We numerically investigate the supershear transition of inplane rupture under slip weakening friction and tectonic loading, using a non-smooth spectral element method. Nucleation of the rupture in the vicinity of an initial stress concentration is consistently solved together with the dynamic propagation. Nucleation is shown to influence the transition to the supershear propagation and, depending upon the initiation process and the level of the stress out of the nucleating asperity, crack-like or pulse-like solutions can be selected in the intersonic regime. Pulses are triggered by dynamic unloading of the traction ahead of the Rayleigh wave and tend to become metastable. The asymptotic numerical solution is always a crack propagating close to P wave speed. The rupture front and the arrest velocities are numerically characterized. We argue that the nucleation phase introduces a trade-off with the state of the stress, in determining the distance at which supershear appears.