Ground deformation associated with the March 1996 earthquake swarm at Akutan volcano, Alaska, revealed by satellite radar interferometry

In March 1996 an intense swarm of volcano-tectonic earthquakes (similar to 3000 felt by local residents, M-max = 5.1, cumulative moment of 2.7 x 10(18 ) N m) beneath Akutan Island in the Aleutian volcanic are, Alaska, produced extensive ground cracks but no eruption of Akutan volcano. Synthetic apert ure radar interferograms that span the time of the swarm reveal complex isl and-wide deformation: the western part of the island including Akutan volca no moved upward, while the eastern part moved downward. The axis of the def ormation approximately aligns with new ground cracks on the western part of the island and with Holocene normal faults that were reactivated during th e swarm on the eastern part of the island. The axis is also roughly paralle l to the direction of greatest compressional stress in the region. No groun d movements greater than 2.83 cm were observed outside the volcano's summit caldera for periods of 4 years before or 2 years after the swarm. We model ed the deformation primarily as the emplacement of a shallow, east-west tre nding, north dipping dike plus inflation of a deep, Mogi-type magma body be neath the volcano. The pattern of subsidence on the eastern part of the isl and is poorly constrained. It might have been produced by extensional tecto nic strain that both reactivated preexisting faults on the eastern part of the island and facilitated magma movement beneath the western part. Alterna tively, magma intrusion beneath the volcano might have been the cause of ex tension and subsidence in the eastern part of the island. We attribute loca lized subsidence in an area of active fumaroles within the Akutan caldera, by as much as 10 cm during 1992-1993 and 1996-1998, to fluid withdrawal or depressurization of the shallow hydrothermal system.