Radar interferometry observations of surface displacements during pre- and coeruptive periods at Mount St. Helens, Washington, 1992-2005

We analyzed hundreds of interferograms of Mount St.
Helens produced from radar images acquired by the ERS-1/2,
ENVISAT, and RADARSAT satellites during the 1992-2004
preeruptive and 2004-2005 coeruptive periods for signs of
deformation associated with magmatic activity at depth. Individual interferograms were often contaminated by atmospheric
delay anomalies; therefore, we employed stacking to amplify
any deformation patterns that might exist while minimizing
random noise. Preeruptive interferograms show no signs of
volcanowide deformation between 1992 and the onset of eruptive activity in 2004. Several patches of subsidence in the 1980
debris-avalanche deposit were identified, however, and are
thought to be caused by viscoelastic relaxation of loosely consolidated substrate, consolidation of water-saturated sediment,
or melting of buried ice. Coeruptive interferometric stacks are
dominated by atmospheric noise, probably because individual
interferograms span only short time intervals in 2004 and
2005. Nevertheless, we are confident that at least one of the
seven coeruptive stacks we constructed is reliable at about the
1-cm level. This stack suggests deflation of Mount St. Helens
driven by contraction of a source beneath the volcano.