2001 to 2004 Inflation Event at Okmok Volcano

Okmok is a shield volcano located along the Aleutian Arc. Frequent eruptions at Okmok make it an ideal volcano to study the dynamics of intrusive and eruptive behavior. Okmok has had an eruption every ten to twenty years for the past several hundred years and possibly longer. The most recent eruption occurred in 1997 and emanated from a cone in the southwest quadrant of its large summit caldera. During this time InSAR observations tell us about surface deformation at the volcano. In 2000 the first of yearly campaign GPS measurements were made and in 2002 continuous GPS were installed at a few sites on the volcano. Using yearly campaign measurements we have observed a large inflation event at Okmok. The event spanned three years with a maximum vertical displacement of 42cm occurring at the center of the caldera. This event is modeled with a series of Mogi sources. Assuming that the pressure change beneath the volcano is purely due to a volume change, then we find a volume increase of 22.5 million cubic meters, over the duration of this event. The location and depth of the source is stable throughout the inflation event. Looking back at previous results from InSAR that span several years prior to the 1997 eruption and several years after we can see that the pressure/volume source beneath Okmok has longterm stability as well. All deformation studies have located a source of inflation or deflation within 1-2km of the center of the caldera. From the time series recorded at the continuous stations we can see that the inflation event has at least two phases and probably three. The shorter history at the continuous sites and the poor temporal resolution of the campaign measurements precludes us from being able to determine the detailed timing of events before the summer of 2002.