Global Positioning System (GPS) measurements registered up to >5m of coseismic displacements during the 28 March 2005 Mw = 8.7 Nias earthquake, Indonesia. The vertical offsets put tight constraints on the northern and southern limit of the rupture. The inferred coseismic slip distribution indicates high slip patches near the epicenter and near the southern extent of the 26 December 2004 Aceh-Andaman rupture, where aftershocks have been abundant. Six months of postseismic time-series are better fit with a logarithmic instead of exponential function, suggesting that the postseismic deformation is likely controlled by afterslip. Our inversion model predicts afterslip to be concentrated both up- and down-dip from patches of maximum coseismic slip where aftershocks are sparse. The shallow afterslip adds further evidence that the earthquake probably did not break the surface (with implications for tsunami generation) and instead caused aseismic deformation in shallow parts of the subduction zone after the event.