Abstract
A major goal in volcanology is to be able to constrain the physical properties of a volcanic system using surface observations. The behaviour of a volcanic system following an eruption can provide powerful constraints on these properties and can provide valuable information for understanding future hazard. We use spatially and temporally dense observations of surface deformation following the 12 June 2011 eruption of Nabro (Eritrea) to place constraints on the mechanics of its subsurface volcanic system. Nabro was imaged 129 times by TerraSAR-X and COSMO-SkyMed satellites during a 15-month period following the eruption. We have produced a detailed time series of the line-of-sight (LOS) displacements at Nabro, finding that the volcano subsides during the entire observation period at a decaying rate. We found significant atmospheric artefacts remained in the data set after a standard spatio-temporal filter was applied. Applying an empirical correction using a linear phase-elevation relationship removed artefacts but also removed real topographically correlated deformation. Instead, we were able to correct each SAR acquisition using independent delay estimates derived from the ECMWF ERA-Interim (ERA-I) global atmospheric model. The corrected time series can be modelled with the deflation of a Mogi source at ∼ 6.4 ± 0.3 km depth. Modelling the time series using viscoelastic relaxation of a shell which surrounds a spherical magma chamber can explain the observed subsidence without a source of further volume loss if the magma is compressible. CO2 outgassing is also a possible cause of continued subsidence. Contraction due to cooling and crystallisation, however, is probably minor. If any post-eruptive recharge of the magmatic system at Nabro is occurring, the rate of recharge must be slower than the post-eruptive relaxation processes. Combined with the lack of pre-eruptive inflation, we suggest that recharge of the magmatic system at Nabro either occurs at a rate that is slower than our detection limit, or it occurs episodically. This case study demonstrates the power of long, dense geodetic time series at volcanoes.
Highlights
The pattern and rate of surface deformation observed after a volcanic eruption are dependent on the magmatic processes occurring at depth and the rheology of the crust and magma
Because the initial acquisitions in each time series are on different dates, we needed to adjust the starting position of the TSX-derived time series of volume so that they were consistent with the CSK time series of volume
Potential processes that could account for post-eruptive subsidence include viscoelastic relaxation (e.g. Newman et al 2001) cooling and contraction of a magma reservoir (e.g. Poland et al 2006; Parker et al 2014) or other mechanisms operating on a closed magmatic system
Summary
The pattern and rate of surface deformation observed after a volcanic eruption are dependent on the magmatic processes occurring at depth and the rheology of the crust and magma. There are numerous processes which may control whether a volcanic edifice undergoes a period of uplift, subsidence or remains at a given height. Edifice uplift due to post-eruptive recharge of a magma chamber has been observed at many volcanoes, including Alu-Dalafilla (Ethiopia) following the 2008 eruption (Pagli et al 2012) and Gabho and Dabbahu (Ethiopia) following the 2005 Dabbahu-Manda-Hararo rifting episode (Wright et al 2006; Grandin et al 2010). Some post-eruption magmatic processes do not result in geodetic signals at the surface. The process could be overlooked if the temporal resolution of the geodetic data is too low (e.g. Chaitén, Chile (Fournier et al 2010))
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