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The Krafla volcanic system has geothermal areas within the Krafla caldera and at Bjarnarflag in the Krafla fissure swarm, 9-km south of the Krafla caldera. Arrays of boreholes extract geothermal fluids for power plants in both areas. We collected and analyzed InSAR, GPS, and leveling data spanning 1993–2015 in order to investigate crustal deformation in these areas. The volcanic zone hosting the geothermal areas is also subject to large scale regional deformation processes, including plate spreading and deflation of the Krafla volcanic system. These deformation processes have to be taken into account in order to isolate the geothermal deformation signal. Plate spreading produces the largest horizontal displacements, but the regional deformation pattern also suggests readjustment of the Krafla system at depth after the 1975–1984 Krafla rifting episode. Observed deformation can be fit by an inflation source at about 20 km depth north of Krafla and a deflation source at similar depth directly below the Krafla caldera. Deflation signal along the fissure swarm can be reproduced by a 1-km wide sill at 4 km depth closing by 2–4 cm per year. These sources are considered to approximate the combined effects of vertical deformation associated with plate spreading and post-rifting response. Local deformation at the geothermal areas is well resolved in addition to these signals. InSAR shows that deformation at Bjarnarflag is elongated along the direction of the Krafla fissure swarm (∼ 4 km by ∼ 2 km) while it is circular at Krafla (∼ 5 km diameter). Rates of deflation at Krafla and Bjarnarflag geothermal areas have been relatively steady. Average volume decrease of about 6.6 × 10 5 m 3 /yr for Krafla and 3.9 × 10 5 m 3 /yr for Bjanarflag are found at sources located at ∼ 1.5 km depth, when interpreted by a spherical point source of pressure. This volume change represents about 8 × 10 −3 m 3 /ton of the mass of geothermal fluid extracted per year, indicating important renewal of the geothermal reservoir by water flow. [ABSTRACT FROM AUTHOR]

Spatial viscosity variations in the crust can be inferred from geophysical imaging and may be essential for interpreting volcano deformations that show more complex behaviours than what simple uni-viscous models predict. In southern Kyushu, Japan, recent seismic imaging implies the presence of a low viscosity zone (LVZ) beneath Aira Caldera, and geodetic data in and around the caldera require uni-viscous model to have lower and higher viscosities earliest and later, respectively, in the period following the 1914 eruption of Sakurajima Volcano. Here, we use a 3D finite element model comprising an elastic layer underlain by a linear Maxwell viscoelastic layer to examine the influence of a LVZ on ground surface displacement in response to two different deformation source modes, i.e., instantaneous source deflation during a major eruption and subsequent continuous source inflation due to magma recharge. A LVZ is introduced into the viscoelastic layer by gradually reducing its viscosity towards the centre. The behaviour of the LVZ model quantified by comparison with the behaviour of a uniform viscosity (UNV) model reveals that, for a given LVZ structure, an apparent UNV model that best represents the LVZ model displacement in response to the instantaneous deflation has lower viscosity than that in response to gradual inflation, i.e., the rate-controlling viscosities of the LVZ model are those in the inner and outer parts of the LVZ for the instantaneous source deflation and subsequent continuous inflation, respectively. Such LVZ model behaviour, for a LVZ spatial extent comparable with the imaged low velocity anomaly, explains well the geodetic data in and around Aira Caldera at any stage after the 1914 eruption, as the predominant viscoelastic response changes with time from that to the instantaneous deflation to that to the subsequent continuous inflation. This study highlights the need for interdisciplinary investigations that integrate geodetic and geophysical datasets to better understand volcanic unrest. • Effects of a low viscosity zone (LVZ) beneath Aira Caldera are examined. • LVZ model predicts an apparent viscosity increase in post-eruption period. • Geophysically inferred LVZ well explains geodetic data in and around Aira Caldera. [ABSTRACT FROM AUTHOR]

• Inflation in 2004–2006/2011–2013 and deflation in 2013–2016 occurred at Fogo volcano • Deformation is modeled by sources located east of Fogo caldera at 3.2–3.7 km depth • A newly intrusion of magma can explain the 2004–2006 inflation • 2011–2013 inflation can relate to magma chamber or hydrothermal system disturbances Ground deformation at São Miguel Island, Azores, has been mapped with GPS (Global Positioning System) geodetic measurements for understanding volcanic unrest. We use yearly measurements from 52 GPS stations covering the central part of the island, recorded between 2004 and 2016. The results show disturbances in the velocity field, with periods of inflation in 2004–2006 and 2011–2013, and a deflation period in 2013–2016. For each period of deformation, the velocity field is modeled by using sources of pressure change embedded within uniform elastic half-space. Point-pressure, prolate spheroid, and penny-shaped sources are considered. We compare the fits to the data using chi-square statistics and F -tests. The best fit modeled sources are located near the eastern and northeastern rim of Fogo volcano caldera, and to the east of it, at depths in the range of 3.2–3.7 km. Best fit point-pressure source models indicate volume increase in the range of 8.8 - 10.4 × 10 6 m 3 in 2004–2006, 0.7 - 5.4 × 10 6 m 3 in 2011–2013, and volume decrease in the range of 3.1 - 4.4 × 10 6 m 3 in 2013–2016 (95 % confidence intervals). The geodetic results, with multiple sources of deformation located in different areas, show that the 2004–2016 activity at São Miguel Island was complex. The location of the 2004–2006 inflation, 3 km northeast of Fogo caldera, and the lack of subsidence reversal can be explained by the permanence of a newly intruded magma body. The location of the 2011–2013 inflation at the eastern rim of Fogo caldera, and the total reversal to subsidence in 2013–2016 can relate to disturbances in a magma chamber under the Fogo caldera or mixed magma-hydrothermal processes. [ABSTRACT FROM AUTHOR]

Abstract: The Krafla rifting episode, which occurred in North Iceland in 1975–1984, was followed by inflation of a shallow magma chamber until 1989. At that time, gradual subsidence began above the magma chamber and has continued to the present at a declining rate. Pressure decrease in a shallow magma chamber is not the only source of deformation at Krafla, as other deformation processes are driven by exploitation of two geothermal fields, together with plate spreading. In addition, deep-seated magma accumulation appears to take place, with its centre ∼10 km north of the Krafla caldera. The relative strength of these sources has varied with time. New results from a levelling survey and GPS measurements in 2005 allow an updated view on the deformation field. Deformation rates spanning 2000–2005 are the lowest recorded in the 30-year history of geodetic studies at the volcano. The inferred rate of 2000–2005 subsidence related to processes in the shallow magma chamber is less than 0.3 cm/yr whereas it was ∼5 cm/yr in 1989–1992. Currently, the highest rate of subsidence takes place in the Leirbotnar area, within the Krafla caldera, and appears to be a result of geothermal exploitation. [Copyright &y& Elsevier]

Abstract: The currently two most important surveying tools in volcano monitoring are observations of seismicity and surface deformation. Magma migration within the Earth''s crust is frequently associated with seismic activity, often occurring as distinct earthquake swarms. Understanding the exact nature and significance of these swarms is important when evaluating volcanic crisis situations. There seems, however, to be no general correlation between the amount of seismic energy release, and the rate and volume of magma on the move, which may complicate an immediate risk evaluation. It has previously been shown that stressing rate appears as the controlling factor on the occurrence of seismicity during intrusion. However, in this paper we emphasize that this is only true under specific circumstances. Other factors influence the evolution and resulting seismic energy release, and each intrusion scenario needs to be evaluated separately. Three Icelandic cases, where seismic swarms have been confirmed through deformation measurements to be related to magmatic movements, were selected. We show how the relationship between volume change and resulting seismicity varies greatly between cases, and suggest that, in a general approach, the most important factor governing the level of seismic energy release is the background stress state. Tectonic setting, regional stresses and tectonic history are therefore all of great importance when evaluating a magmatically induced seismic crisis. The stressing rate only plays the dominant role when either a) stress in the intruded area is close to the yield strength before intrusion or possibly b) when the intrusion is extraordinarily voluminous and rapid. [Copyright &y& Elsevier]

Abstract: The Askja central volcano in northern Iceland has been continuously subsiding at least since 1983. GPS and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the Askja caldera, without any eruptive activity. We have performed an interferometric analysis of Synthetic Aperture Radar images (InSAR) of the area, utilizing data from the ERS satellites. We observe subsidence of the Askja caldera and its fissure swarm, up to a distance of 25 km from the volcano. We evaluate the geometry of the magma chamber at Askja, from a combined inversion of satellite radar interferograms and GPS measurements. Several models were tested, including a Mogi point source as well as an ellipsoidal source. The use of an ellipsoidal source instead of a Mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure, whereas these parameters are not independently resolved if a Mogi source is used. Two-source models were also considered in order to explain the additional subsidence observed along the Askja fissure swarm. We tested a model using two Mogi sources at different depths, a shallow ellipsoidal cavity with a deeper Mogi source, and then a shallow Mogi source with a deeper elongated ellipsoid, oriented along the fissure swarm. Results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera. Residual subsidence occurs along the Askja fissure swarm suggesting the existence of a deeper source of contraction. We interpret this signal in terms of subsidence of the plate boundary. [Copyright &y& Elsevier]

The Hengill triple junction, SW Iceland, is subjected to both tectonic extension and shear, causing seismicity related to strike-slip and normal faulting. Between 1994 and 1998, the area experienced episodic swarms of enhanced seismicity culminating in a ML=5.1 earthquake on June 4, 1998 and a ML=5 earthquake on November 13, 1998. Geodetic measurements, using Global Positioning System (GPS), leveling and Synthetic Aperture Radar Interferometry (InSAR) detected maximum uplift of 2 cm/yr and expansion between the Hro´mundartindur and Grensdalur volcanic systems. A number of faults in the area generated meter-scale surface breaks. Geographic Information System (GIS) software has been used to integrate structural, field and geophysical data to determine how the crust failed, and to evaluate how much of the recent activity focused on zones of pre-existing weaknesses in the crust. Field data show that most surface effects can be attributed to the June 4, 1998 earthquake and have occurred along or adjacent to old faults. Surface effects consist of open gashes in soil, shattering of lava flows, rockfall along scarps and within old fractures, loosened push-up structures and landslides. Seismicity in 1994–1998 was distributed asymmetrically about the center of uplift, with larger events migrating toward the main fault of the June 4, 1998 earthquake. Surface effects are most extensive in the area of greatest structural complexity, where N- and E-trending structures related to the transform boundary intersect NE-trending structures related to the rift zone. InSAR, GPS, and field observations have been used in an attempt to constrain slip along the trace of the fault that failed on June 4, 1998. Geophysical and field data are consistent with an interpretation of distributed slip along a segmented right-lateral strike-slip fault, with slip decreasing southward along the fault plane. We suggest a right step or right bend between fault segments to explain local deformation near the fault. [Copyright &y& Elsevier]

Temporal variation of ground surface displacement rates at volcanoes may represent changes to the volumetric magma supply rate over time. If crustal rock responds elastically to a magmatic deformation source, the volume change of the source has a simple one-to-one relation to the surface deformation. For viscoelastic crustal rock, however, the correlation is not always straightforward, particularly at volcanoes where crustal viscosity is lowered by a high geothermal gradient. For a continuous inflation of the deformation source, the viscoelastic surface displacement depends on the balance between the uplift due to magma supply and subsidence due to viscoelastic relaxation. In this study, using a 3D finite element model composed of an elastic layer underlain by a linear Maxwell viscoelastic layer with a spatially uniform viscosity, we estimate the temporal variation of volumetric magma supply rate into the upper crust beneath Aira caldera in southern Kyushu, Japan, after the 1914 eruption of Sakurajima volcano. It is found that the geodetic data, including levelling and GNSS displacement fields, require a sill-like magmatic deformation source at a depth of 11 km to inflate at the following rates for six different periods: (I) ~6.9–9.4 × 106 m3/yr in 1914–1934, (II) ~9.1–16.7 × 106 m3/yr in 1934–1960, (III) ~1.6–3.8 × 106 m3/yr in 1960–1968, (IV) ~8.1–11.0 × 106 m3/yr in 1968–1976, (V) ~ −1.0–2.2 × 106 m3/yr in 1976–1997, and (VI) ~5.8–9.4 × 106 m3/yr in 1997–2007. The constrained viscosity (η c) of the viscoelastic crust ranges from ~5 × 1018 Pa s to ~1020 Pa s or more. The lowest and highest values produce the best fit to the data in the earliest period and in period V showing surface subsidence, respectively, but for the other periods any viscosity is acceptable unless it is lower than ~1018 Pa s. The lowest viscosity providing the best fit in the earliest period indicates that viscoelastic displacement in response to the 1914 eruption plays an important role in the surface deformation field. The observed surface subsidence in period V can be explained either by inflation or deflation of the deformation source, depending on whether η c is lower or higher than ~1019 Pa s, respectively. The subsequent surface recovery in period VI requires an increase in the inflation rate for any case of η c. The volume of magma accumulated since the 1914 eruption (ΔV) is predicted to be ~0.6–0.8 km3 in 2020. The estimated inflation/deflation rate of the deformation source, in comparison with the observed volumetric eruption rate, reveals that a significant eruption occurs only when ΔV is ~0.4 km3 or more. However, even if ΔV is beyond the critical value, the eruptive activity does not depend on ΔV. The temporal inflation rate inferred in this study gives an opportunity to discuss eruptive dynamics in relation to the observed eruptive activity, such as a quantitative relationship between magma supply rate into, and discharge rate from, a deformation source. • Variable inflation rate of a deformation source beneath Aira caldera is constrained. • The inflation rate ranges from ~ − 1.0 to ~16.7 × 106 m3/yr since the 1914 eruption. • Crustal viscosity is constrained to be greater than ~5 × 1018 Pa s. • A magma accumulation ΔV since the 1914 eruption is ~0.6–0.8 km3 in 2020. • The eruptive activity is significant only when ΔV ≥ ~0.4 km3. [ABSTRACT FROM AUTHOR]

Iceland, rising to more than 2 km above sea level, originates from the interaction of a mid-oceanic ridge and excessive mantle upwelling. The plate boundary between the North American and Eurasian plates traverses Iceland as a series of seismic and volcanic rift zones. The building blocks of the volcanic zones are about 32 volcanic systems, typically consisting of a central volcano often with a caldera and an associated fissure swarm (one or more). Recent revision of geometry and nature of the volcanic systems include new constraints on the fissure swarms of Western Volcanic Zone, and the one active in 2014–2015 associated to the Bárðarbunga central volcano. Countrywide seismic network, and historical earthquake activity, shows that seismicity is most pronounced in transform zones in south and north Iceland, and at central volcanoes undergoing volcanic unrest. In a two year period 2015–2016, the most seismically active volcanoes were Katla and Bárðarbunga. Measurements of ground deformation, using both GPS geodesy and interferometric analysis of synthetic aperture radar images acquired by satellites (InSAR) reveal well the present day plate spreading process in Iceland. Geodetically measured spreading rate across Iceland is consistent with global plate motion models (MORVEL2010: 18–19 mm/yr in direction N(100–105)°E), accommodated by a single rift zone in north Iceland, overlapping rifts in south Iceland, and a highly oblique rift on the Reykjanes Peninsula. Glacial isostatic adjustment (GIA) in response to ice retreat since 1890 is an additional important processes on a regional scale in Iceland, responsible for rise of central part of Iceland of >30 mm/year. • Geodesy reveals style of crustal strain accumulation in Iceland. • Iceland spreads 18–19 mm/year in direction ~N(100–105)°E. • Plate boundary deformation zones are 50–100 km wide. • Central part of Iceland rises >30 mm/year due to glacial unloading. • Strain build-up across transforms is released in M6–7 earthquakes. [ABSTRACT FROM AUTHOR]

Ground deformation in utilized geothermal areas is often attributed either to pressure decrease or temperature decrease in the geothermal reservoir. A new geothermal power plant at Reykjanes began operation in May 2006 and local deformation caused by geothermal utilization was observed shortly thereafter. We use images acquired by the Envisat and TerraSAR-X (TSX) satellites, between 2003 and 2016, as well as available GNSS data, to derive constraints on the cumulative ground displacement at the Reykjanes geothermal area, Iceland, and compare these results to production data acquired from observation wells in this region. We employ interferometric analysis of synthetic aperture satellite radar images (InSAR), using a combined persistent scatterer and small baseline approach, on both ascending and descending Envisat and TSX satellite tracks covering the 2003–2016 period. Time series of range change along line-of-sight (LOS) from the ground to the satellite show the characteristics of on-going ground deflation in the vicinity of the Reykjanes power plant. In the 2005–2008 period, the main area of deformation was 4 km long by 2.5 km wide, aligned along the Reykjanes fissure swarm, but in the period 2009–2016 it is more circular in shape and ~2 km wide. LOS displacement rates have remained relatively steady in time, although slightly faster in the 2005–2008 period than the 2009–2016 period. The average LOS velocities from ascending and descending tracks are decomposed into estimates of near-vertical and near-east displacements. The inferred maximum subsidence since the start of production is ~260 mm. Horizontal displacements show contraction towards the center of deflation of up to ~140 mm. Geodetic modeling is undertaken using sources of simple geometry within an elastic halfspace to determine the optimal sources for the observed contraction throughout 2005–2016. For the earlier period modeled utilizing ENVISAT interferograms (16 June 2005–16 August 2008) the optimal source is a Yang model with a strike of 58° and a source depth of 2.2 km. The calculated volume change associated with the observed contraction is −2.3 × 106 m3. For the latter period, utilizing TSX interferograms (24 September 2009–17 August 2016), the preferred source is a Mogi-type model at a depth of 1.2 km and the modeled volume change is −1 × 106 m3. • Extended InSAR and GNSS time series at Reykjanes Peninsula reveal variations in rates and spatial extent of deformation induced by geothermal utilization. • Geodetic modeling using sources of simple geometries in a uniform elastic halfspace suggest that the optimal source explaining the observed deformation is an ellipsoid with a northeast strike for the period 2005–2008. • A spherical type source is preferred for the period 2009–2016. • The source has significantly contracted during the observation period. • The data suggest a non-linear correlation between pressure change and displacement rates. [ABSTRACT FROM AUTHOR]

• Basaltic dikes propagating through hyaloclastite interact with pre-existing fractures. • Dikes can propagate along fractures, be deflected or arrested in front of fractures. • Dike orientation and fracture interaction seem to be controlled by tectonic stress. Magma in the Earth's crust is commonly transported through dikes. Fractures and faults, which are common in the shallow crust, form structural weaknesses that can act as energy-efficient propagation pathways. Although examples of this are known from active and extinct volcanoes in varying host rocks, the conditions and mechanisms of how and when dikes are influenced by these structures are not yet fully understood. This study investigates how basaltic dikes propagating through hyaloclastite in the shallow crust interact with pre-existing fractures. Using virtual 3D-models from drone-based photogrammetry, we mapped basaltic dikes exposed in a caldera-filling hyaloclastite in the extinct Dyrfjöll volcano, NE-Iceland, to measure the orientations of fractures and dikes, and quantify their interactions. We observe 39 changes in strike among 45 dikes and found a strong control of the governing stress field on orientations and interactions. Three types of dike-fracture interaction were identified: (1) Dikes propagating along pre-existing fractures. This is most frequently observed for dikes following the tectonic stress field. (2) Dikes with an abrupt change in strike occurring near or at a crosscutting fracture, but without magma flow into the fracture. (3) Dikes arrested at a crosscutting fracture. Such dikes may develop offshoots near the dike tip, which may approach the fracture at different angles and be able to cut across. Understanding how dikes interact with pre-existing fractures in moderately fractured host rock such as hyaloclastite is relevant for hazard assessment and monitoring of volcanically active areas. [ABSTRACT FROM AUTHOR]

After more than 200 years of quiescence, in July 2011 an intense seismic swarm was detected beneath the center of El Hierro Island (Canary Islands), culminating on 10 October 2011 in a submarine eruption, 2 km off the southern coast. Although the eruption officially ended on 5 March 2012, magmatic activity continued in the area. From June 2012 to March 2014, six earthquake swarms, indicative of magmatic intrusions, were detected underneath the island. We have studied these post-eruption intrusive events using GPS and InSAR techniques to characterize the ground surface deformation produced by each of these intrusions, and to determine the optimal source parameters (geometry, location, depth, volume change). Source inversions provide insight into the depth of the intrusions (~ 11–16 km) and the volume change associated with each of them (between 0.02 and 0.13 km 3 ). During this period, > 20 cm of uplift was detected in the central-western part of the island, corresponding to approximately 0.32–0.38 km 3 of magma intruded beneath the volcano. We suggest that these intrusions result from deep magma migrating from the mantle, trapped at the mantle/lower crust discontinuity in the form of sill-like bodies. This study, using joint inversion of GPS and InSAR data in a post-eruption period, provides important insight into the characteristics of the magmatic plumbing system of El Hierro, an oceanic intraplate volcanic island. [ABSTRACT FROM AUTHOR]

Following volcano deflation since 1989, the deformation pattern in Krafla caldera, North Iceland, changed in 2018. Geodetic measurements reveal a difference in surface velocity fields for 2015–2018 and 2018–2020 periods, reflecting a change in the deformation pattern. The difference velocity field broadly fits deformation caused by a spherical pressure source within a uniform elastic half-space, with a volume change of 2.6–3.8 × 105 m3/yr and centre depth of 2.1–2.5 km, which is close to the brittle-ductile boundary in the area, at a depth of 1.8 to 2.2 km. Potential processes causing the deformation change are evaluated: magmatic processes such as magma inflow or accumulation of volcanic gas, changes in the geothermal area because of change in geothermal production, or a combination of these. In particular, we evaluate if the change in deformation may relate to about 0.1 MPa/yr pressure increase in the geothermal system as measurements in monitoring well KG-10 indicate, eventually due to changes in the geothermal exploitation strategy at the Krafla power plant. Modelling shows that inferred volume change may be due to a spherical source with 1.4 km radius with 0.1 MPa/yr pressure change if the surrounding crust has a Young's modulus E of about 7 GPa. However, the average regional Young's modulus for the upper crust in Iceland has been estimated to be 30 GPa. We use the Finite Element Method (FEM) to assess the influence on the displacement due to the presence of a local crustal volume 5 × 5 × 4 km (horizontal dimensions × depth), which envelops the source (within the Krafla caldera), with E = 7 GPa in the central area and 30 GPa in the far field, in a three-dimensional model. Such a model can reproduce significant features of the observed deformation. There are no changes in seismicity in 2018. In late 2019, the earthquake rate increases following a slight decrease over few months. The seismic moment release is relatively steady until the end of 2019, when the rate increases and is relatively constant in 2020. Gravity measurements in late 2019, when compared to limited measurements in 2018, are inconclusive regarding the nature of the deformation, but useful for further monitoring. No noticeable changes have been observed in the chemical composition of fumarole discharge in the Krafla field that relate to new intrusions. • Ground deformation changed in 2018 at Krafla caldera at a similar time as pressure increased in the geothermal system. • Inversion results locate a point-pressure source at 2.1–2.5 km depth, close to the brittle-ductile boundary at Krafla • Changes in the geothermal exploitation strategy at the Krafla power plant occur at the time of deformation and pressure changes • FEM model with a pressure source and intra-caldera lowered Young's modulus can explain the geodetic observations. [ABSTRACT FROM AUTHOR]

Tulu Moye is an actively deforming volcanic complex with a geothermal field in the Main Ethiopian Rift. We use InSAR between 2014 and 2022, integrated with other geophysical data, to investigate the temporal and spatial characteristics of the deformation signal in the area, and to model its source. Velocity maps and time series analysis show a deformation signal consistent with uplift at a velocity of up to 50 mm/yr in the satellite Line-of-Sight (LOS) in 2014–2017, then decreasing to 12 mm/yr until 2022. The centre of deformation is located about 10 km west of a main geothermal drilling site at Tulu Moye, between the Bora, Berecha, and Tulu Moye volcanoes, with a NW-SE elongation direction. Our best-fit model suggests that the deformation is caused by an 8.7 km by 1.2 km sill situated ∼7.7 km below the surface (∼5.9 km below sea-level), elongate in the N54°W direction and dipping S11°W, and experienced an average velocity of volume change of ∼8.9 × 106 m3/yr in 2014–2017. The surface projection of the sill overlaps with local transverse faults and hydrothermal manifestations. The sill is ∼1–2 km below clusters of microseismic swarms and a region of high resistivity, both indicating hydrothermal fluid flow. The location and geometry of the sill correlates with the upper edge of high conductivity interpreted as a zone of partial melt, and we therefore attribute the uplift at the Tulu Moye volcanic complex to inflow of magma in the sill. We also suggest that the transverse caldera rims faults may restrict magma flow, and also facilitate both vertical and lateral hydrothermal fluid flow. • Tulu Moye is an actively deforming volcanic complex in the Main Ethiopian Rift. • InSAR data show deformation signal in Tulu Moye volcanic complex in 2014–2017. • The deformation showed an uplift at a velocity of up to 50 mm/yr. • Okada rectangular dislocation sill model is assumed to model the source. • The modelled source of the deformation showed a magmatic sill situated at 5.9 km. [ABSTRACT FROM AUTHOR]

Abstract: Anisotropy of magnetic susceptibility (AMS) and rock magnetic studies have been made on three outcrops separated by 12km along strike (NNE–SSW) on the Streitishvarf composite dike in east Iceland. Samples for this study have been collected from the inner quartz-porphyry part of the dike, which show clear field evidence of a lateral flow component from north to south at one of the sites. This flow component is consistent with margin AMS results from all three sites. The quartz-porphyry has a substantial bulk magnetic susceptibility (10−2 SI) mainly carried by magnetically soft titanium-poor titanomagnetite (MDF ~15mT). The ferrimagnetic grains yield a characteristic remanent magnetization in all three sites which gives a virtual geomagnetic pole at latitude 52.6° S and longitude 319.6° E. The degree of anisotropy is low (P J =1.033) and the magnetic fabrics shifts from oblate to prolate shapes depending on dike margin and outcrop. The magnetic fabric has been interpreted according to the imbrication model, using the minor susceptibility axis as shear plane indicator. The absolute directions given by the minor susceptibility are then quantified using vector algebra. The magma flow is indicated as an upward directed flow, flowing from north to south with an inclination between 30° and 64°, with a 95% confidence ellipse of 3°–9°. A model for the intrusion of the Streitishvarf dike has been constructed where a magma pocket with felsic magma is punctured by a mafic dike, enabling the felsic magma to rise and extend to the south within the pathway created. The results of this study confirm the applicability of AMS in studies of magma flow directions in igneous dikes of felsic composition. [Copyright &y& Elsevier]

Abstract: Here we review the achievements of volcano geodesy in Iceland during the last 15 years. Extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques, including leveling, electronic distance measurements, campaign and continuous Global Positioning System (GPS) geodesy, and interferometric analysis of synthetic aperture radar images (InSAR). Results from these measurements provide a comprehensive view of the behavior of Icelandic volcanoes. Between inflation, intrusion, and eruption episodes, volcanoes are likely to deflate or show no sign of seismic activity. Subsidence rates are often in the range of a few millimeters to a few centimeters a year, reducing progressively with time since the last eruption or intrusion at the volcano. Subsidence can be caused by cooling and contraction of magma, outflow of magma, it can be related to plate spreading. Volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near-surface locations. Such magma recharge has been observed geodetically at Hengill, Hekla, Eyjafjallajökull, Katla, Grímsvötn, and Krafla volcanoes, with inflow inferred to last from a few months up to two decades. In the last 15 years, five volcanic eruptions, three intrusive events and two >M6 earthquakes have occurred. In recent years, the Grímsvötn and Katla volcanoes have exhibited continuous inflation of a few centimeters per year, which at Grímsvötn culminated in an eruption on 1 November 2004. Hekla and Torfajökull volcanoes have inflated at rates an order-of-magnitude less. Subsidence is occurring presently at the Askja and Krafla volcanoes. Within the period of geodetic measurement, signals consistent with no deformation are typical for most of the 35 active volcanoes in Iceland. [Copyright &y& Elsevier]

Abstract: Askja caldera in northeast Iceland has been in a state of unrest for decades. Ground-deformation surveys show that the rate of deformation, i.e., deflation, is much higher then observed at any other dormant volcano in Iceland. This work presents the results from microgravity and deformation studies at Askja from 1988 to 2003. The deflation reaches a maximum of −0.46 m in the centre of the caldera, relative to a station outside the caldera, during the study period. The source of deformation is inferred to be at ∼3 km depth and a recent study infers a second deeper source at ∼16 km depth. The deflation is consistent with a subsurface volume change of −0.018 km3. We find a net microgravity decrease of 115 μGal in the centre of the caldera relative to the same station. This corresponds to a subsurface mass decrease of 1.6×1011 kg between 1988 and 2003 based on the use of a point source model. A combination of magma drainage and cooling and contraction of the shallow magma reservoir at 3 km depth is our favoured model, consistent with the integrated observations. We suggest that extensional tectonic forces generate space in the ductile part of the crust to accommodate ongoing magma drainage from the shallow magma chamber. [Copyright &y& Elsevier]

Geodetic modelling has become an established procedure to interpret the dynamics of active volcanic plumbing systems and magma transfer within the crust. Most established geodetic models implemented for inverting geodetic data share similar physical assumptions: (1) the Earth's crust is modelled as an infinite, homogeneous elastic half-space with a flat surface, (2) there is no anisotropic horizontal stress to simulate tectonic stresses, (3) the source boundary conditions are kinematic, i.e. they account for an instantaneous inflation or deflation of the source. Field and geophysical observations, however, provide evidence that significant inelastic shear deformation of the host rock can accommodate the propagation of dykes and sills. We show that inelastic processes accommodating the emplacement of dykes in the brittle crust have large implications for dyke-induced surface deformation patterns. We present two quantitative laboratory experiments of dyke emplacement, during which the syn -emplacement surface deformation is monitored. In one experiment, the host material is elastic gelatine, whereas in the other experiment the host material is cohesive Coulomb, plastic silica flour. Even if both experiments produce sub-vertical dykes of similar shapes, their emplacement mechanisms and their associated surface deformation strongly differ. In the gelatine experiment, the dyke propagates as a tensile fracture in a dominantly elastic host, and the surface deformation exhibits two uplifting bulges separated by a trough parallel to, and above the apex of, the underlying dyke. Conversely, in the silica flour experiment, the dyke propagates as viscous indenter through a dominantly plastic host, and the surface deformation exhibits a single uplifting area that narrows through time. The comparison of our experiments shows that (1) plastic deformation (e.g. , shear failure, compaction) of the host has large effects on dyke-induced surface deformation patterns and needs to be considered in geodetic models, and (2) dyke emplacement mechanisms matter in geodetic modelling, strongly suggesting that commonly used kinematic geodetic models such as the opening rectangular dislocation model (Okada 1985) are limited for revealing the physics and dynamics of volcano plumbing systems. Finally, our silica flour experiment shows that pure uplift geodetic signals can result from the emplacement of a dyke emplaced as viscous indenter, whereas such signals are commonly modelled using geodetic models of inflating spherical/elliptical or horizontal planar source. Our experiments call for the design of new geodetic models that account, even partly, for the plastic deformation component of the Earth's brittle crust. • Laboratory experiments of dyke intrusion and dyke-induced surface deformation • Experiments simulate two distinct dyke emplacement mechanisms • Each emplacement mechanism triggers drastically distinct surface deformation • Dyke emplacement can trigger uplift only, contradicting theoretical predictions • Geodetic models need to include inelastic behaviour of the Earth's crust [ABSTRACT FROM AUTHOR]

Ascent of magma through Earth's crust is normally associated with, among other effects, ground deformation and gravity changes. Geodesy is thus a valuable tool for monitoring and hazards assessment during volcanic unrest, and it provides valuable data for exploring the geometry and volume of magma plumbing systems. Recent decades have seen an explosion in the quality and quantity of volcano geodetic data. New datasets (some made possible by regional and global scientific initiatives), as well as new analysis methods and modeling practices, have resulted in important changes to our understanding of the geodetic characteristics of active volcanism and magmatic processes, from the scale of individual eruptive vents to global compilations of volcano deformation. Here, we describe some of the recent developments in volcano geodesy, both in terms of data and interpretive tools, and discuss the role of international initiatives in meeting future challenges for the field. [ABSTRACT FROM AUTHOR]

Abstract: Hekla is one of Iceland's most active volcanoes; its eruptions, characterized by surface fissuring and repeated lava flows during its post-glacial activity, have built up an 800m high elongated mountain. Since 1970 it has erupted every ~10years; the previous repose interval averaged ~60years. For the last eruption in 2000 we constrain the magma geometry by using a wide variety of deformation data: campaign GPS; an InSAR interferogram; dry tilt data, and borehole strain data. The dike that causes surface fissuring extends no more than ~0.5km in depth, and the reservoir depth is ~10km. These are connected by a conduit of small lateral extent. Data for previous eruptions are consistent with this model. We propose that the marked change in eruption interval is because this conduit remains liquid during the short interval between recent eruptions; only a small pressure increase is required to rupture the thin crustal seal. Such a state is consistent with precursory seismicity being confined to very shallow depths and may be applicable to other volcanoes that undergo abrupt changes in eruption interval. [Copyright &y& Elsevier]

Abstract: Many calderas in the world show long-term unrest in the form of elevated rates of deformation and seismicity, related to pressure changes and magma movements within their magmatic plumbing systems. We present new observations of the style of deformation at the Askja caldera, Iceland, since 2000, using interferometric analysis of synthetic aperture radar images (InSAR) acquired by the Canadian RADARSAT-2 satellite. When combined with previously acquired detailed geodetic observations, by various techniques, we obtain an overview of Askja''s behaviour through more than four decades. The combined dataset reveals that, during this non-eruptive period, Askja continuously subsides at a rate of 2.5–3cm/yr in 2000–2009, somewhat lower than the ~5cm/yr rate inferred for the 1983–1998 period. This behaviour of Askja is evaluated and compared to those of other restless calderas. The wrapped interferograms show three main features: (1) concentric fringes depicting subsidence in the centre of the main Askja caldera, (2) oval fringes elongated along the rift portraying subsidence and (3) subsidence in an area north of the Vatnajökull glacier. The average line-of-sight (LOS) velocity from ground to satellite was used as input for inverse modelling, of a deflating pressure source beneath the caldera, embedded in a homogeneous, elastic half-space. Two different source geometries were tested: a point pressure source and a horizontal penny shaped crack. The concentric fringes centred in the Askja caldera are best fit by a point source located at 65.05°N 16.78°W, at a depth of 3.2–3.8km with a volume decrease of 0.0012–0.0017km3/yr from 2000 to 2009, consistent with previous studies. Provisional 2D FEM models including structural complexities in the crustal layers indicate that the tectonic setting of Askja plays an important role in the continuous, long-term high subsidence rates observed there. In order to fully understand the cause and effects of the complicated tectonic setting we encourage the use of a more realistic rheological model of the area, which could lead to reinterpretation of previous model results. [Copyright &y& Elsevier]

One of the World's premier field geologists, Kristján Sæmundsson led immense geological mapping programs and authored or co-authored nearly all geological maps of Iceland during the past half century, including the first modern bedrock and tectonic maps of the whole country. These monumental achievements collectively yield the most inclusive view of an extensional plate boundary anywhere on Earth. When Kristján began his work in 1961, the relation of Iceland to sea-floor spreading was not clear, and plate tectonics had not yet been invented. Kristján resolved key obstacles by demonstrating that the active rifting zones in Iceland had shifted over time and were linked by complex transforms to the mid-ocean spreading ridge, thus making the concept of sea-floor spreading in Iceland acceptable to those previously skeptical. Further, his insights and vast geological and tectonic knowledge on both high- and low-temperature geothermal areas in Iceland yielded a major increase in knowledge of geothermal systems, and probably no one has contributed more than he to Icelandic energy development. Kristján's legacy is comprised by his numerous superb maps on a variety of scales, the high quality papers he produced, the impactful ideas generated that were internationally diffused, and the generations of colleagues and younger people he inspired, mentored, or otherwise positively influenced with his knowledge and generous attitude. • Kristján Sæmundsson led immense geological mapping programs for Iceland during the past half century. • He was a pioneer in relating Iceland to sea-floor spreading and plate tectonics. • his insights yielded a major increase in knowledge of geothermal systems and contributed hugely to Icelandic energy development. [ABSTRACT FROM AUTHOR]