Station AC17

Questions for Ronni Grapenthin

How did you become a scientist/engineer?

I was interested in nature and exploring as a kid, but never thought about becoming a scientist. This was mainly because I had no idea what scientists do. Even as a college student in computer science I didn't think of becoming a scientist until I got actively involved in research. That happened during an exchange semester in Iceland where I began studying crustal deformation and understood that seasonal snow deposits on Icelandic ice caps result in strong seasonal signals in GPS time series. At that point I understood what scientists do: create new knowledge, find new stuff, solve puzzles—and I was hooked.

I became a geophysicist because I wanted to combine my interests in computer science, data analysis and the outdoors.

What is your job like?

During my time as a graduate student in Alaska I got to travel to many places in the state as well as several times to Kamchatka, Russia, to do field work. Once the instruments are deployed, the work consists of data collection (either occasional site visits, or remote downloads), and mostly data processing and analysis: We comb through the data, try to find interesting signals from which we hope to understand something about the Earth. Occasionally a volcano erupts, which in the case of Redoubt 2009 got me involved in the excitement and produced a significant amount of the data I used in my dissertation.

What are you hoping to learn from your research?

In geodetic volcano monitoring, we try to link surface deformation to changes of a magma reservoir at depth. We are most interested in the geometry, depth, and volume changes of the reservoirs, as this will teach us something about possible future scenarios. From AC17 we learned that magma accumulated deep enough under Redoubt before the 2009 eruption that it shows up in the records of AC17. We hope this will be helpful knowledge during future eruptions.

Eruption of Mount Redoubt

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.

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