Measuring and modelling the crustal response to the 2011 eruption of Nabro volcano, Eritrea

Nabro volcano, situated to the east of the Afar Rift Zone, erupted on 12 June 2011. Eruptions at such off-rift volcanoes are infrequent, and consequently the plumbing systems are poorly understood. In this thesis I present post-eruption InSAR and seismic data to delineate the plumbing system of Nabro. I also investigate the temporal evolution of the system. I discuss my findings in reference to the tectonics of the Afar Rift Zone, off-rift volcanism and compare the findings to volcanoes world wide. I present 6 weeks of continuous seismic activity from an array of 7 seismic stations deployed following the eruption. For the analysis I locate and relocate hypocentres, determine focal mechanisms, calculate b-values and cross-correlate waveforms. I have relocated the hypocentres of 456 earthquakes and used the spatial pattern to interpret the local and regional crustal response to the eruption. The shallow earthquakes beneath Nabro's caldera delineate a NE-SW thrust fault which dips 45 degrees to the SE and extends across the caldera floor. This accommodates the stress change following the eruption, rather than movement on ring faults. The NE-SW fault plane is not associated with measurable surface deformation, indicating that it does not contribute much to the caldera deformation. A 10 km deep cluster highlights potential magma migration pathways directly beneath Nabro. On the flanks of the volcano, a linear pattern of earthquakes illuminate possible minor faults. There is also a cluster of earthquakes beneath Mallahle caldera at a depth of 6 km; the b-value for this cluster is 0.97 and is lower than that for clusters under Nabro (b=1.3). This implies that the earthquakes generated at Mallahle are not dominated by magmatic processes and occur in rock with a stronger rheology. Therefore, the seismicity I observed is likely due to changes in the stress field resulting from the subsidence at Nabro, and not caused by magma movement beneath Mallahle. TerraSAR-X and COSMO-SkyMed were both tasked to prioritise 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. I processed the 25 images acquired by TerraSAR-X between 1 July 2011 and 5 October 2012 on descending orbit 046, to create 34 interferograms. I complemented these with 19 images from ascending orbit 130 spanning 6 July 2011 to 10 October 2012 from ascending orbit 130, which I used to create 21 interferograms. I produced velocity ratemaps and time series using pi-RATE, showing subsidence of 25 cm/yr offset by 2 km to the SW of Nabro's caldera. 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. I combined the InSAR data sets using a modelling approach to produce a detailed time series of the deflation of a Mogi source at 6.4 km depth. The time series shows that the volcano continued to subside for the entire period of observation, with the most rapid subsidence in the first 12 weeks, followed by subsidence at a slowly declining rate. I assessed the impact of atmosphere delays, using 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 two atmospheric artefacts apparent in the COSMO-SkyMed time series. This result highlighted the importance of applying atmospheric corrections using independent sources of information. This contrasts with a standard approach of filtering in space and time which did not completely remove these atmospheric errors. Without the ERA-I correction the time series appeared to show pulses of recharge; with the correction continued subsidence is observed. I explore mechanisms that might explain the long-lived subsidence at Nabro volcano. In particular, I tested models of thermal contraction, degassing, fluid migration and viscoelasticity. Degassing is the most likely cause of deformation, although contraction due to cooling may also contribute. The long term post-eruption subsidence is unusual in comparison to other active volcanoes. I suggest that the low magma supply rate, combined with the high rate of passive degassing, induces an overall subsidence of the ground surface above Nabro