Global Positioning System (GPS) measurements made in Sulawesi, Indonesia from 1992 to 1999 detected co-seismic and transient post-seismic deformation related to the 1 January, 1996, $M_w=7.9$ earthquake on the North Sulawesi (Minahassa) trench. These motions are superimposed on the long term secular motion (40 mm/yr) of the left-lateral Palu fault in central Sulawesi, and continued for about 1.5 to 2 years. Following the earthquake, a string of earthquakes (of magnitude larger than 6) migrated along the Minahassa trench, from west to east. Subsequently, two earthquakes of magnitude larger than 6 occurred on or near the Palu Fault migrating towards the south. Modeling the increase in Coulomb stress generated by the successive earthquakes agrees with the hypothesis of interacting events. An unclamping effect, possibly due to fluid migration in the Palu area, is also suggested by the stress computations and the detected (GPS) displacements.