Magma Supply Dynamics of Okmok Volcano Inferred From Interferometric SAR

Okmok volcano, located in the central Aleutian arc, is a shield volcano topped with a 10-km-wide caldera. The caldera-forming eruptions occurred about 8000 and 2400 years ago. Historic eruptions occurred in 1931, 1936, 1938, 1943, 1945, 1958, 1960, 1981, 1983, 1986, 1988, and most recently during February-April 1997. More than 70 Interferometric SAR (InSAR) images were used to study the transient deformation of the volcano before, during, and after the 1997 eruption. Co-eruptive deflation of 1.4 m was detected. Point-source models suggest a magma reservoir resides at a depth of 3 km beneath the center of the caldera, which is about 5 km northeast of the eruptive vent. Pre-eruptive inflation from 1992 to 1996 was observed. The inflation rate decreased from about 10 cm/year during 1992-1993 to a nearly negligible magnitude during 1995-1996. The post-eruption inflation rate during 1997 and 2002 generally decreased with time, although the rate during 2000-2001 was exceptionally low. The location of the magma source during the pre-, co-, and post-eruption inflation periods is essentially constant. Interferograms spanning 1997-2002 captured at least two distinct deformation processes: (1) volcano-wide inflation due to influx of magma into the reservoir and (2) subsidence of the 1997 lava flows most likely due to thermal contraction. By the summer of 2002, about 1/3 of the magma from the 1997 eruption was replenished within the shallow reservoir. The decaying inflation rate is consistent with a simple fluid flow model driven by a pressure gradient between a deeper magma source and the 3-km-deep shallow reservoir. Continuous monitoring of deformation at Okmok volcano will lead to an improved understanding on how this volcano works.