory experiments reproducing seismic slip conditions show extreme frictional weakening due to the activation of lubrication processes. Due to a substantial variability in the details of the weakening transient, generalization of experimental results and comparison to seismic observations have not been possible so far. Here we show that during the weakening, shear stress.. is generally well matched by a power law of slip u in the form tau proportional to u(-alpha) (with 0.35 < alpha < 0.6). The resulting fracture energy G(f) can be approximated by a power law in some aspects in agreement with the seismological estimates G'. It appears that G(f) and G' are comparable in the range 0.01 < u < 0.3 m. However, G' surpasses G(f) at larger slips: at u approximate to 10 m, G' approximate to 10(8) and G(f) approximate to 10(6). Possible interpretations of this misfit involve the complexity of damage and weakening mechanisms within mature fault zone structures.