1983–2003 decaying rate of deflation at Askja caldera: Pressure decrease in an extensive magma plumbing system at a spreading plate boundary

New deformation data from the Askja volcano, Iceland, show that the volcano's caldera has been deflating continuously for over 20 years, and confirm that the rate of subsidence is slowing down. The decay in subsidence rate can be fitted with a function of the form e ^{− t / τ} , where τ is 39 years. Reanalysis of GPS data from 1993–1998 show that these data can be fitted with a model calling for two Mogi point sources, one shallow, and another one much deeper (16.2 km depth). Pressure decrease occurs in both sources. The deeper source is responsible for observed horizontal contraction towards Askja at distances that cannot be explained by the shallower source. Plate spreading of 19 mm/year distributed evenly over about 100-km-wide zone is also favoured by the data.     

Temporal development of the 1999 intrusive episode in the Eyjafjallajökull volcano, Iceland, derived from InSAR images

Eyjafjallajökull volcano, located in southern Iceland, is characterized by its quiet nature. Only about a handful of earthquakes associated with the volcanic system had been detected prior to the 1990s. Earthquake swarms did, however, occur in 1994 and 1999. Here we investigate the spatio-temporal evolution of a magmatic intrusion associated with the 1999 earthquake swarm via analysis of produced surface deformation. A series of interferometric synthetic aperture radar (InSAR) images, spanning various periods of the intrusion, show that in 1999 surface deformation occurred mainly on the southern flanks of the volcano. The deformation amounts to more than 20 cm of range change. Inverse modeling resolves the deformation source to be a sill intrusion at 6.3 km depth. Sill opening was up to 1 m and the total intruded volume amounts to ～0.03 km³. The InSAR data display a migration of the center of deformation through time, enabling us to create time-dependant sill-opening models. Furthermore, we investigate the spatio-temporal distribution of earthquakes and find that the distribution supports the InSAR derived model and additionally provides indications for a possible site of a feeder channel. Magmatic flow-rate estimates indicate an initial intrusion rate of 4–6 m³/s, declining over a few weeks.     

Depletion of background galaxies owing to the cluster lens CL0024+1654: U- and R-band observations

We have obtained U - and R -band observations of the depletion of background galaxies resulting from the gravitational lensing of the galaxy cluster CL0024+1654 ( z =0.39). The radial depletion curves show a significant depletion in both bands within a radius of 40–70 arcsec from the cluster centre. This is the first time that depletion is detected in the U band. This gives independent evidence for a break in the slope of the U -band luminosity function at faint magnitudes. The radially averaged R -band depletion curve is broader and deeper than in the U band. The differences can be attributed to the wavelength dependence of the slope of the luminosity function and to the different redshift distribution of the objects probed in the two bands. We estimate the Einstein radius, r _E, of a singular isothermal sphere lens model using maximum-likelihood analysis. Adopting a slope of the number counts of α =0.2 and using the background density found beyond r =150 arcsec, we find r _E=17±3 and 25±3 arcsec in the U and R bands, respectively. When combined with the redshift of the single background galaxy at z =1.675 seen as four giant arcs around 30 arcsec from the cluster centre, these values indicate a median redshift in the range 〈 z _S〉≈0.7 to 1.1 for the U _AB≥24 mag and R _AB≥24 mag populations.     

Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground- and satellite-based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003 to 2007, we observe a net micro-gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that, here, magma movement and/or a steam cap in the geothermal system of Askja at depth are responsible for the observed micro-gravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular, variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation.     

Plate boundary deformation and continuing deflation of the Askja volcano,North Iceland,determined with GPS, 1987–1993

GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of 0.8 strain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja.     

Natural hazards in Nordic Countries

Compared to many areas of the world, the human losses caused by natural hazards are smaller in Nordic countries. This is mainly due to the low population density in the exposed areas. However, the economic losses are significant and the geohazards picture varies among the countries. The predominant natural hazards in Nordic countries are floods, landslides, and, with the exception of Denmark, snow avalanche. Volcanoes and earthquakes are major geohazards in Iceland, and parts of Norway are susceptible to seismic activity. Slidetriggered tsunamis also represent a threat to parts of the coastal areas of Nordic countries and Greenland.     

Probabilistic model for eruptions and associated flood events in the Katla caldera,Iceland

Eruptions in the subglacial Katla caldera, South Iceland, release catastrophic jokulhlaups (meltwater floods). The ice surface topography divides the caldera into three drainage sectors (Ko, So and En sectors) that drain onto Myrdalssandur, Solheimasandur and Markarfljot plains, respectively. In historical times, floods from the Ko sector have been dominant, with only two recorded So events. Geological records indicate that floods from the En sector occur every 500–800 years. A probabilistic model for an eruption is formulated in general terms by a stochastic parameter that simulates a series giving the time interval in years between two consecutive events. The model also contains a Markovian matrix that controls the location of the event and thereby what watercourse is hit by the flood. A record of Katla eruptions since the 8th and the 9th century a.d., and geological information of volcanogenic floods towards the west over the last 8,000 years is used to calibrate the model. The model is then used to find the probabilities for floods from the three sectors: Ko, So and En. The simulations predict that the most probable eruption interval for the En sector and the So sector is several times smaller than the average time interval, implying infrequent periods of high activity in these sectors. A correlation is found between the magnitude of eruptions and the following time intervals. Using the statistical approach and considering this magnitude–time interval correlation, the probability of an eruption in Katla volcano is considered to be 20% within the next 10 years. This compares to a probability of 93% if only a simple average is considered. These probabilities do not take account of long-term eruption precursors and should therefore be regarded as minimum values.     

Effects of eelgrass (Zostera marina) canopy removal and sediment addition on sediment characteristics and benthic communities in the Northern Baltic Sea

The eelgrass Zostera marina is a key structural and functional species across the European coastline. The separate and interactive effects of eelgrass canopy removal and sediment addition on the sediment characteristics and the structure of benthic communities were studied in a factorial field experiment in the Northern Baltic Sea in July–August 2006. The removal of eelgrass canopy temporarily increased the sediment oxygen consumption, reduced the content of fine particles (<100 μm) and organic matter in the sediment, and increased the share of sand fraction (250–500 μm). Sediment addition increased the content of fine particles (<100 μm) and reduced the share of sand fraction (250–1000 μm). The effects were strongest in the presence of eelgrass canopy. Benthic invertebrates and macroalgae were affected by eelgrass canopy removal but not by sediment addition. The removal of eelgrass canopy significantly decreased benthic species richness and invertebrate and macroalgal densities. To conclude, our experiment demonstrates that Z. marina defines the patterns of benthic macroalgae and invertebrates but has moderate effects on sediment structure and metabolism in the Northern Baltic Sea.     

Comment on " Volume of magma accumulation or withdrawal estimated from surface uplift or subsidence, with application to the 1960 collapse of Kīlauea volcano" by P. T. Delaney and D. F. McTigue

 In volcanoes that store a significant quantity of magma within a subsurface summit reservoir, such as Kīlauea, bulk compression of stored magma is an important mode of deformation. Accumulation of magma is also accompanied by crustal deformation, usually manifested at the surface as uplift. These two modes of deformation – bulk compression of resident magma and deformation of the volcanic edifice – act in concert to accommodate the volume of newly added magma. During deflation, the processes reverse and reservoir magma undergoes bulk decompression, the chamber contracts, and the ground surface subsides. Because magma compression plays a role in creating subsurface volume to accommodate magma, magma budget estimates that are derived from surface uplift observations without consideration of magma compression will underestimate actual magma volume changes. Received: 30 September 1998 / Accepted: 27 July 1999