Geodetic constraints on magma movement and withdrawal during the 2006 eruption of Augustine Volcano: Chapter 17 in The 2006 eruption of Augustine Volcano, Alaska

For the first time in the United States, a modern geodetic network of continuously recording Global Positioning System (GPS) receivers has measured a complete eruption cycle at a stratovolcano, Augustine Volcano in Alaska, from the earliest precursory unrest through the return to background quiescence. The on-island network consisted of five continuously recording, telemetered GPS stations, four continuously recording, nontelemetered stations, and about 10 campaign bench marks. The continuous network recorded several distinct and conspicuous signals over the course of the unrest and eruption, starting with a months-long precursory inflation centered beneath the volcano at around sea level. Nearly coincident with the highest volumetric eruption rates, this inflation gave way to a more deep seated deflation that we interpret as a major withdrawal (approx. 25 million m3 of compressed magma) from a nearly cylindrical magma reservoir centered about 5 km below sea level. Detailed analysis of the geodetic time series reveals additional nuance, including the probable upward propagation of a small dike into the edifice in the 60 days or so before the onset of large-scale explosive activity. Comparisons of the geodetic data and their resulting interpretations with other data, such as earthquake hypocenters and petrologically inferred magma-pressure histories, reveal a kinematic, if not mechanical, account of the 2006 eruption that details the shape and location of the magma source region, the means and velocity of magma transport, and the establishment of a short lived volume- (or pressure-) buffering capability held within the magma reservoir. The cumulative deformation over the course of the eruption shows a large signal close in and high on the edifice that decays rapidly with distance. This pattern indicates a small permanent increase in the edifice volume (beyond that added by the surficial lava dome) but also shows that without close-in (<2.5 km from the summit) stations, the eruption might have been invisible to campaign GPS stations alone.