High-resolution InSAR time series analysis following the 2011 eruption of Nabro Volcano, Eritrea: Implications for the mechanisms of post-eruptive subsidence

During continental rifting the majority of active volcanism typically becomes focused in a central rift axis. However, volcanic structures are also commonly found offset from the axis of spreading. Eruptions at such off-rift volcanoes are infrequent, and consequently the crustal processes are poorly understood. Nabro volcano, situated to the east of the Afar Rift Zone (ARZ), erupted on 12 June 2011. The eruption was only the second eruption of an off-rift volcano associated with the ARZ in modern times (since Dubbi volcano in 1861). We have recently published a study [Hamlyn et al., JGR 2014] comparing the post eruption ground displacement observed through TerraSAR-X InSAR with seismicity recorded at a local seismic array that we deployed following the eruption. This has provided new insights into the controls of caldera faults on subsidence. We now present a detailed analysis of the InSAR derived, time series of post-eruption subsidence using both TerraSAR-X and COSMO-SkyMed observations. We test competing hypotheses for the origin of the subsidence and stress the importance of atmospheric correction using independent data when interpreting topographically-correlated deformation at volcanoes. Following the June 2011 eruption at Nabro, TerraSAR-X and COSMO-SkyMed were both tasked to prioritize the acquisition of SAR data over the volcanic centre. During the following 15 months, Nabro was imaged 129 times by these satellites, with an acquisition every 5 days on average. Using a Small Baseline Subset (SBAS) approach, we processed the 25 images acquired by TerraSAR-X between 1 July 2011 and 5 October 2012 on descending orbit 046, to create 34 interferograms. We complemented these with 19 images from ascending orbit 130 spanning 6 July 2011 to 10 October 2012 from ascending orbit 130, which we used to create 21 interferograms. We produced velocity ratemaps and time series using π-RATE, showing subsidence of up to 25cm/yr centred on Nabro [Hamlyn et al., JGR 2014]. COSMO-SkyMed satellite also imaged the volcano on a descending track between 26 June 2011 and 18 July 2012 within the Italian Space Agency project SAR4Volcanoes: a total of 64 images were acquired and used to produce 171 interferograms. Each dataset were used to create a detailed time series of incremental deformation of the Nabro caldera using an SBAS approach. We combine these data sets using a modelling approach to produce a detailed time series of the deflation of a Mogi source at ~7 km depth. The raw and smoothed (temporal window of 70 days, spatial window of ~3km) time series show that the volcano continued to subside for the entire period, with a rapid subsidence in the first 12 weeks, followed by a slowly declining rate. In addition there are three apparent pauses in the subsidence. As the subsidence rate is approximately correlated with the topography of the volcano, we were concerned about the influence of atmospheric delays on our geophysical signal. To assess the impact of atmosphere delays, we use the outputs from ERA-Interim (ERA-I), a global atmospheric model computed by the European Centre for Medium-range Weather Forecasting (ECMWF), to correct each SAR acquisition. The atmospheric correction noticeably reduced the scatter in the time series, and removed the three apparent atmospheric artefacts. This result stresses the importance of applying atmospheric corrections using independent sources of information – the standard approach of filtering in space and time did not remove these atmospheric errors and without the ERA-I correction we would have produced a significantly different interpretation of the InSAR observations. Finally, we explore alternative mechanisms that might explain the long-lived subsidence at Nabro volcano. In particular, we test models of thermal contraction and cooling, visco-elastic relaxation, and degassing. The Nabro eruption was one of the largest volcanic emissions of SO2 since the 1991 eruption of Mt. Pinatubo. Therefore we compare the InSAR derived subsidence to the SO2 emissions observed by the IASI instrument. A correlation between the rate of SO2 emission and the rate of subsidence suggests a common underlying process, possibly magma cooling.