Watching Magma from Space
By Zhong Lu, Charles Wicks, Daniel Dzurisin, Wayne Thatcher, Jeffrey T. Freymueller, Stephen R. McNutt, and Dörte Mann

Westdahl is a broad shield volcano at the western end of Unimak Island in the Aleutian chain. It has apparently been dormant since a 1991-92 eruption and seismicity levels have been low. However, satellite radar imaging shows that in the years following 1992 the upper flanks of Westdahl have risen several centimeters, probably from the influx of new magma deep below its summit. Until now, deep magma reservoirs have been difficult to detect beneath most volcanoes. But using space geodetic technologies, specifically interferometric synthetic aperture radar (InSAR), we have discovered a deep magmatic source beneath Westdahl.
 
Although the closest people live to Westdahl is 100 kilometers, the volcano lies along the major aircraft route connecting North America to Asia and poses a significant aviation hazard. A 1991 eruption produced ash plumes that rose to 7 kilometers in altitude, prompting the U.S. Federal Aviation Agency to divert air traffic.
 
Volcano hazard monitoring and mitigation is currently primarily on techniques that provide information over two different time scales. Stratigraphic studies reveal a volcano’s long-term eruptive history, while monitoring shallow seismicity and ground deformation typically documents activity in the days to months before an eruption. Because a volcano can erupt a cubic kilometer or more of magma in a brief span of hours to days, large volumes of magma must accumulate somewhere in the middle to lower crust between large eruptions. Detecting a deep-seated deformation signal years to decades before the onset of shallow unrest would thus allow scientists to focus their monitoring efforts and permit public officials to better mitigate volcano hazards.
 
In our research, supported by the U.S. Geological Survey and NASA, we used InSAR to image progressive inflation of Westdahl over the six-year period following its most recent eruption. InSAR measures deformation by mapping changes in the ground-to-satellite distance between two satellite images, producing an interferogram.
 
The change in distance is shown using a repeating cycle of colors (each cycle is called a fringe), making a contour map of the displacement of the ground in the satellite look direction. We used interferograms for several time periods. The interferogram for 1993-98 reveals two to three concentric fringes, which indicate uplift of the ground surface, around most of the volcano. We constructed a numerical model to explain the observations, and identified an inflating magmatic source located almost directly beneath the summit of Westdahl volcano at a depth of about 9 kilometers.
 
Because InSAR cannot measure deformation in areas covered by snow and ice, we did not have usable signals in parts of the summit area. Data from Global Positioning System (GPS) surveys carried out in 1998 and 1999 complemented our
InSAR data. GPS measures 3-D displacements of the ground, and GPS measurements can be made in areas dominantly covered with snow and ice. Initial GPS results from two sites on the eastern flank of Westdahl show motion outward and upward, as our model predicted. GPS observations this summer of sites within
the summit region will allow us to test our model more completely. In addition, a seismic network deployed by the Alaska Volcano Observatory in 1998 will provide a short-term warning capability for future eruptions.
 
The new space geodetic techniques may extend the warning time before eruptions to years or even decades, filling a crucial gap between traditional volcano monitoring of shallow precursors (hours to months of warning) and the long-term volcano hazards assessments based on eruptive history.
 
Deformation at Westdahl extends more than 10 kilometers from the summit in all directions. InSAR seems to be the best tool available for detecting deep, aseismic magma accumulation because it measures broad, subtle deformation of the ground surface. InSAR data paired with GPS data is even more useful.
 
We also seek to track magma as it ascends from mid-crustal reservoirs to the surface, a necessary (but not sufficient) prelude to eruption. InSAR data from volcanoes typically provide only a year-to-year snapshot of deformation. A combination of periodic mapping using InSAR, continuous GPS monitoring at permanent sites, and local seismic monitoring could provide both the temporal and spatial resolution needed to identify restless volcanoes long before they erupt, and to closely follow their subsequent unrest.

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