GPS measurements during and soon after the earthquake sequence were compared with GPS measurements of 1995 and 1999 to study the surface deformation. Model estimation based on these are in good agreement with the seismic modelling above (Árnadóttir et al. 2001). The same is true for InSAR observations based on comparing satellite images a few years before the earthquakes (Pedersen et al. 2001). Coulomb stress changes in the SISZ due to the two earthquakes (Árnadóttir et al. 2003) have been estimated based on distributed slip model derived from joint inversion of InSAR and GPS data (Pedersen et al. 2003). Among results of this study is that static stress change caused by the June 17 earthquake is enough to promote failure on the June 21 fault.