Synthetic aperture radar interferometry of Okmok volcano, Alaska: Radar observations

ERS-1/ERS-2 synthetic aperture radar interferometry was used to study the 1 997 eruption of Okmok volcano in Alaska. First, we derived an accurate digi tal elevation model (DEM) using a tandem ERS-1/ERS-2 image pair and the pre existing DEM. Second, by studying changes in interferometric coherence we f ound that the newly erupted lava lost radar coherence for 5-17 months after the eruption. This suggests changes in the surface backscattering characte ristics and was probably related to cooling and compaction processes. Third , the atmospheric delay anomalies in the deformation interferograms were qu antitatively assessed. Atmospheric delay anomalies in some of the interfero grams were significant and consistently smaller than one to two fringes in magnitude. For this reason, repeat observations are important to confidentl y interpret small geophysical signals related to volcanic activities. Final ly, using two-pass differential interferometry, we analyzed the preeruptive inflation, coeruptive deflation, and posteruptive inflation and confirmed the observations using independent image pairs. We observed more than 140 c m of subsidence associated with the 1997 eruption. This subsidence occurred between 16 months before the eruption and 5 months after the eruption, was preceded by similar to 18 cm of uplift between 1992 and 1995 centered in t he same location, and was followed by similar to 10 cm of uplift between Se ptember 1997 and 1998. The best fitting model suggests the magma reservoir resided at 2.7 km depth beneath the center of the caldera, which was simila r to 5 km from the eruptive vent. We estimated the volume of the erupted ma terial to be 0.055 km(3) and the average thickness of the erupted lava to b e similar to 7.4 m.