Páll Einarsson

The sub-glacial Grimsvotn volcano, one of Iceland's most active volcanoes, erupted in 1983, 1998 and 2004. Since 1998, annual GPS measurements have been conducted at the only available nunatak at the volcano, located on the rim of its caldera. A clear pattern of deformation is observed that can be attributed to magma inflow and outflow, uplift due to glacial thinning,

ABSTRACT The Askja central volcano is located on the divergent plate boundary in North Iceland. The last major rifting episode happened in Askja in 1874-1876, but the most recent eruption in Askja occurred in 1961. Askja has been continuously deforming since crustal deformation measurements started in the area in 1966. GPS and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 at the volcano, without any eruptive activity. Interferometric analyses of Synthetic Aperture Radar images (InSAR), acquired by the ERS-1and ERS-2 satellites, have been conducted. The interferograms span the 1992-2000 period and cover a large area in central Iceland. A deformation signal around Askja, showing subsidence, is clearly evident in five different interferograms from two different Track/Frame pairs. The interferograms cover all of the Askja area and span different time periods. The observed deformation signal consists of a concentric fringe pattern, centered at the Askja caldera with a 20 km diameter, and slightly elongated in the north direction. The fringes are most closely spaced about 3 km from the center of deformation, suggesting that deformation gradients decrease near the center of Askja. Two approaches have been taken to model the InSAR data. Firstly, we assumed a Mogi model in an elastic half-space. A preliminary model indicates a 3 km source depth and a maximum vertical subsidence of 0.23 m, centered in the main caldera, from 1992 to 1998. An alternative approach is to use the finite element method in order to include topography. An axisymmetric model of a deflating spherical source has been constructed using the ANSYS software. Initially, a model that reproduces the Mogi source has been created, in order to compare analytical to finite element solutions. As agreement between the two sets of solutions was good, we have also constructed a model with a cone shape topographic relief, crudely approximating Askja. Results indicate that the maximum vertical displacement is not located above the center of the source, like in the half-space solutions, but on the sides of the volcano.

ABSTRACT Crustal deformation studies of Icelandic volcanoes conducted for over 30 years provide an extensive data set, complementary to seismic observations. The geodetic studies are continuously expanding and cover now most of the active volcanic areas. Current measurements include campaign and continuous GPS, extensive InSAR studies, optical leveling tilt, and borehole strain. The measurements have revealed the different styles of deformation at more than 15 volcanoes. They help quantify the location and volume of magma intrusions, as well as deflation and dike volumes associated with eruptions. The conventional elastic spherical-source Mogi model has successfully been used to fit many inflation/deflation episodes. The estimated volumes of transported magma is typically a small fraction of a cubic kilometer. Despite precise measurements of surface deformation fields, uncertainties on the estimated volumes are large because of possible inelastic effects and potentially complicated source geometry. The measurements have shown that the plumbing systems of the volcanoes are widely different, being influenced by the tectonic setting and time since last major magma recharging of the systems. Many of the volcanoes are presently not deforming, some do have persistent local deformation sources due to shallow magma chambers whereas others deform episodically. Some of the continuously active sources are deflationary and are at least partly related to cooling of a magma chamber. For example, Askja volcano has been deflating at least since 1983 at a rate of about 5 cm/year. In many cases magma movements have triggered seismic activity, but in greatly varying amounts depending on several factors such as depth of the deformation source and the regional stress field. One of the smallest observed intrusion volumes, that of the Hengill volcanic area in 1994-1998, was associated with the highest seismic activity, because of high regional stress. A summary of recent observations and interpretations will be presented.

ABSTRACT The Eyjafjallajökull volcano has been exhibiting intermittent unrest for the past 18 years. The most recent intrusive episode was in 1999. The current activity initiated in January 2010 with intense seismic activity and inflation observed by continuous GPS stations. The rate of deformation rapidly increased in early March until a basaltic flank eruption started on Fimmvörðuháls on 20 March. The eruption continued until 12 April without much observed deflation. Two days later, a second more exposive eruption started at the ice covered summit of Eyjafjallajökull. Very fine ash was produced during a phreatomagmatic eruption phase, where magma of trachy-andesitic composition came into direct contact with glacial melt water, causing unprecedented disruption to air traffic in Europe. During the first week of the second eruption, the geodetic data show motion cosistent with a volume decrease in a magma chamber at ~4 km depth under the summit caldera. The three previous historical eruptions of Eyjafjallajökull in 920, 1612 and 1821-1823 were followed by subsequent eruptions of Katla. The geodetic observations, however, do not detect any measurable deformation at Katla through April 2010.

Glaciers in Iceland began retreating around 1890 and the Vatnajökull ice cap reduced by 182 km3 from 1890 to 1978. This unloading of the crust induces uplift around the ice cap. Results from Sigmundsson and Einarsson (1992) indicate that the land around Vatnajökull is rising at a rate of 5-15 mm per year. Recent GPS measurements in the area, by

Glacial rebound at the end of the Weichselian glaciation was completed in Iceland around 9000 years BP, but extensive ice volume retreat has occurred in Iceland in historical times. Since 1890 the Vatnajokull ice cap has lost about 300 km3 of ice. Unloading of the crust due to ice melting induces glacio-isostatic deformation around the ice cap. Ongoing uplift around

A hydro-power plant (Kárahnjúkavirkjun) is currently under construction in northeastern Iceland. As a part of that project a major water reservoir (Hálslón) will be constructed north of Vatnaj\ddot{\textrm{o}}kull ice cap at the eastern edge of the rift zone, at the plate boundary, in North Iceland. Geological observations made during the construction time led to the discovery of Holocene activity on

ABSTRACT The subglacial Grímsvötn volcano in Vatnajökull ice cap erupted in 1998, 2004, and 2011. We present results of Global Positioning System (GPS) geodetic measurements since 1997 conducted at one site on the volcano's caldera rim, which protrudes through the ice cap. The volcano contains a complex of three calderas (<12 km2 each). The observed displacement can be attributed to several processes: i) transport of magma in and out of a shallow chamber under the center of a caldera complex (inflation, deflation, accompanied by in- and outward horizontal displacements), ii) isostatic uplift due to gradual thinning of the ice cap, iii) annual variation in snow load, iv) crustal plate movements. Annual GPS measurements started after the 1998 eruption and a continuous GPS-monitoring station has been operative shortly before the 2004 eruption. During inflation periods the vertical displacement is a joint result of a glacial isostatic adjustment due to thinning of the glacier and inflow of magma to a shallow magma chamber, while the horizontal displacement component is predominately attributed to magma pressure changes. The horizontal displacement of the GPS-site on the caldera rim is directed outward during inflation and inward (directly opposite) accompanying the eruptions. This pattern has been repeated for each eruption cycle, pointing to location of one and the same magma chamber in the center of the caldera complex. Erupting vents in 1998, 2004 and 2011 were located at the foot of the southern Grímsvötn caldera rim. The earthquake pattern was similar prior to the two most recent eruptions: a slow increase in number of events during the years before the eruptions and practically none following the eruptions. The continuous GPS data after the eruption in 2011 suggest a fast pressure recovery of the shallow magma chamber similar to that following the 1998 and 2004 eruptions, although the 2011 eruption is the best observed. The regularity of the crustal deformation and the earthquake pattern prior to the past two eruptions led to both successful long- and short-term predictions and warning of the events. Grímsvötn volcano has shown to be in a state of continuous magma accumulation at shallow depths that results in eruptions when the strength of the crust is overcome by the magma pressure.