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MAGMA MOTION Sintesi della relazione

Project ID: 23855
Finanziato nell'ambito di: FP6-MOBILITY
Paese: France

Final Activity and Management Report Summary - MAGMA MOTION (The use of geodetic data to establish the geometry and depth of magma reservoirs and the rate of magma movement towards the Earth ...)

The Askja Volcanic centre, in the Northern Volcanic Zone (NVZ) in Iceland, hosts three calderas. The main caldera, called 'Askja', has been subsiding at least since 1983 and probably even since 1973. The last eruption at Askja took place in 1961. Excluding erupting volcanoes, Askja has displayed a significantly higher rate of ground deformation, i.e. subsidence, than anywhere else in Iceland or even the world. Satellite images provide an excellent tool to monitor this unique deformation. Interferometric Synthetic aperture radar (InSAR) is a remote sensing technique which combines two Synthetic aperture radar (SAR) images which cover the same area but are acquired at different times. We calculated the phase difference of the waves returning to the satellite, which is directly related to the ground movement.

Here, for the first time, we have used RADARSAT-2 images from the Canadian Space Agency to calculate InSAR images of the NVZ in Iceland. Seventeen RADARSAT-2 images enable the construction of several reasonable to good quality interferograms covering the Askja caldera and surroundings. The Stanford Method for persistent scatterers (StaMPS) was adapted for general use with RADARSAT-2 data and used to create time series of RADARSAT-2 interferograms covering the 2000 - 2010 period. The time series of RADARSAT interferograms shows that Askja caldera continues to subside into summer 2010.

The interferograms show three main features:
i) concentric fringes depicting subsidence in the centre of the main Askja caldera;
ii) oval fringes elongated along the rift portraying subsidence; and
iii) subsidence in an area north of the Vatnajökull glacier.
The average Line-of-sight (LOS) velocity from ground to satellite was used as input for inverse modelling, of a deflating pressure source beneath the caldera, embedded in a homogeneous, elastic half-space. Two different source geometries were tested to mimic the magma chamber beneath Askja: a spherical Mogi source and a penny shaped crack. The concentric fringes centred in the Askja caldera are best fitted by a Mogi source located at 65.05N 16.78W, at 3.5 km depth with a volume change of -0.0014 km3/yr from 2000-2009, consistent with previous studies. In the last few years, previously undetected activity has been observed in the Askja area.

Since 2006 small magnitude lower-crustal earthquake swarms were observed and, since the beginning of 2007, 20 km to the east of Askja, intense persistent deep-seated seismic activity has been recorded. Both of these observations, even though very different in character, suggest magma movement took place at deep levels in the Askja area. Furthermore, micro-gravity data of 2008-2009 were interpreted in terms of a mass increase beneath Askja caldera. None of these processes significantly affected the main subsidence pattern, suggesting the independence of the pressure changes at the Askja volcanic system. Precise levelling has been conducted at Askja intermittently since 1966.

We separated (InSAR) LOS changes into mostly vertical and mostly horizontal components to facilitate comparison to precise levelling data. The combined InSAR and precise levelling dataset reveals that, during this non-eruptive period, Askja continuously subsides at a rate of 2.5-3 cm/yr in 2000 - 2010, somewhat lower than the ~5 cm/yr rate inferred for the 1983 - 1998 period. The comparison of Askja's behaviour with that of other restless calderas worldwide suggests that the unique location of Askja, on a spreading segment and above a centre of up-welling (a so called hot-spot) plays a major role in establishing the deformation pattern. Provisional two-dimensional FEM models including structural complexities in the crustal layers indicate that the tectonic setting of Askja plays an important role in the continuous, long-term high subsidence rates observed there. In order to fully understand the cause and effects of the complicated tectonic setting we encourage the use of a more realistic rheological model of the area which could lead to reinterpretation of previous model results.


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