Volcanic and tectonic deformation on Unimak Island in the Aleutian Arc, Alaska

+3.3 � 10 6 m 3 yr � 1 . The GPS data indicate that inflation may have slowed down slightly compared with interferometric synthetic aperture radar (InSAR) observations between 1993 and 1998. The accumulated subsurface volume increase during the GPS and InSAR observation period (1993–2001), � 70 � 10 6 m 3 , already accounts for at least 15% more than the erupted volume from the last eruption in 1991–1992. Fisher caldera shows subsidence and contraction across the caldera center. The data are fit best with a rectangular dislocation source at a shallow depth. It is 14 km long and 0.5 km wide, dips 80� to the NW, and strikes N35� E, with rather large uncertainties for most of these parameters. Its volume decrease is 2.0 � 10 6 m 3 yr � 1 . The main mechanisms to explain the subsidence and contraction are degassing and contractional cooling of a shallow magma body and depressurization of Fisher’s hydrothermal system, possibly triggered by an earthquake in 1999. At the 95% confidence level, no significant strain accumulation due to subduction is observed across the entire island. The low coupling across the rupture zone of the 1946 earthquake is a strong argument for the idea that the earthquake and tsunami did not result from a purely double-couple (earthquake) source. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 8135 Tectonophysics: Evolution of the Earth: Hydrothermal systems (8424); 8145 Tectonophysics: Evolution of the Earth: Physics of magma and magma bodies; 8150 Tectonophysics: Evolution of the Earth: Plate boundary—general (3040); KEYWORDS: volcanic deformation, caldera deformation, Aleutian Arc, Alaska, Unimak Island Citation: Mann, D., and J. Freymueller, Volcanic and tectonic deformation on Unimak Island in the Aleutian Arc, Alaska, J. Geophys. Res., 108(B2), 2108, doi:10.1029/2002JB001925, 2003.