Following large earthquakes, the state of stress of the surrounding earth crust and mantle is modified, triggering aftershocks and various aseismic processes, in the so called postseismic phase. Aftershocks represent a small portion of the energy released in this postseismic phase, and aseismic processes, among which afterslip (transient frictional sliding on the fault), viscoelastic relaxation in the mantle, and/or poroelastic rebound are considered to dominate. These mechanisms involve different spatial and temporal scales, and it is not trivial to differentiate between them based on the deformation signal observed at the surface (Ingleby & Wright, 2017).A detailed analysis of the temporal evolution of the deformation is necessary to identify the underlying mechanisms at play at both short (minutes to days) and long (years) timescales. The early stages of the postseismic

The early stage of the postseismic phase is characterized by a large deformation rate. Its analysis is thus key to decipher the role played by different mechanisms (afterslip and viscoelasticity) at various time scales. Here, we process GPS data to obtain 30-seconds kinematic position time series recording the surface deformation following the Mw 9.0 Tohoku-Oki megathrust earthquake (2011), and combine them with static solutions over 9 years. We analyze the temporal evolution of the time series and use these observations to image the postseismic slip. We find that the first month of deformation following Tohoku-Oki can be explained by an afterslip mechanism, that exhibits an "Omori-like" decay, with a p-value around 0.75 almost everywhere with the exception of a small region around Ibaraki prefecture where p˜1 is observed. This p<1 indicates that the postseismic displacements do not increase logarithmically with time as predicted by rate-and-state rheology. Instead, we argue that early afterslip is associated to a transient brittle creep mechanism. We use numerical simulations to show that an exponent of p<1 can be explained by a combination of thermal activation of local slips and elastic interactions. Over longer time scales, an additional mechanism is required to explain the observed deformation signal, and the transient brittle creep mechanism is combined with viscoelastic relaxation modeled by a Newtonian flow. The spatial analysis reveals two distinct afterslip regions, a major one on the North, associated with a p-value around 0.75, and a smaller one close to the Ibaraki aftershock, associated to p˜1.