AVO operates networks of continuously recording Global Positioning System (GPS) receivers at Augustine, Spurr,
Akutan, and Okmok volcanoes. The GPS instruments can measure their positions to within fraction of a centimeter
and thus detect very small movements of the Earth's crust. These movements, referred to as ground deformation, can
arise from many sources, including plate tectonics, earthquakes, and volcanic activity. GPS has proven to be an
important tool for mid- to long-term monitoring of volcanoes, since these instruments are sensitive to slow, steady
magmatic recharge that otherwise would go unnoticed.
AVO also conducts periodic "campaign" GPS surveys, where instruments are deployed at specific benchmarks on a
volcano and left for a few days to collect data. The volcano is then visited a year or two later and the difference
in the position of the benchmark gives useful information about how a volcano has behaved over that interval. It is
especially important to establish "baseline" campaign GPS measurements at potentially active volcanoes, so that when
a volcano becomes restless, scientist shave an accurate comparison point from a period of relative inactivity.
Ground deformation can also be measured with satellite radar interferometry (InSAR). Though not as accurate as GPS,
the satellite measurements have the advantage of broad spatial coverage. The combination of GPS and InSAR is especially
powerful because the two data sets complement one another's strengths and weaknesses.
Tiltmeters are also routinely used to monitor volcanic deformation, though not yet in Alaska (tiltmeter networks
are planned for near-term deployment). These instruments measure extremely small changes in the ground's tilt or
slope. The sensitivity of tiltmeters is truly astonishing - they measure in units of microradians, which are millionths
of a radian. To put this in perspective, 1 microradian is the amount of tilt caused by sliding a dime under one end of a
board that is a kilometer long. A normal tiltmeter can measure a tilt 10 times smaller than this. Like GPS instruments,
tiltmeters are useful to detect and quantify inflating or deflating magma chambers, though the tiltmeter is much more
attuned to very fast or abrupt deformation.
For more information about using GPS satellite data to measure deformation at volcanoes, see
For more information about using InSAR data to measure deformation at volcanoes, see
To read about deformation at Peulik volcano: