Inflation-Deflation Episodes in the Hengill and Hrómundartindur Volcanic Complexes, SW Iceland

Cécile Ducrocq,Bjarni R Kristjánsson,Daniel Juncu,Gunnar Gunnarsson,Sigrún Tómasdóttir,Freysteinn Sigmundsson,Sigrún Hreinsdóttir,Thóra Árnadóttir,Hanna Blanck,Vincent Drouin,Halldór Geirsson

doi:10.3389/feart.2021.725109

Abstract

Non-eruptive uplift and subsidence episodes remain a challenge for monitoring and hazard assessments in active volcanic systems worldwide. Sources of such deformation may relate to processes such as magma inflow and outflow, motion and phase changes of hydrothermal fluids or magma volatiles, heat transfer from magmatic bodies and heat-mining from geothermal extraction. The Hengill area, in southwest Iceland, hosts two active volcanic systems, Hengill and Hrómundartindur, and two high-temperature geothermal power plants, Hellisheiði and Nesjavellir. Using a combination of geodetic data sets (GNSS and InSAR; Global Navigation Satellite Systems and Interferometry Synthetic Aperture Radar, respectively) and a non-linear inversion scheme to estimate the optimal analytical model parameters, we investigate the ground deformation between 2017–2018. Due to other ongoing deformation processes in the area, such as plate motion, subsidence in the two geothermal production fields, and deep-seated source of contraction since 2006, we estimate 2017–2018 difference velocities by subtracting background deformation, determined from data spanning 2015–2017 (InSAR) or 2009–2017 (GNSS). This method highlights changes in ground deformation observed in 2017–2018 compared to prior years: uplift signal of ∼10 km diameter located in the eastern part of the Hengill area, and geothermal production-related temporal changes in deformation near Húsmúli, in the western part of the Hengill area. We find an inflation source located between the Hengill and Hrómundartindur volcanic complexes, lasting for ∼5 months, with a maximum uplift of ∼12 mm. Our model inversions give a source at depth of ∼6–7 km, located approximately in the same crustal volume as an inferred contracting source in 2006–2017, within the local brittle-ductile transition zone. No significant changes were observed in local seismicity, borehole temperatures and pressures during the uplift episode. These transient inflation and deflation sources are located ∼3 km NW from a source of non-eruptive uplift in the area (1993–1999). We consider possible magmatic and hydrothermal processes as the causes for these inflation-deflation episodes and conclude that further geophysical and geological studies are needed to better understand such episodes.

Highlights

Uplift and subsidence episodes without eruptive activity are common in several active volcanic systems (e.g., Etna, Campi Flegrei, Yellowstone, Okmok, Alutu; Italy, United-States, Ethiopia Lima et al, 2009; Chang et al, 2010; Biggs et al, 2011; Walwer et al, 2019)
The Interferometry Synthetic Aperture Radar (InSAR) and Global Navigation Satellite Systems (GNSS) times series analysis (Geodetic Data) highlights two main ground deformation signals in the Hengill area in 2017–2018 that differ from the general trend of the prior years (Figure 6)
Our statistical analysis (χ2]; Table 2) indicates that a contracting prolate spheroid (Húsmúli source) and an expanding point source in Ölkelduháls is the simplest model combination explaining satisfactorily the changes in deformation observed in the Hengill area between 2017 and 2018

Summary

Introduction

Uplift and subsidence episodes without eruptive activity are common in several active volcanic systems (e.g., Etna, Campi Flegrei, Yellowstone, Okmok, Alutu; Italy, United-States, Ethiopia Lima et al, 2009; Chang et al, 2010; Biggs et al, 2011; Walwer et al, 2019). Studies have shown that more complex mechanisms, often linked to highenthalpy fluids, volatiles, and their interaction with magma and magmatic gases, can generate transient uplift motions followed by gradual subsidence Uplift episodes can resemble precursory signals for an eruption. It is important for hazard assessments of volcanic systems to better understand the sources of deformation, motivating detailed documentation of inflation-deflation episodes at volcanic systems

Results

Discussion

Conclusion