Crustal Deformation and Fault Mechanics

Measuring volcano deformation using persistent scatterers

Persistent scatterer (PS) analysis of InSAR data is a very sensitive technique for measuring steady or functionally-simple deformation with time when applied to urban areas. Applying these methods to estimate deformation on volcanoes is, however, more challenging because a) the majority of volcanoes are not urbanized and therefore lack the man-made structures which are recognized by the PS algorithm, and b) deformation tends to proceed at an irregular rate.

We are developing a new method for identifying PS pixels in a series of interferograms, based on a combination of their amplitude and phase characteristics, that is applicable to the study of natural targets. The phase-based method avoids one major problem with the existing algorithm: low amplitude pixels with actual phase stability are not identified. Our method also uses the spatial correlation of the phases rather than a well-defined phase history so that we can observe temporally-variable processes. The algorithm involves removing a residual topographic component of the phase for each PS, assumed proportional to the interferometric baseline, and then unwrapping the phase of the PS interferogram stack both temporally and spatially. Our technique finds scatterers with stable phase characteristics, independent of amplitudes associated with man-made objects. It is applicable to areas where conventional InSAR fails due to complete decorrelation of the majority of scatterers, yet a few stable scatterers may be distributed amongst them.

We created and analyzed a stack of 21 interferograms for Long Valley Caldera in California, and identified 23,000 PS pixels in the study region, as opposed to about 400 found with Feretti’s (2001) algorithm. The resulting unwrapped phases, when transformed into estimates of line-of-sight displacements, agree with GPS, leveling and EDM measurements made over similar time intervals, validating the technique. Furthermore, the dense spatial coverage of the PS allows us to refine models of the sources of deformation within the caldera.

References

Hooper, A., H. Zebker, P. Segall, and B. Kampes, A new method for measuring deformation on volcanoes and other natural terrains using InSAR persistent scatterers. Geophys. Res. Lett., 31, doi:10.1029/2004GL021737, 2004.