Changes in the magma system during the 2008 eruption of Okmok volcano, Alaska, based on GPS measurements

Abstract

The 2008 eruption of Okmok volcano was preceded by 6–7 months of immediate precursory inflation, which followed ∼3 years of quiescence during which no significant magma was intruded at shallow depth. Although GPS data for the precursory period are too sparse to derive a unique source model, the data show that either the center of pressurization shifted from its 1997–2005 location during the immediate precursory time interval or the shape of the pressure source changed from a sphere to something else. Deflation during the eruption resulted from a decrease in pressure from a source estimated to be at 2.1 km below sea level using a Mogi model. Although source depth estimates from a Mogi model can be biased due to elastic layering or heterogeneity, the GPS data require the 2008 eruptive source to be shallower than the preeruptive inflation source. During the eruption, the GPS time series indicate three distinct pulses of deflation rather than a single, smoothly decaying process, with the initial pulse being the largest. Reinflation of the volcano commenced within at most 3 weeks after the end of ash emissions and was evident at a near‐source site while a far‐field site continued to deflate for an additional 4–5 weeks. The asynchronous transition from deflation to inflation, controlled by the distance of the site from the source, indicates that paired deep deflation and shallow inflation is required to explain the observations.