Deformation Of Alaskan Volcanoes Measured Using Sar Interferometry And Gps

Abstract

Geodetic measurements using the Global Positioning System (GPS) and synthetic aperture radar interferometry (InSAR) show deformation of Okmok, Westdahl, and Fisher volcanoes in the Alaska-Aleutian arc. This thesis shows the variety of deformation signals observed, presents models for the observations, and interprets them in terms of underlying processes. InSAR data show deflation of Okmok caldera during its last eruption in 1997, preceded and followed by inflation of smaller magnitude. Modeling shows that the main deformation source, interpreted as a central magma reservoir, is located at 2.5 to 5.0 km depth beneath the approximate center of the caldera, and 5 km away from the active vent. Mass balance calculations and comparison with the long-term eruptive frequency indicate that Okmok may be supplied with magma continuously from a deep source. GPS measurements between 1998 and 2001 show inflation of Westdahl volcano, with a source located about 7 km beneath the summit. The combined subsurface volume increase measured during the GPS and an earlier InSAR observation period [Lu et al., 2000a] accounts for at least 15% more than the volume erupted from Westdahl in 1991--92, suggesting that an eruption of that size could occur at any time. Neighboring Fisher caldera shows subsidence and contraction across the caldera center that is not related to any eruptive activity. The main mechanisms to explain this deformation are degassing and contractional cooling of a shallow magma body, or depressurization of Fisher's hydrothermal system, possibly triggered by an earthquake in the vicinity of the caldera in 1999. A systematic coherence analysis of SAR interferograms documents the cooling history of the 1997 Okmok lava flow. The flow is incoherent directly after emplacement, but coherence increases as more time has passed since the eruption, and also the shorter the period spanned by the interferogram. Coherence is regained three years after the eruption. This corresponds to the time when the 20 m thick flow has solidified, indicating that flow mobility is the dominant factor degrading coherence on young lava flows. Based on these results, InSAR coherence analysis can be used to derive the minimum thickness of a lava flow.