Dynamic deformation of Seguam Island, Alaska, 1992-2008, from multi-interferogram InSAR processing

We generated a time-series of ERS-1/2 and ENVISAT interferometric synthetic aperture radar (InSAR) images to study ground surface deformation at Seguam Island from 1992 to 2008. We used the small baseline subset (SBAS) technique to reduce artifacts associated with baseline uncertainties and atmospheric delay anomalies, and processed images from two adjacent tracks to validate our results. Seguam Island comprises the remnants of two late Quaternary calderas, one in the western caldera of the island and one in the eastern part of the island. The western caldera subsided at a constant rate of ~. 1.6. cm/yr throughout the study period, while the eastern caldera experienced alternating periods of subsidence and uplift: ~. 5. cm/year uplift during January 1993-October 1993 (stage 1), ~. 1.6. cm/year subsidence during October 1993-November 1998 (stage 2), ~. 2.0. cm/year uplift during November 1998-September 2000 (stage 3), ~. 1.4. cm/year subsidence during September 2000-November 2005 (stage 4), and ~. 0.8. cm/year uplift during November 2005-July 2007 (stage 5). Source modeling indicates a deflationary source less than 2. km below sea level (BSL) beneath the western caldera and two sources beneath the eastern caldera: an inflationary source 2.5-6.0. km BSL and a deflationary source less than 2. km BSL. We suggest that uplift of the eastern caldera is driven by episodic intrusions of basaltic magma into a poroelastic reservoir 2.5-6.0. km BSL beneath the caldera. Cooling and degassing of the reservoir between intrusions result in steady subsidence of the overlying surface. Although we found no evidence of magma intrusion beneath the western caldera during the study period, it is the site (Pyre Peak) of all historical eruptions on the island and therefore cooling and degassing of intrusions presumably contributes to subsidence there as well. Another likely subsidence mechanism in the western caldera is thermoelastic contraction of lava flows emplaced near Pyre Peak during several historical eruptions, most recently in 1977 and 1992-93.