| Summary: | 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 ...
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