Deformation Measurements for the September 2005 AFAR Rifting Event from Sub-Pixel Correlation of SPOT Images

The rifting of the ~60km long Dabbahu segment of the AFAR rift, associated with a large number of Mw=3.5- 5 earthquakes and a volcanic eruption, is the largest such land event in the era of satellite geodesy. We matched at a sub-pixel level SPOT4 10m optical satellite images spanning the event in order to measure horizontal displacements in the rupture zone. The latest improvement of the technique has removed the CCD artifacts of the sensor and by averaging across a swath we obtain measurement uncertainties of the order of ~20cm (1/50th pixel). The displacements show a maximum opening of up to 8m, agreeing well with the horizontal components of a 3D deformation field estimated from SAR interferograms and radar range (accurate to ~2m) and azimuth (accurate to ~60-70cm) offsets (Wright et al., 2006). Large deformations in the rift zone itself causes decorrelation in the interferogram and loss of data. Using horizontal offsets from optical matching, we are able to determine an enhanced 3D deformation field, particularly in the area of maximum rupture, and hence an improved elastic dislocation model of the event.