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

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.

Seismicity and 3-D body-wave velocity models across the Hengill geothermal area, SW Iceland

“…Prestress: The fields are close to each other and would be expected to be under the same tectonic stress. However, there may be some variation in stress caused by the production, as evidenced by the deformation (subsidence) above the fields (Juncu et al, 2017(Juncu et al, , 2020Ducrocq et al, 2021). Considering this further is beyond the scope of this paper, but should be analyzed in a future publication.…”

“…Over the past years, the Hellisheiði and Nesjavellir production fields have seen substantial subsidence linked to the power plant operations (Ducrocq et al, 2021). Sleggja, an area with intermittent injection within the production field, lies at the northern end of the subsidence signal (in 2018-2019).…”

“…A Bouguer gravity map and an aeromagnetic map have been published and interpreted qualitatively (Björnsson et al, 1986;Hersir et al, 1990). GPS and satellite images have been used to observe surface deformation related to pressure changes in geothermal reservoirs (e.g., Budzinska, 2014;Juncu et al, 2017;Ducrocq et al, 2021). Surface faults are defined from field mapping and aerial photographs (e.g., Saemundsson, 1992;Clifton et al, 2002;Saemundsson et al, 2016).…”

Evolution of Stresses Over Conjugate Faults in Hjalli‐Ölfus, South Iceland

“…North‐northwest of the SISZ lies the Western Volcanic Zone (WVZ) that runs parallel to the EVZ, and spreads at one‐third to half the rate of its eastern counterpart (Árnadóttir et al., 2008; Gudmundsson, 2007; Perlt & Heinert, 2006). The southern end of the WVZ hosts the Hengill volcanic system that has witnessed notable inflation‐deflation episodes over recent decades (Ducrocq et al., 2021; Feigl et al., 2000; Sigmundsson et al., 1997). The active central volcanic system, Hengill, lies north of the western part of the SISZ (regionally known as Ölfus), with its fissure swarms extending into western Ölfus.…”

Joint Bayesian Modeling of Velocity Break Points, Noise Characteristics, and Their Uncertainties in GNSS Time Series: Far‐Field Velocity Anomalies Concurrent With Magmatic Activity in Iceland

“…However, such approach obstructs the finding of break points. In contrast, piecewise functions are transparent for estimating both break points and velocity change values (e.g., Davis et al., 2006; Ducrocq et al., 2021). Detecting break points for piecewise functions is a non‐linear problem because the location of a break point affects velocity estimation.…”