Surface deformation caused by shallow magmatic activity at Okmok volcano, Alaska, detected by GPS campaigns 2000-2002

Annual GPS campaigns were carried out at Okmok volcano in the Aleutian Islands, Alaska, between 2000 and 2002. Surface deformation detected by these measurements reveals that Okmok volcano has been inflating over these 3 years at a variable inflation rate. The horizontal displacements show a radial outward pattern, and there has been significant uplift of the caldera center. The uplift of the caldera center relative to the caldera rim was similar to 2.1 cm during 2000-2001, and similar to 6.7 cm during 2001-2002. The latter rate is quite consistent with that deduced from InSAR measurements spanning 1997-2000, but the deformation rate during 2000-2001 was much slower than during the preceding and succeeding periods. Shallow pressure source was inferred at a depth of similar to 3.1 km beneath the approximate center of the caldera. The location of the source, similar to 5 km laterally from the active vent, is consistent with that inferred from InSAR data during 1997-1998. The total increase in volume during 2000-2002 of the inferred source is similar to 0.44 x 10(7)m(3), which is 3-8% of the amount of volume erupted in 1997. The GPS and InSAR data show that magma accumulation beneath Okmok was steady in rate and location during 1997-2002, except for a pause at some time between 2000 and 2001.