Volcano Geodesy 101
Rob R. asks:
I’ve been following along with the recent happenings at Yellowstone (that is, as best as I can as a layman) but haven’t seen that site [data from the Yellowstone GPS network] before. Could you explain (or link to) what I’m seeing there and what a “change in surface topography” might look like? Thanks in advance.
Sure thing! What you are looking at on that page is a network of GPS receivers cemented to various points in the Yellowstone caldera and surrounding area. The USGS has written a nice overview of GPS and other geodetic volcano monitoring techniques, and I won’t duplicate their efforts here.
The instrument in this photo is the GPS station from White Lake, Wyoming. For the past several years, it’s been moving upwards at a rate of about 4 cm/yr – if it keeps going, which it obviously won’t, it would take about 10 billion years to get to the moon.
This is the cartoon version of what it is looking for:
It’s not just an expanding magma chamber that will produce this kind of surface change, though. You can also use long-term GPS series (and other types of survey data) to see ground water levels going up and down, the crust gradually relaxing after an earthquake, and the slow creep of plate tectonics.
As for what actual GPS data from an eruption might look like, well, here’s an example from the 2005-2006 eruption of Augustine, in Alaska:
Figure source: Mattia et al., 2008
The space between the dotted lines represents 5 cm of movement. The different colors of dots represent different kinds of data processing; the red ones are the current “gold standard” daily averages.
GPS data is very noisy, compared to the signals earth scientists are looking for. This is in large part due to fluctuations in atmospheric conditions which slow down or speed up the signal in an unpredictable way. One way people get around the noise is to use data acquired over a long period of time, hoping that the weather will cancel itself out. Another is to compare the station of interest to a nearby reference. The Alaska Volcano Observatory has been experimenting with ways of getting reliable positions using an hour or less worth of data, which will enable them to detect changes in the ground surface much more quickly than currently possible with GPS.
- Mario Mattia, Mimmo Palano, Marco Aloisi, Valentina Bruno, Yehuda Bock (2008). High rate GPS data on active volcanoes: an application to the 2005-2006 Mt. Augustine (Alaska, USA) eruption Terra Nova, 20 (2), 134-140 DOI: 10.1111/j.1365-3121.2008.00798.x