Abstract One of the main goals of volcano geodesy is to improve the understanding of how an increase in pressure related to magma accumulation causes ground deformation in order to evaluate volcanic unrest. The inversion methods used for this purpose rely on a parametrization of the shape of the crustal volume in which pressure changes due to magma inflow/outflow (the magma domain), to search for the optimal parameters that minimize the difference between model predicted and measured ground displacements. However, these methods assume a predefined shape of the magma domain, which limits their applicability. Here, we propose a new shape optimization framework that can invert these sources without such prior, formulating a reconstruction problem to infer the complete shape of the magma domain. First, we validate this approach using a synthetic test case and then apply it to observations of the Svartsengi volcanic system in Iceland.

Abstract The Krafla volcanic system in Iceland, a subaerial segment of the divergent boundary between the North American and Eurasian plates, offers a unique opportunity to investigate volcano‐tectonic processes. We present a six‐segment kinematic back slip plate boundary model constrained jointly by long‐term deformation velocity fields from Global Navigation Satellite System (2002–2024) and Sentinel‐1 SAR interferograms (2015–2023). The inferred plate boundary axis passes through the middle of the Krafla caldera and follows recent eruptive fissures. Its orientation varies between N1.2°–N10.8°E within and south of the central volcano but rotates to NNW‐SSE north of it. For this preferred axis geometry, the inferred spreading rate is mm/yr in an azimuth of ° (formal uncertainties corresponding to 95% confidence interval of the Bayesian inversion results). Locking depth varies significantly along the plate boundary axis: it is shallowest within and just north of the Krafla caldera ( km) and deepens to and km to the north and south, respectively. This variation is consistent with the depth distribution of seismicity and reflects an up‐doming brittle‐ductile transition along the fissure swarm. These first‐order features remain robust, although uncertainties increase when the axis location is allowed to vary. After correcting vertical velocities for glacial isostatic adjustment and regional subsidence caused by extensional rheological anomalies, residual velocities reveal three areas of local deflation: two related to the Krafla and Bjarnarflag geothermal fields and one at the northernmost part of the 1975–1984 Krafla lava field. Therefore, detailed plate boundary models can provide insights into crustal rheology, volcano‐tectonic interactions and local processes.

Changes at the surface of a volcanic edifice, such as snow or hydrological loading, ice cap melting, and flank destabilization, can cause significant surface deformation. Understanding the contribution of surface processes to ground deformation is therefore important for monitoring the state of the underlying volcanic system. The Katla Volcano in Iceland lies under Mýrdalsjökull, the fourth largest glacier in Iceland, and undergoes the largest seasonal deformation of all Icelandic volcanoes: up to 4 cm horizontally and 3 cm vertically at the Austmannsbunga (AUST) Global Navigation Satellite System (GNSS) station. The last confirmed eruption of Katla occurred in 1918. Since then, episodes of elevated seismicity and jökulhlaups (sudden glacial outburst floods) have been recorded at the volcano, the most noticeable in 1955, 1999, 2011, and 2024. During the 2024 jökulhlaup, horizontal displacement of up to 7 cm was recorded at AUST. In this work, elastic three-dimensional finite element method models were implemented to quantify surface deformation from seasonal load changes. The models include realistic bedrock topography and ice unloading based on recent data collected at Mýrdalsjökull. A deformation model considering only seasonal snow unloading can reproduce within uncertainty the observed GNSS vertical surface displacements. It can also explain the horizontal signals at GNSS stations located outside the glacier, although not at GNSS stations located on nunataks inside Mýrdalsjökull. An additional deformation source must be considered to explain the residual cm-scale horizontal displacement at these stations. We model the residual signal using a thermo-poro-elastic cylindrical source. The best-fit source is a cylinder located at the surface that produces cm-scale horizontal deformation with limited vertical deformation. The shallow source depth derived from the inversion and the surface deformation recorded at AUST during the jökulhlaup series in 2024 and 2025 suggests that seasonal deformation is not strictly related to magmatic activity. We infer that changes within the hydrologic system of the glacier are responsible for most of the horizontal deformation at Katla.

Askja is one of the most monitored volcanoes in Iceland. Since 1966, annual ground deformation measurements have been carried out in Askja along a leveling line. In 1993 the first Global Navigation Satellite System (GNSS) measurements were made in Askja and in 1992 the first Interferometric Synthetic Aperture Radar (InSAR) images of Askja were gathered. Since 2021 there has been uplift at Askja volcano, after decades of subsidence. The uplift is monitored with GNSS and InSAR measurements. The net uplift from June 2021 to December 2025 is approximately 90 cm with a decreasing rate. Previous geodetic models of the observed ground deformation inferred an inflation source at a median depth of 2.7 – 2.8 km. Gravity surveys have been carried out regularly since 1988, and annually since 2018. Gravity measurements show mass or density changes in the sub-surface. From 1988 – 2017 there was a net gravity decrease, while measurements from 2017 – 2023 show a net gravity increase during that period.We carried out GNSS campaigns and gravity surveys in August of 2024 and 2025. We measured 18 gravity stations and 20 GNSS stations scattered around Askja. The gravity was measured with two relative spring gravimeters (Scrintex CG5 and CG6). Gravimeters are very sensitive and prone to sudden data tares, to mitigate this we used two meters. We can evaluate the uplift between years with GNSS and InSAR data and apply the theoretical Free Air gradient to correct for the gravity change due to elevation change. The yearly uplift rate 2023 - 2025 is up to about 10 cm/year. After correcting for the height changes, preliminary evaluation suggests that the net gravity change from 2023-2025 does vary between stations, with increase at some stations and decrease at others. By analyzing the gravity change we are adding another parameter to our dataset, which helps us to identify the process responsible for the current uplift episode.

The hypothesis of the ISVOLC project is that retreat of Icelandic glaciers since the end of the 19th Century has the potential to impact both volcanic and seismic activity. As volcanic activity increased significantly during (and in the

Subsidence in already emplaced lava can be caused by contraction as they cool, degassing and collapse. In this study, we present preliminary estimates of short-term volume change associated with the January (14 Jan, 07:58 UTC – 16 Jan, 01:08 UTC) and February (8 Feb, 06:03 UTC – 9 Feb, afternoon) 2024 eruptions at the Sundhnúkagígar crater row, Iceland. Surface elevation changes were derived from a series of multi-temporal pre- and post-eruptive Digital Elevation Models (DEMs) based on stereo imagery collected by UAV and manned aircraft, using a photogrammetric workflow in the Agisoft Metashape software. Volume changes were quantified from DEMs of Difference (DoDs) by integrating surface elevation changes over the mapped lava field, accounting for random, spatially correlated, and systematic errors. Positive and negative lava volume estimates represent the areal integration of surface uplift and subsidence respectively, as a result of the eruption, rather than strictly representing net mass addition or removal at a given location. Positive changes may reflect lava emplacement or internal redistribution of previously erupted material, whereas negative changes indicate thermal contraction, lava drainage, degassing, or collapse of the cooling lava surface. All reported volumes refer to changes integrated over the mapped lava field only. During the January eruption, rapid emplacement between 14 and 15 January resulted in a dominance of positive volume change, with 0.463 ± 0.0005 Mm³ of positive and −0.233 ± 0.0004 Mm³ of negative volume change. Between 15 and 17 January (approximately 48 h after eruption onset), volume changes were dominated by surface lowering, with −0.094 ± 0.0009 Mm³ negative versus 0.047 ± 0.0006 Mm³ positive volume change, reflecting contraction and internal redistribution as the dominating processes. From 17 January to 13 February, volume changes were minor, with 0.049 ± 0.001 Mm³ positive and −0.040 ± 0.001 Mm³ negative. For the February eruption, the analysis was constrained by the geological setting and the short repose time between eruptions, as rapid resurfacing of the lava field by subsequent eruptive activity limited the temporal persistence of measurable surface changes. On 8 February, comparison of DEMs acquired at 13:15 and 17:05 UTC shows a dominance of negative volume change, with −1.65 ± 0.01 Mm³ of negative versus 1.35 ± 0.01 Mm³ of positive volume change. Between 8 February (17:05 UTC) and 13 February, negative changes −2.07 ± 0.06 Mm³ exceeded positive changes 1.15 ± 0.04 Mm³. Ongoing work aims to further refine these results by quantifying vertical surface subsidence rates to better characterize post-eruptive surface change behaviour.

Geodetic observations, coupled with modelling of the detected signals, can help discriminate between different processes contributing to measured surface deformation during a volcanic eruption, providing insight into its evolution, the associated magma transport, and processes occurring in the subsurface. Global Navigation Satellite System geodesy and Interferometric analysis of Synthetic Aperture Radar satellite images reveal gradual deflation during the six-month-long 2021 eruption in Geldingadalir at Mt. Fagradalsfjall, in SW-Iceland. The co-eruptive deflation shows three temporal phases: T1, 19 March–10 May; T2, 11 May–31 July; T3, 1 August–18 September, correlating with changes in the effusion rate, eruptive style, and geochemistry of the erupted basalt. Effects of lava loading are evident in the geodetic observations. We remove this signal with a Finite Element Method model and infer geodetic sources driving the observed ground deformation, by testing point-pressure, sill, and ellipsoid models. Model fit and parameter evaluation indicate an ellipsoidal source (centered at 7–8 km depth, and deflating volume change of 21–25 Mm3) fits the data marginally better, though geochemical evidence supports a sill source ( 12–14 km depth and deflating volume change of 21–27 Mm3). Inflation was detected after the eruption and can be modelled at a similar depth as the co-eruptive source. Understanding co- and post-eruptive ground deformation patterns and their correlation with other observables at volcanoes, e.g., effusion rate and geochemistry, is essential to unveil the architecture of the underlying magmatic plumbing system and hazard assessment.

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 Sundhn & uacute;kur 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) x 10(6) m(3), with the first event being by far the largest and longest (similar to 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.

Widespread andesitic volcanism with several eruption centres occurred during the Middle Miocene in the Cserhat Hills, central-northern Hungary. In this time, an extensive dyke system developed in the area, where some dykes have exposed maximum length of 23 km, and maximum width of 25 m. This dyke system shows a change in its strike from E-W to NNW-SSE. Here we integrate new and previous field observations to derive structural maps and study dykes and fractures in the Cserhat Hills. K/Ar geochronology is used to understand the temporal evolution of regional fault patterns before, during and after the formation of the dykes and also to gain insights into the interaction between the dyke emplacement and the regional stress field. Fault-slip data were collected at 27 different sites along the dykes and were combined with reinterpreted datasets from 16 sites located at a distance from dykes. The field observations, integrated with the geochronological data sets suggests that dykes with different orientations were emplaced in two different eruptive cycles around 15.4 and 14.7 Ma. The deformation history of the Pannonian Basin involved a clockwise change in regional minimal stress axis, probably as a result of regional vertical-axis counter-clockwise block rotation. Our field observations suggest strike-slip stress regime may occur near propagating dyke tips, and the direction of minimal stress axis may have locally rotated counter-clockwise where dykes changed their strikes and emplaced along pre-existing fractures, mostly normal faults.

The Surtsey Island was built from 130 m water depth during the period 1963–1967. Two tephra cones formed above sea level reaching a height of 170 m a.s.l. The final surface area of Surtsey reached 2.65 km2 with a volume of 0.8 km3 (dense rock equivalent). Repeated levelling and Global Position System (GPS) campaigns have been carried out every 5–10 years to monitor the internal deformation and subsidence of the island. The first levelling measurements were made in the summer of 1967. Surtsey follows an exponentially decaying subsidence curve, with total subsidence reaching 1 m since 1967. The 2000–2023 GPS surveys confirmed that Surtsey moves horizontally with the Eurasian plate. The average subsidence rate for the three GPS monitoring sites during 2000-2023 was 3.8 mm/yr, a total of 9 cm. About 2/3 of the total subsidence is contributed by compaction of sedimentary and volcaniclastic material, thermal contraction, and palagonization. Up to 1/3 of the total subsidence can be explained by loading of the island on the asthenosphere.

The Reykjanes Peninsula in SW Iceland is transacted by a divergent plate boundary with oblique spreading. Volcanic unrest periods, marked by fissure eruptions widely across the peninsula, seem to occur with regular intervals, approximately every 800-1000 years. These volcanic unrest periods have durations of 100 up to 400 years. In 2020, it became clear that magma was on the move again after 800 years of quiescence, as repeated uplift was measured in the vicinity of the Svartsengi geothermal area. Minor subsidence was recorded between uplift periods and seismicity increased again, only after the previous state of uplift had been surpassed, in line with the so-called Kaiser effect. A year later, a dike intrusion in Fagradalsfjall triggered earthquake activity tens of km away as stored tectonic stresses along the peninsula were released. After a clear decline in earthquake activity, an eruption took place in Geldingdalir, Fagradalsfjall, on 19 March 2021, the first one in over 6000 years in that region. At the time of writing, 4 volcanic eruptions have occurred since 2021 and in total roughly 20 magmatic intrusive events have taken place on the Reykjanes Peninsula.Recently an escalation in volcanic activity has been observed. In late October 2023, the 5th period of uplift started in Svartsengi signifying faster magma inflow rates than previously inferred. Seismicity increased and was widespread in line with increased stresses above an inflating sill at about 5 km depth. On 10-11 November, during nearly 12 hours of intense seismic activity, the magma found its way from the magma storage beneath Svartsengi some 2 km laterally towards the center of an old crater row and creating a 15 km long shallow dike. Subsidence was observed above Svartsengi as the magma was drained from beneath and a graben formed beneath the coastal town of Grindavík where extensive faulting caused considerable damage. On 18 December, a similar but smaller magma intrusive event originating in Svartsengi occurred, causing an eruption approximately at the center of the original dike. This time, earthquakes only occurred about 90 minutes before the eruption onset and no clear trend of earthquakes migrating from Svartsengi towards the laterally offset dike were detected. At the time of writing (10 January, 2024), a similar amount of magma volume is inferred to have accumulated beneath Svartsengi since shortly before the last eruption, however, seismicity is still at normal background levels. The volcano monitoring team at the Icelandic Meteorological Office in close collaboration with geoscientists at the Insitute of Earth Science at the University of Iceland and HS Orka, have been under immense pressure to interpret the ongoing activity. A vital part has been to interpret seismicity rates and earthquake locations and any changes thereof, along with modeling dike and sill inflow rates from geodetic measurements. We show that meaningful interpretation of earthquake activity can only be done when jointly interpreted together with deformation and stress models as stress changes heavily influence earthquake locations and the temporal onset of earthquake activity.

Axial rift volcanoes characterised by an active magmatic and hydrothermal system offer a unique opportunity to study the interaction between these processes. The Dallol volcanic and hydrothermal area is situated in the Afar rift, on the axis of Erta Ale ridge, in a depressed salt plain. Dallol has been experiencing deformation at least since the first dike intrusion observed by InSAR in 2004. Here, we present the results of a new InSAR analysis of Dallol between 2014 and 2023, and inverse modelling of the observed deformation. We used SAR data from the ESA´s Sentinel 1A/B ascending (014) and descending (079) orbits to produce over 651 interferograms. Then we obtained InSAR average velocity maps revealing the presence of three closely spaced and concentric deformation signals of a range increase, consistent with subsidence, of up to 40 mm/yr in the satellite Line-of-Sight (LOS). The main deformation signal corresponds to the Dallol crater, while the two smaller maxima occur on the bishophite precipitating Black Mountain area south of Dallol and at the location of a circular pool at the edge of the salt plain, west of the crater. Our modelling results indicate that the deformation sources can be explained by contractions of three Okada tensile dislocation sources situated at different shallow depths, ranging between 0.7-1.7 km, with a length of 1-3 km and volume decrease of 1-3x10-4 km3/yr. Time series analysis also shows that the subsidence pattern was about linear while small seasonal fluctuation patterns are identified at the two smaller maximas. We interpret that the main subsidance at the Dallol crater is likely caused by the depressurisation of shallow sills, while a possible contribution to the defomration from the hydrothermal system due to seasonal flooding is envisaged for the other two maximas.

How can we not afford to scientifically probe magma? Fifteen years of accidental drilling encounters with magma have shown that it can be done safely with recovery of magmatic and partial melt samples quenched in situ. More could be gained if preceded by thorough scientific preparation and followed by long-term monitoring. Through the panoply of instruments now available, we can measure temperature, pressure, strain, heat and mass transport and changes over time. In 2009, the Iceland Deep Drilling Program well #1 reached rhyolitic magma at 2100 m depth under Krafla Caldera. The project was exemplary in sharing provocative results, but only hints at what is possible. Equilibrium temperatures were estimated by traditional petrologic techniques to be 850 – 1100 C. Pressure estimates range from 40 – 90 MPa with both extremes seemingly problematic, because for the first time we know the depth of a magma body to 4 significant figures. The lowest value is below lithostatic and the highest could be inherited from deeper levels. Now it appears that the lower pressure is what magma “feels”. But without drilling, would traditional estimates be good enough? Magma is somewhere between 1500 – 4000 m depth and with temperature corresponding to some type of magma? Actually, we would not even know that shallow magma is there but now in hindsight we see it geophysically. Ground-truth testing is how methodologies are improved. Our situation is like speculating about the nature of the Moon without sampling it. The cost of probing Earth’s magma is high and the probability of success uncertain, but far less so on either count than for extraterrestrial exploration. On Earth we are more restrained by self-imposed limits than by our technical capabilities. Besides understanding the differentiation of our planet, we have two compelling reasons for bold exploration: 1) We need the baseload, magma resource with its far higher temperature, energy density, and more extensive thermal fracturing than conventional geothermal; 2) We need to raise the level of reliability of eruption forecasts by testing our magma-dynamic models directly, thereby saving countless lives. As with other endeavors that are expensive for a single country to undertake but that benefit all humankind, a way forward is through an international infrastructure, where teams of scientists can conduct experiments with magma and superhot fluids. This is analogous to particle accelerators and the complement to outer space travel: inner space. The Krafla Magma Testbed is a much-needed step and an opportunity for all planetary, magma, volcano, and hydrothermal scientists to test their methods and ideas. KMT will drill a doublet of wells to magma for long-term monitoring and experimentation, respectively. The project, now organized as a legal entity within the Iceland Geothermal Research Cluster (GEORG), in partnership with the National Power Company of Iceland (Landsvirkjun), Iceland Energy GeoSurvey (ISOR), and a multinational team of scientists and engineers, under the aegis of the International Continental Scientific Drilling Program (ICDP), is ready. Magma could have been intentionally explored before. It is time to ask, “Why not now?”

Following an increase in seismic activity in December 2019, a pressure increase began in the center of the Svartsengi volcanic system in January 2020, as inferred from geodetic observations. The first diking event occurred, however, in the nearby Fagradalsfjall volcanic system, about 10 km east of Svartsengi, 24 February – 19 March 2021, when an ~9 km long dike gradually formed with geodetically inferred initial volume increase rates up to 35 m3/s, during the first week of diking. The total dike volume was ~34 Mm3, based on joint inversions of InSAR and GNSS observations that have been extensively used to study this and later events in the area. This dike intrusion culminated in an eruption on 19 March 2021. The initial dike had minor incremental volume increase in association with opening of additional vents above the dike during the 6-month-long 2021 eruption, with near-surface opening in the top few hundred meters. Three additional dike intrusions occurred in the Fagradalsfjall area between December 2021 to July 2023, with initial magma flow rates between 22 to 70 m3/s. The Fagradalsfjall dikes were fed through a channel with an inferred cross-sectional area of about ~2-4 m2, passing through the lower crust from a source near the crust-mantle boundary, with a geodetically imaged deflation source at about ~12-13 km depth. Since late 2023, activity has been focused at the Svartsengi system, with 9 diking events and 7 eruptions in 2023-24. Initial diking there occurred on 10-11 November 2023 with inferred peak flow rates of ~7400 m3/s when an ~15 km long dike formed, following magma accumulation near the brittle-ductile boundary at about 4-5 km depth. The inferred cross-sectional area of the limiting part of the channel from the Svartsengi magma domain feeding the zone where dikes have formed in 2023-24 is on the order of ~2000 m2 or about 2-3 orders of magnitude larger than that inferred at Fagradalsfjall. This and the different depth of magma storage in the plumbing systems at Fagradalsfjall and Svartsengi explains their different behaviour in recent years, that are coupled in such a manner that only one of of the systems has been primarily magmatically active at each time since 2020.

This repository contains the data and software used in the article “Characterisation of the plumbing system beneath Askja Caldera, Iceland, revealed by microgravity and deformation data during uplift between 2022 and 2023”, submitted to JGR: Solid Earth. The study investigates the compressibility of the mush zone where new magma was intruded during the 2022–2023 uplift period. To achieve this, we jointly inverted gravity and deformation data to estimate the volume and mass change of the intruded magma, allowing us to infer both the intrusion density and the mush compressibility. We used GNSS time series operated by the Icelandic Meteorological Office (IMO). Microgravity data was taken in August 2022 and August 2023. The microgravity data taken in August 2022 is originally available in (Koymans et al., 2023; https://doi.org/10.4121/912b666b-95c0-4f93-9485-e98256517991.v1), but we have also upload the data here. We also present the LOS velocities after corrections and the decomposed velocities into east-west and near-vertical component. Volume and mass changes of the magma body beneath Askja Caldera were estimated using MATLAB code developed by Nikkhoo and Rivalta (2023). This code was integrated into the GBIS software (Geodetic Bayesian Inversion Software; Bagnardi and Hooper, 2018) to enable joint inversion of deformation and gravity data. The original version of GBIS is available at https://comet.nerc.ac.uk/geodetic-bayesian-inversion-software-gbis/. The modified version of GBIS used in this study is included in this repository.

Ground deformation during an eruption may help to interpret physical processes related to the plumbing system. Here, we report the co-eruptive deformation of the Fagradalsfjall (SW-Iceland) eruption in 2021, that occurred in an oblique rift zone. The eruption lasted six months, 19 March - 21 September, following several weeks of intense seismic activity. The spatial and temporal analysis of Global Navigation Satellite System, GNSS, and Interferometric Synthetic Aperture Radar, InSAR, (by Sentinel-1) observations of ground displacements locate changes in the deformation pattern during the eruptive period. Three temporal changes in the subsidence rates and horizontal motion towards the eruptive area are identified: the first period, T1, 19 March–12 May; the second period, T2, 12 May–30 July, and the third period, T3, 30 July–21 September. The maximum deformation rate (20 mm/yr in line-of-sight) is observed in T2 and coincides with the average effusive rate increase (from 8 m3/s in March–April to 9–13 m3/s in May, Pedersen et al., 2022). We jointly inverted the GNSS and InSAR data to place constraints on the size and location of the source of subsidence during the six-month eruption. Initial modelling result (InSAR and GNSS) indicates a point-source at mid-crust level, 9.5-10.5 km depth and a volume decrease of 18-21 × 106 m3. The deflation volume estimated is significantly lower than that of the lava flow field, with a bulk volume of 150 ± 3 × 106 m3of lava (Pedersen et al., 2022). A residual signal is observed in our model, centered above the source location and around the eruptive center. Both the residual signal and the lower-than-expected volume change suggest additional inflow from a deeper source in agreement with evidence of physical mixing of magma from a mantle supply after the start of the eruption (Halldórsson et al., 2022). A link to a deeper source influences the influx rate of the magma and, consequently, the magma available. Both local seismicity rate and seismic moment release gradually decreases between the onset of the eruption and late April. Afterwards, both show a relatively constant rate, until the end of the eruption. Pedersen, G., Belart, Joaquín M.C., Óskarsson, B., Gudmundsson, M. et al (2022). Volume, Effusion Rate, and Lava Transport During the 2021 Fagradalsfjall Eruption: Results From Near Real‐Time Photogrammetric Monitoring. Geophys. Res. Lett., 49. 10.1029/2021GL097125. Halldórsson, S.A., Marshall, E.W., Caracciolo, A. et al. (2022). Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland. Nature 609, 529–534 https://doi.org/10.1038/s41586-022-04981-x

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.

During plate spreading, large volumes of magma can be extracted from the upper mantle and intrude the crust. Geophysical and geochemical studies at active magmatic rifts and passive margins show that crustal intrusions mainly occur in the form of transient sill-like bodies. The sills pond at various crustal levels, potentially feeding shallower plumbing systems, dike intrusions and surface eruptions. Trans-crustal magma migration and intrusion thus have a key role in controlling extension, strain localization and subsidence during rifting. However, a clear understanding of the mechanisms of sill intrusion, their connection to upper mantle processes, as well as the spatial and temporal response of the sills to a new arrival of magma is still limited by the paucity of direct observations. In this study, we provide one of the few direct InSAR observation of rift-scale deformation caused by magma inflow from the upper mantle to multiple crustal sills in the Central Afar (CA) rift. We used InSAR time-series from 255 ESA Sentinel-1 interferograms during 2014-2021 and combined them with available GNSS measurement to retrieve the 3D velocity field and the temporal evolution of surface deformation in CA. We observed four uplift patterns with rates of ~5 mm/yr, that we inverted using four inflating Okada tensile dislocation sources (sills). Our best-fit model shows four sills elongated in a NW-SE direction, similar to the rift trend, and opening rates ranging between 16 and 44 mm/yr. The sills are located at various crustal depths but mainly in the mid-to-lower crust, following the thinning of the crust imaged seismically in CA. Cross-correlation of time-series also show that the uplift above the four sills starts simultaneously in December 2016 and continue until March 2021. We interpreted the simultaneous inflation of four distant sills as the result of a shared pressurization event caused by an episodic magma inflow from a common source in the upper mantle. Our results show that magma supply from the mantle beneath continental rifts is episodic, and occurs across large spatial scales but short temporal scales over which deep crustal magma ponding takes place. Such process could explain how the thick intruded crust common at magma-rich rifted margins is created and could help in understanding the long-term dynamics of rifting episodes and volcanism.

Magma transport through the Earth's shallow crust can be affected by pre-existing weaknesses like faults. Consequently, fault-channeled magma may reach the surface in unexpected locations. Hence, better understanding of magma-fault interaction is needed to improve hazard assesment. We investigate the effect of host rock cohesion and magma viscosity on intrusion-fault interaction using laboratory experiments. Vegetable oil and glucose syrup, serving as low- and high-viscosity analogue magmas, were injected into intact and faulted granular materials with variable cohesion (mixtures of silica flour and micro-glass beads), serving as a brittle plastic model crust. High-cohesion models produced sheet intrusions, that propagated along fault segments upon intersection. Low-cohesion models produced low-aspect ratio intrusions low width/thickness ratio. Without tectonic stresses, the cohesion strongly controls intrusion-fault interaction, while tested model magma viscosities exerted a weaker control. Our findings show that intrusion-fault interaction is a highly complex process and important to consider at active volcanoes.

The Dallol volcano on the axis of the Erta Ale ridge (Afar rift) offers an ideal opportunity to study the interaction between magmatic and hydrothermal processes. Volcanic activity in Dallol has developed in an area below sea level with a salt plain. Dallol has been actively deforming since InSAR measurements started in the area in 2004. However, the source of deformation under Dallol remains unclear. We present a new InSAR study of Dallol from 2014 to 2023 showing at least three concentric deformation signals of range increase consistent with subsidence with rates ranging 23–43 mm/yr in the satellite Line‐of‐Sight. The main subsidence occurs at Dallol volcano, and two smaller maxima occur at the Black Mountain and the Bubbling Pool areas to the south and southwest of Dallol, respectively. Our modeling indicates that the deformation is caused by contraction of three sill‐shaped sources (Okada tensile dislocations) at depths ranging 0.6–1.5 km, each with a volume contraction in the range 1.8–5.5 × 10 4 m 3 /yr. Time series analysis shows that the subsidence at Dallol volcano and Black Mountain was continuous and linear in time. Furthermore, an integrated observation of InSAR with the geology, resistivity image and seismic reflection of the area suggest that the 1.5 km deep source under Dallol is the cooling and contraction of a magma reservoir. At Black Mountain (1 km deep) and Bubbling Pool (0.6 km deep), the data suggest that subsidence is due to either a pressure decreases in the shallow hydrothermal system and/or salt dissolution.