Constraints and conundrums resulting from ground-deformation measurements made during the 2004-2005 dome-building eruption of Mount St. Helens, Washington

A prolonged period of dome growth at Mount St. Helens
starting in September-October 2004 provides an opportunity
to study how the volcano deforms before, during, and after an
eruption by using modern instruments and techniques, such as
global positioning system (GPS) receivers and interferometric
synthetic aperture radar (InSAR), together with more traditional ones, including tiltmeters, triangulation, photogrammetry, and time-lapse photography. No precursory ground
deformation was detected by campaign GPS measurements
made in 2000 and 2003, nor by a continuous GPS station
(JRO1) operating ~9 km to the north-northwest of the vent
area since May 1997. However, JRO1 abruptly began moving downward and southward, toward a source centered about
8 km beneath the volcano, concurrently with the start of a
shallow earthquake swarm on September 23, 2004. The JRO1
velocity slowed from ~0.5 millimeters per day (mm/d) in late
September–early October 2004 until spring 2005. Thereafter,
it was essentially constant at ~0.04 mm/d through December
2005. In similar fashion, the growth rate of the welt on the
south crater floor slowed from 8.9 m³/s during October 4–11
to 6.4 m³/s during October 11-13, 2004; this trend continued
after emergence of the first lava spine on October 11. The
volumetric extrusion rate decreased from 5.9 m³/s during
October 13-November 4, 2004, to 2.5 m³/s during December
11, 2004-January 3, 2005, and for the remainder of 2005, it
was in the range 2.0-0.7 m³/s. Fifteen continuous GPS stations, installed soon after the eruption began, showed radially
inward and downward ground motions through December
2005. Likewise, InSAR observations spanning the first year of the eruption indicate broad subsidence centered near the vent.
Model-derived estimates of source-volume decrease from
September 23, 2004, to October 31, 2006, are 16-24×10⁶ m³,
substantially less than the volume erupted during the same
period (87×10⁶ m³
through October 21, 2006). The discrepancy can be explained by a combination of magma expansion
and recharge in the source region.
Lack of precursory deformation at JRO1 suggests that
the conduit is poorly coupled to the rest of the edifice, so the
rising magma column was able to push ahead older conduit material rather than intruding it. Constraints on conduit
length and radius require that reservoir magma (as opposed
to conduit-filling magma) reached the surface early during
the eruption, probably soon after CO₂
emission rates peaked
in early October 2004. If rapid emergence of spine 3 (the first
whaleback-shaped extrusion) in late October 2004 marked
the arrival of reservoir magma, then the volume of conduit
material flushed from the system was about 20×10⁶ m³
--the
volume of surface deformation plus spines on November 4,
2004. The corresponding radius for a cylinder extending from
the surface to depth d = 5 km is 35.7 m, or 28.2 m for d = 8
km. The average ascent rate through the conduit, assuming
reservoir magma began its rise on September 23, 2004, was
120 m/d for d = 5 km, or 190 m/d for d = 8 km. Observed lineal extrusion rates were 2-10 m/d, so the conduit must widen
considerably near the surface. Equating magma flux through
the conduit to that at the surface, we obtain a vent radius of
125 m and an extrusion rate of 5.7 m³/s--both values representative of the early part of the eruption.
Lack of precursory inflation suggests that the volcano was poised to erupt magma already stored in a crustal
reservoir when JRO1 was installed in 1997. Trilateration
and campaign GPS data indicate surface dilatation, presumably caused by reservoir expansion between 1982 and 1991,
but no measurable deformation between 1991 and 2003. We
conclude that all three of the traditionally reliable eruption precursors (seismicity, ground deformation, and volcanic
gas emission) failed to provide warning that an eruption was
imminent until a few days before a visible welt appeared at
the surface--a situation reminiscent of the 1980 north-flank
bulge at Mount St. Helens.