We investigated the 2004 Parkfield earthquake using a multiscale slip inversion. The multiscale approach allows us to both focus on the details of the early stages of the rupture process, and to model the entire earthquake at a larger scale. Our model indicates that the Parkfield earthquake began with bilateral high-speed rupture with fast slip rate, up to 4 m/s, and fast propagation velocity of about 3.0 km/s. Rupture propagation to the southeast was arrested 3 s after onset, while propagation to the northwest continued, generated a large slip patch at ∼5 s, and ceased within 10 s. A similar rapid initiation was observed for the 2004 mid-Niigata Prefecture, Japan, earthquake in a previous study. High-speed initial rupture suggests that large earthquakes may initiate and grow in the same way as small earthquakes, and that the final size is difficult to predict from the early stages of an earthquake.