2023–2024 inflation-deflation cycles at Svartsengi and repeated dike injections and eruptions at the Sundhnúkur crater row, Reykjanes Peninsula, Iceland

• Extensive geodetic dataset used to evaluate volcanic inflation-deflation cycles. • Improved understanding of magma accumulation prior to diking events and eruptions. • Improved understanding of the style of magma transfer during repeated rifting events. • Evolution of volume changes within a magma domain during inflation-deflation cycles. • Implications for forecasting future diking events and eruptions. Series of inflation-deflation cycles have occurred during 2020–2024 in the center of the Svartsengi volcanic system, SW-Iceland. Since 27 October 2023, continuous inflation has been interrupted by deflation periods when nine dike injections and seven eruptions have occurred from 10 November 2023 to 8 December 2024 at the Sundhnúkur crater row and its extension. Extensive observations of ground deformation using GNSS (Global Navigation Satellite System) geodesy and interferometric analysis of synthetic aperture satellite (InSAR) images is here used to improve understanding of the dynamics of magma accumulation and transfer, both prior to and during repeated rifting events. Joint inversions of the GNSS and InSAR data, considering a deformation source within a uniform elastic half-space, infer pressure changes at about 4–5 km depth near the regional brittle-ductile boundary, with inflow causing volume increase rates of 2.4–9 m 3 /s. Geodetic modelling using GNSS has been undertaken in near real-time throughout the events, using deformation sources in fixed locations inferred in earlier joint inversions. The deflation periods began rapidly when a dike propagated from the eastern edge of the magma accumulation area. The estimated volume of dikes is in the range (1–133) × 10 6 m 3 , with the first event being by far the largest and longest (∼15 km). Geodetic observations have contributed to success in forecasting diking/eruption onset in the medium and short term, using the expectation that a correlation exists between volume loss in the magma domain during a deflation event and subsequent volume recharge to the system before the next event is triggered.