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The crust around the rupture zone of the 1992 Landers earthquake has continued to deform in the years following the earthquake at rates ∼3 times greater than pre‐earthquake rates. We use a combination of Global Positioning System (GPS) and synthetic aperture radar (InSAR) data collected during a ∼3‐year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities. In order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation. The combination of GPS and InSAR data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models. The data particularly require the presence of a highly ductile uppermost mantle …