Deformation of Grimsvotn volcano, Iceland: 1998 eruption and subsequent inflation

Grimsvotn is among the most active volcanoes in Iceland, with the most recent eruption in December 1998. The volcano is situated above the central part of the Iceland mantle plume and is nearly covered by the Vatnajokull ice cap. Only a part of the southern caldera rim is exposed above the ice. This makes it possible to follow the magmatic movements of a subglacial volcano. Its deformation has been measured using geodetic GPS measurements 7 times in the time period 1992-2001. The GPS measurements show that the volcano inflated prior to the 1998 eruption. Subsidence of more than 0.15 meters at the GPS-station during the eruption was followed by re-inflation, initially at a rate of 2 cm/month, then declining to 0.5 cm/month. The 1998 eruption caused horizontal displacement of more than 0.25-meters inward to the caldera. After the eruption measured uplift and outward displacement indicates readjustment and inflow of magma. The center of subsidence and uplift originates in the same area in the caldera. Utilizing both the measured horizontal and vertical movements at a single station it is possible to model a Mogi-type point source making an assumption about the horizontal position of the source. The source was assumed to be located under the center of the Grimsvotn caldera. The model of the subsidence indicates a depth of at least 1.6 km. The suggested amount of magma escaping from the chamber during the eruption is comparable to the estimates of the erupted volume. The inflation of the volcano continues, but has not yet reached the pre-eruption level.