Changes in seismicity and observed deformation related to inflation at the Theistareykir high-temperature geothermal field, NE Iceland, in 2023-2024

Theistareykir is one of five active volcanic systems of the Northern Volcanic Zone, NE Iceland, along with Krafla, Fremrinámar, Askja and Kverkfjöll, from north to south, respectively. The Theistareykir volcanic system includes a N-S trending rifting fissure swarm, approximately 70-80 km long and 7-8 km wide, extending through it. There are neither postglacial eruptive fissures nor a clear caldera formation at Theistareykir, with the latest eruption occurring ~2,400 years ago. Theistareykir comprises a high-temperature geothermal system which has been systematically explored over the past 50 years, with around 20 exploration and production wells drilled to date. Since 2017, geothermal energy has been utilised at Theistareykir with a 90 MWe power station currently operated by Landsvirkjun, the National Power Company of Iceland. Extensive monitoring of the geothermal field is carried out through e.g., various geophysical measurements. An inflation was observed to start at Theistareykir at the beginning of 2023, with the centre of uplift approximately 2.5 km west of the Theistareykir power plant. Earthquake activity in Theistareykir occurs in more or less three separated clusters, and coinciding with the start of inflation, an increase in seismicity rate was observed within the northernmost cluster, with the largest earthquake reaching ML 2.2 at 5.2 km depth. Earthquake depths within this cluster range between ~5-7 km, deepening towards north. No significant change is observed in faulting mechanisms within this cluster, despite the inflation, with oblique strike-slip events most common. The vertical component of the continuous Global Navigation Satellite System (GNSS) station in Theistareykir (THRC), located ~0.5 km northeast of the uplift centre, indicates a best-fit onset time of the inflation around 9 February 2023, and an initial uplift of 21.5 mm/yr. Synthetic Aperture Radar Interferometry (InSAR) of Sentinel-1 images was used to measure the inflation. The anomaly is ~10 km wide and the uplift in its centre is ~12-15 mm between the summers of 2022 and 2023. Knowing that the uplift started around the beginning of 2023, the actual uplift rate is therefore ~20-25 mm/yr at the uplift centre. This is similar to the two previous inflation episodes observed in the area in 1995-1996 and 2006-2009. Geodetic modelling, using the InSAR data, indicates that a model with a point source pressure within a uniform elastic halfspace can explain the observations. The inferred source has a centre depth in the range of 4.4-6.2 km (95% confidence interval), and a volume change of (1.1-2.5) x 106 m3 (95% confidence interval) until the end of summer 2023. Our aim is to understand better the activity and identify the driving processes, and their implications for the geothermal field. Results will be presented in the context of past earthquake and deformation data.