Magma Supply, Storage and Pathways From Deformation Studies at Alaskan Volcanoes

Deformation observations (GPS and InSAR) at several Alaskan volcanoes reveal the locations of magma reservoirs, rates of magma accumulation, and pathways of magma transport. These data show that there are many similarities in the behavior of individual volcanoes, but also some notable differences. In particular, steady magma accumulation in shallow (3-7 km) reservoirs is common even years before eruption, and refilling of magma reservoirs commences immediately after the eruption. At Okmok caldera, deformation is related to influx and outflow of magma. Observations spanning its 1997 eruption indicate that magma was stored at 3-5 km depth before the eruption, and that the replenishment of Okmok's shallow magma reservoir is a continuous process. Similar observations are made at Westdahl volcano, where re-inflation started immediately after the last eruption in 1992, and has continued aseismically since then at a roughly constant rate. The rates of average subsurface volume change are similar for Okmok and Westdahl, with reservoirs at 3-5 km and 7-8 km respectively. From their eruptive histories and the size of recent eruptions, we infer that the average magma supply rates have been fairly constant during the past few decades. Significant horizontal magma transport, presumably via a dike, is required by the offset between the center of deformation and the eruptive vent at Okmok. At Fisher caldera, GPS measurements show subsidence and contraction across the caldera center. In absence of eruptive activity, the main mechanisms to explain the observations are degassing and contractional cooling of a shallow magma body, or depressurization of Fisher's hydrothermal system. The direction of principal stress derived from the orientation of the presumed dikes and magma bodies at Okmok and Fisher are not parallel to plate motion, suggesting that the local stress system differs from the stresses generated by plate interaction.