Analysis of GPS-measured deformation associated with the 2004-2006 dome-building eruption of Mount St. Helens, Washington

Detecting far-field deformation at Mount St. Helens
since the crater-forming landslide and blast in 1980 has been
difficult despite frequent volcanic activity and improved
monitoring techniques. Between 1982 and 1991, the systematic extension of line lengths in a regional GPS trilateration network is consistent with recharge of a deep magma
chamber during that interval. The rate of extension, however,
averages only 3 mm/yr, and some of this apparent deformation may result from systematic scale error in the electronic
distance measurements. Subsequent GPS surveys and data
from a continuous GPS station, located 9 km north of Mount
St. Helens and operating since 1997, show no significant
volcanic deformation until the start of unrest on September
23, 2004. The current eruption has been accompanied by
subtle but widespread inward and downward movement of
GPS monitoring stations, exponentially decreasing with time
and totaling as much as 30 mm. The observed deformation is
consistent with the predictions of an elastic half-space model
of a vertically elongate magma chamber with its center at
a depth of around 7 to 8 km and with a total cavity-volume
loss of about 16–24×10⁶ m³
. The discrepancy between the
estimated cavity-volume loss and the >83×10⁶-m³
volume
of the erupted dome can be explained, for the most part, by
exsolution of gas in the stored magma and by minor input of
new magma during the eruption.