AC17 was installed to measure subduction zone deformation, but the station also happens to be just 28 km northeast of the summit of Mount Redoubt. Redoubt erupted from March 23 to about mid June in 2009. AC17 was the only GPS instrument near the volcano recording continuously during the months prior to the eruption.
Although scientists didn't recognize it at the time, the station's data reveals subtle motion outward from the volcano beginning as early as May 2008, over one year before the eruption. At the beginning of this period, there was no associated earthquake activity, making GPS the first instrumentation to detect activity at the volcano.
When the eruption began, the motion of AC17 reversed (Figure 3). We link the
inflation, the
motion leading up to the eruption, to an intrusion of magma, filling the reservoir feeding the volcano. We link the
deflation during the eruption to the "evacuation" of material from the magma reservoir.
Looking closely at the satellite data provided a possible new way to use GPS in volcano monitoring. When we looked at the records of the closer GPS stations, which were installed just prior to the eruption, we found evidence of ash plumes in the data. For example, when we plot site RVBM's position over the course of the day, we see that it seems to move significantly during the explosions from 14:00 to 14:40 UTC (Figure 5). Why does the site move? It turns out that it doesn't. We found that the plume affects the GPS radio signal, effectively slowing it down and making the site
look like it's moving. In the future we hope to use this in near real time volcano monitoring.
For the video story of a plane's encounter with Redoubt's ash during the 2009 eruptions, see Frontier Scientists'
Redoubt's Ash. For more information see the USGS
Alaska Volcano Observatory website and the National Weather Service's
Mt. Redoubt page.