Although several studies suggest that a slow slip followed the seismic slip on the northern half of the 1300-km long plate interface that ruptured during the great Sumatra-Andaman earthquake, the onset and duration of the slow slip remain unresolved. In this paper we analyze a tide gauge record at Port Blair, situated about 1000 km north of the epicenter. Our preferred model, which fits the observed data well, consists of a mixed mode of slip: ~half of the total slip occurring seismically in less than 5 min after the rupture arrival and the rest developing in the next 30 min. A scenario of slow slip on the northern half of the rupture area, that is consistent with seismic data, and GPS data, consists of slip on the time scale of 1 hour over the rest.