An analysis of the first two years of postseismic surface deformations from GPS reveals that afterslip is the only mechanism significantly contributing to postseismic deformation following the 2004 M6 Parkfield, California earthquake. Finite element modeling shows this event to have been too small to significantly stress the lower crust and upper mantle, thus viscoelastic relaxation did not lead to detectable surface displacements. Similarly, coseismically induced pressure changes in the upper crust were not sufficient to induce a measurable poroelastic response. From 10 days to two years after the Parkfield earthquake, postseismic displacements at all GPS stations experience the same characteristic decay time (∼2 weeks). This suggests that only afterslip was activated and the distribution of slip remained unchanged in this time period. Afterslip was found to be broadly distributed in the upper 15 km of the crust and associated with a moment release of much greater magnitude than the coseismic rupture.