Moving towards detecting and understanding volcanic unrest prior to eruptions, there has been significant improvements in understanding the structure and dynamics of magma plumbing systems. However, deciphering subsurface processes during inter-eruptive periods often remains challenging, and whether magma is involved or not is an essential question to be able to assess the degree of activity of a volcano. In this thesis, I explore the integration of surface deformation and temporal gravity, using the Askja volcano (Iceland) as a case study.
Magmatic processes are usually closely linked to pressurization-depressurization mechanisms, which can translate into subsurface volume changes and lead to surface deformation responses. Because magmatic processes are diverse, integrating temporal gravity can help narrowing down the list of possible on-going processes. Indeed, temporal gravity is related to subsurface mass change, and for example, a surface uplift associated with a gravity increase could be caused by a magma intrusion, whereas an uplift with no mass change could be caused by gas pressurization resulting from temperature increase. Additionally, comparing both signals can help evaluating the contributions of external processes, such as of hydrothermal or tectonic origin.
The Askja volcano, which is one of the most active volcanoes in Iceland, has erupted at least 40 times in the last 1,100 years. Some events were very powerful, such as the 1875 VEI-5 caldera-forming Plinian event and the most recent event was a basaltic fissure eruption, which occurred in 1961. Since at least 1983, the main caldera has been subsiding and all the previous studies that applied analytical modelling to surface deformation records at Askja agree that the subsidence can be best explained over periods of 10 years or less, by a deflating source, located at 3-3.5 km depth beneath the caldera centre. The constrained linear volume changes have diminished from about -0.002 km^3 yr^{-1} near 2000 to about -0.001 km^3 yr^{-1} near 2010.
In parallel, gravity measurements, which were recorded between 1988 and 2010, highlighted a gradual gravity decrease of about 140 microgal up to 2007, centred on the main caldera, and a gravity increase of about 60 microgal was observed between 2007 and 2009, while the subsidence continued. Due to the lack of spatial coverage, no analytical model could be performed using the gravity results, hence there were no constraints on the depth and magnitude of mass changes. Due to the correlation in locations, previous studies assumed that mass and volume changes were related to the same process, occurring at 3-3.5 km depth. Based on this assumption it was suggested that the main process causing the subsidence was a magma drainage down to deeper levels with possible additional effects from magma crystallisation. A magma intrusion at the shallow reservoir and/or mass variations in the hydrothermal system were proposed to explain the temporary gravity increase. Finally, the likely high contribution of plate spreading as a cause of subsidence was demonstrated using finite-element modelling, considering the caldera and shallow chamber as zones of weak materials, embedded in a two-layer crust model with a visco-elastic lower crust.
In this thesis, I take the analyses of both surface deformation and temporal gravity at Askja a step further, to clarify the causes of subsidence. I use the Interferometric Synthetic Aperture Radar (InSAR) technique to investigate the spatial and temporal signature of the long-term subsidence, considering a 15-year-long time period. This technique, which was used at Askja in two previous studies, can measure surface deformation at the centimetre scale over large areas with spatial resolution of tens to hundreds of meters. My results show that the caldera is steadily subsiding as a whole, and can be fitted by an exponential decay with relaxation time of about 42 years. Using the Bayesian inversion modelling approach paired with the Markov chain Monte Carlo sampling, and incorporating the exponential behaviour of the subsidence, I refine the depth of the shallow reservoir with narrower bounds compared with previous studies: when assuming a point pressure source, which can reproduce well the circular spatial deformation pattern observed in the caldera, there is 95% chance that the reservoir is located at 3+/-0.1 km beneath the caldera centre and the exponential volume decrease has total amplitude of 0.07+/-0.01 km^3.
In parallel, I investigate the spatial and temporal evolution of gravity changes over 2015-2017, from a larger gravity network and using improved methodologies compared with previous studies. My results show a spatial-bowl shape signature over 2015-2016, with maximum decrease of about 100+/-30 microgal at the caldera centre. Although this annual change is spatially correlated with the synchronous subsidence, the following annual gravity change, showing negligible variations across the caldera, is not. This suggests that both signals do not relate to the same processes, and the difference in magnitude of gravity changes compared with previous studies is due to the choice of reference station. I then further investigate the link between gravity changes and deformation, by performing the first gravity inversion at Askja, and using the Bayesian inversion modelling approach paired with the Markov chain Monte Carlo sampling. Even though poorly constrained, the inversion suggests that, when assuming a spherical source, the gravity changes over 2015-2016 have 95% chance to be due to a mass decrease within 1.5x10^{12}-7.5x10^{10} kg and located within 2.7-9.9 km. These large confidence intervals are due to the large uncertainties of the gravity results. Assuming magma drainage, the mass change derived from the volume decrease constrained from the exponential deformation is outside the 95% confidence interval of mass change constrained from gravity. The uncorrelated temporal variations and discrepancy in magnitude between both types of signals therefore suggest that magma drainage is unlikely to be responsible for the subsidence at Askja. Alternatively, the steady and gradually decaying subsidence could be driven by extension due to plate spreading, which would induce pressure decrease at the shallow reservoir, and crystallisation processes could also participate. On the other hand, the gravity changes could be due to mass fluctuations in a hydrothermal system just above the shallow magma reservoir.
To precisely extract gravity changes related to magma movements and/or hydrothermal mass variations and fully integrate errors in my gravity analysis, I have developed a statistical approach that estimates the total error budget associated with temporal gravity, when using spring relative gravimeters. In this thesis, I present the method in detail, providing equations for users to estimate case-by-case error budgets, and I also provide ranges of best-to-worst case scenarios, to guide users on where to focus effort to minimizing errors, depending on the magnitude of the signal of interest. My results show that the choice of gravimeter is essential to minimize vibration noise and errors due to imprecise levelling, which can both reach hundreds of microgals. Similarly, monitoring the temporal evolution of calibration factor should be usual practice, especially when studying gravity time-series spanning several years. Finally, I demonstrate that a bulk estimate of errors due to unknown meterological effects, which can reach a few tens of microgals, can be derived from base station measurements spanning at least a few days.
