About 10 years ago, a remarkable new technique for measuring ground deformation from Earth orbit burst on the scene with all the drama of a major earthquake. Geodesists around the world were struck by the remarkable detail visible in the image, which resembled the displacement pattern predicted by theoretical models of such an earthquake. The pattern had never before been fully observed in the field, because conventional ways of measuring ground deformation were capable of filling in only a few pieces of the puzzle. Suddenly, all of the pieces fell into place and the race was on to apply the amazing new technique, called satellite radar interferometry, to other sources of ground deformation, including volcanoes. This is thus a remote sensing technique that uses radar satellite images.
Volcanoes deform constantly.
Obviously during eruptions, the magma injected from down below has to work its way through surrounding rock, and this creates slow ground movements that can be detected with InSAR . But also when a volcano is in relative quiescence, we can monitor the injection of magma at depth because that deformation too reaches the surface. Again, this tool allows us to understand better how volcano work. It is too broad and low to be noticed from the ground, and several close aerial inspections of the area have revealed no unusual surface features. The images that reveal the 10-centimeter uplift near South Sister were obtained in 1996 and 2000.