Volcano number:311290
Country:United States
Geodetic measurements?Yes
Deformation observation?Yes
Measurement method(s):InSAR, GPS - continuous, GPS - campaign
Duration of observation:1992 - 2010
Inferred cause of deformation:Magmatic, Surface deposits
Characteristics of deformation:

The center of Okmok caldera uplifted >14 cm during 1992–1993 and ~6 cm during 1993–1995. The caldera then subsided 1 – 2 cm in 1995 – 1996 prior to the start of an eruption in 1997. During this eruption the caldera subsided more than 1.2 m. Prior to the 1997 eruption, the source volume increased by ~0.006 km³. During the 1997 eruption the volume decrease was ~0.047 km³.

Soon after the 1997 eruption the volcano re-inflated at rates decreasing from ~10 cm/yr in 1997 – 1998 to ~4 cm/yr in 2000 – 2001. The inflation rate increased again in 2001 – 2003 reaching a maximum of ~20 cm/yr before slowing to 10 cm/yr in 2003 – 2004. Between 2004 – 2005 the caldera floor subsided 3 – 5 cm, before uplifting a similar amount during 2005 – 2006. Between the summers of 2007 and 2008, an additional 15 cm of uplift occured prior to another eruption in July 2008.  It is estimated that 0.037 – 0.052 km³ of magma accumulated beneath Okmok between the 1997 and 2008 eruptions, with variations in the deformation rate attributed to variable rates of magma accumulation beneath the caldera.

During the 2008 eruption of Okmok the volcano again experienced volcano-wide subsidence. The caldera floor also experienced localised subsidence of over 1 m including ~75 cm of deformation that occurred during the first 13 hours of the eruption. This localised subsidence of the caldera floor is modelled using a collapsing tabular source located at 0.5 km depth with a volume decrease of 0.005 km³. This source may reflect a loss of groundwater, which interacted with rising magma.

The source of deformation at Okmok is modelled as a spherical zone of shallow (~3 km depth) magma storage with radius ~1 km located beneath the caldera.  This source has uplifted as magma accumulates and deflated rapidly as material is erupted. It has also deflated at a lower rate between eruptions. This slow rate of subsidence is attributed to degassing of magma as it cools and crystallises.

Residual fringes in interferograms prior to both eruptions are attributed to cooling and compaction of lavas, with the largest subsidence rates (~1.5 cm/yr) occurring at the thickest parts of the flows.

Reference:Lu, Z., and Dzurisin, D., 2014, InSAR imaging of Aleutian volcanoes: Chichester, UK, Springer-Praxis, 390 p.
Reference:Miyagi, Y., Freymueller, J. T., Kimata, F., Sato, T., & Mann, D. (2004). Surface deformation caused by shallow magmatic activity at Okmok volcano, Alaska, detected by GPS campaigns, 2000–2002. Earth, Planets and Space,56(10), e29–e32.
Reference:Biggs, J., Lu, Z., Fourneir, T., & Freymueller, J. (2010). Magma flux at Okmok Volcano, Alaska from a joint inversion of continuous GPS, campaign GPS and InSAR. Journal of Geophysical Research, 115, B12401. doi:10.1029/2010JB007577.
Reference:Freymueller, J. T., & Kaufman, A. M. (2010). Changes in the magma system during the 2008 eruption of Okmok volcano, Alaska, based on GPS measurements. Journal of Geophysical Research, 115 (B12415), doi:10.1029/2010JB007716.
Reference:U.S. Geological Survey Alaska Volcano Observatory (AVO) website
Location:-168.13, 53.43

Photo os the SW side of Okmok caldea. Source C. Read, U.S. Geological Survey AVO website