SW Iceland Research Articles

Stacking of Distributed Dynamic Strain Reveals Link Between Seismic Velocity Changes and the 2020 Unrest in Reykjanes

AbstractIn this study, we measure seismic velocity variations during two cycles of crustal inflation and deflation in 2020 on the Reykjanes peninsula (SW Iceland) by applying coda wave interferometry to ambient noise recorded by distributed dynamic strain sensing (also called DAS). We present a new workflow based on spatial stacking of raw data prior to cross‐correlation which substantially improves the spatial coherency and the time resolution of measurements. Using this approach, a strong correlation between velocity changes and ground deformation (in the vertical and horizontal direction) is revealed. Our findings may be related to the infiltration of volcanic fluids at shallow depths, even though the concurrent presence of various processes complicates the reliable attribution of observations to specific geological phenomena. Our work demonstrates how the spatial resolution of DAS can be exploited to enhance existing methodologies and overcome limitations inherent in conventional seismological data sets.

Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part II

Changes from dyke to sill propagation in the shallow crust are often caused by dissimilar layer properties. However, most previous studies have not considered the influence of glacial loading and unloading on dyke and sill deflection processes. Here, we attempt to collectively explore mechanical (layer stiffness) and geometrical (dyke dip, layer thickness) realistic parameters subject to two different magma overpressure values (namely 5 MPa and 10 MPa) that promote dyke-sill transitions in both non-glacial and glacial settings. To do this, we use as a field example, the Stardalur laccolith: a multiple stacked-sill intrusion located in SW Iceland. The laccolith lies near the retreating Langjökull glacier and was emplaced at the contact between a stiff lava layer and a soft hyaloclastite layer. We initially model two different stratigraphic crustal segments (stratigraphy a and b) and perform sensitivity analyses to investigate the likely contact opening due to the Cook-Gordon debonding and delamination mechanism under different loading conditions: magma overpressure, regional horizontal extension, glacial vertical load and a thin elastic layer at the stratigraphic contact. Our results show that contact opening (delamination) occurs in both non-glacial and glacial settings when the dissimilar mechanical contact is weak (low shear and tensile stress, zero tensile strength). In non-glacial settings, stiff layers (e.g., lavas) concentrate more tensile stress than soft layers (e.g., hyaloclastites/breccia) but accommodate less total (x–y) displacement than the surrounding host rock (e.g., soft hyaloclastites) in the vicinity of a dyke tip. Yet, a thicker hyaloclastite layer in the stratigraphy, subject to higher magma overpressure (Po = 10 MPa), may encourage dyke-sill transitions. Instead, in glacial domains, the stress conditions imposed by the variable vertical pressure of the ice cap result in higher tensile stress accumulation and displacement in stiff layers which they primarily control sill emplacement.

Lithogeochemistry of the Nesjavellir geothermal field, SW Iceland—Implications for elemental mass movement in active geothermal systems

The lithogeochemistry of altered basalts was investigated to assess elemental mass changes in the active meteoric water dominated geothermal system at Nesjavellir, SW Iceland. The study revealed that elemental mass changes upon hydrothermal alteration is rather limited and the extent of metasomatism is controlled by the type of alteration minerals. Large mass movement was detected in volatiles (C and S), likely derived from magma degassing. The geothermal system at Nesjavellir may thus provide an ideal locality to investigate the natural sequestration of deep-sourced volatiles and deliver insights in quantifying the input of mantle degassing on the budget of geothermal systems.

Widespread fracture movements during a volcano-tectonic unrest: the Reykjanes Peninsula, Iceland, from 2019–2021 TerraSAR-X interferometry

Tectonic controls on dyke emplacements, eruption dynamics and locations have been observed in multiple volcanic areas worldwide. Mapping of active structures is therefore key for assessing potential tectonic and volcanic hazards in active regions. We used wrapped interferograms from the TerraSAR-X satellite to map active fracture movements over a 2-year period of a volcano-tectonic unrest at the onshore Reykjanes Peninsula plate boundary in SW Iceland. As of 1 December 2023, the unrest has included at least six inflation events and five dyke injections resulting in three eruptions of the Fagradalsfjall volcanic segment. In addition to the deformation associated with the 2019–2021 inflation events and intrusions, the interferograms reveal fracture movements over a wide area surrounding the active plate boundary segment. This first-order mapping of active fractures complements previously mapped structures, as InSAR allows for the detection of subtle ground movements, even in areas where young lava flows cover older structures. Our fracture data therefore fill in some of the apparent voids in previous fracture and fault maps of SW Iceland. Furthermore, our investigation reveals aseismic movement on previously unknown fractures directly beneath the town of Grindavík, as well as a N45∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ $$\\end{document} E striking fracture co-located with the longest lasting volcanic vent of the subsequent 2021 eruption. The mapping method we present in this study is relevant for active volcano-tectonic regions where InSAR can be applied to detect small-scale fracture movements to advance understanding of ongoing unrest and volcano-tectonic hazards.

Authigenic mineralization in Surtsey basaltic tuff deposits at 50 years after eruption

Alteration of basaltic glass and in situ mineral growth are fundamental processes that influence the chemical and material properties of Earth’s oceanic crust. These processes have evolved at the basaltic island of Surtsey (SW Iceland) since eruptions terminated in 1967. Here, subaerial and submarine lapilli tuff samples from a 192 m-deep borehole drilled in 2017 (SE-02b) are characterized through petrographic studies, X-ray powder diffraction analyses, and SEM–EDS imaging and chemical analyses. The integrated results reveal (i) multi-stage palagonitization processes in basaltic glass and precipitation of secondary minerals from matrix pore fluids, (ii) multi-stage crystallization of secondary phillipsite, analcime and Al-tobermorite in the vesicles of basaltic pyroclasts and (iii) variations in palagonitization processes as a function of thermal and hydrological domains. Although temperature appears to be an important factor in controlling rates of secondary mineralization, the chemistry of original basaltic components and interstitial fluids also influences reaction pathways in the young pyroclastic deposits. The integration of systematic mineralogical analyses of the 50-year-old tuff from one of the most carefully monitored volcanic sites on Earth, together with temperature monitoring in boreholes since 1980, provide a reference framework for evaluating mineralogical evolution in other Surtseyan-type volcanoes worldwide.

Transdimensional ambient-noise surface wave tomography of the Reykjanes Peninsula, SW Iceland

SUMMARY Ambient noise seismic tomography has proven to be an effective tool for subsurface imaging, particularly in volcanic regions such as the Reykjanes Peninsula (RP), SW Iceland, where ambient seismic noise is ideal with isotropic illumination. The primary purpose of this study is to obtain a reliable shear wave velocity model of the RP, to get a better understanding of the subsurface structure of the RP and how it relates to other geoscientific results. This is the first tomographic model of the RP which is based on both on- and off-shore seismic stations. We use the ambient seismic noise data and apply a novel algorithm called one-step 3-D transdimensional tomography. The main geological structures in the study area (i.e. covered by seismic stations) are the four NE–SW trending volcanic systems, orientated highly oblique to the plate spreading on the RP. These are from west to east; Reykjanes, Eldvörp-Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. Using surface waves retrieved from ambient noise recordings, we recovered a 3-D model of shear wave velocity. We observe low-velocity anomalies below these known high-temperature fields. The observed low-velocity anomalies below Reykjanes and Eldvörp-Svartsengi are significant but relatively small. The low-velocity anomaly observed below Krýsuvík is both larger and stronger, oriented near-perpendicular to the volcanic system, and coinciding well with a previously found low-resistivity anomaly. A low-velocity anomaly in the depth range of 5–8 km extends horizontally along the whole RP, but below the high-temperature fields, the onset of the velocity decrease is shallower, at around 3 km depth. This is in good agreement with the brittle–ductile transition zone on the RP. In considerably greater detail, our results confirm previous tomographic models obtained in the area. This study demonstrates the potential of the entirely data-driven, one-step 3-D transdimensional ambient noise tomography as a routine tomography tool and a complementary seismological tool for geothermal exploration, providing an enhanced understanding of the upper crustal structure of the RP.

Surface fractures generated during the 2021 Reykjanes oblique rifting event (SW Iceland)

We use a comprehensive dataset of field observations, high spatial resolution drone orthomosaics and digital terrain models (DTMs) to map, quantify and characterize the extensive ground fracturing related to the 2021 seismo-tectonic and volcanic activity in the Reykjanes Peninsula (Iceland). The dataset, spans an area of about 30 km^2, where we map nearly 20 000 ground cracks with metric to decametric lengths and centimetric extensional offsets, revealing a dominant dextral shear, in agreement with published seismic data. Although striking in a direction similar to the volcanic systems in the Reykjanes Peninsula (N030–040), most fractures appear as en-échelon structures globally aligned along N-S-striking fault zones up to 3–4 km long. By examining the timing of ground fracturing through repeated field observations, seismic data and InSAR images, we associate a fracture zone with most earthquakes of M_omega ge 5.0 that occurred in the month preceding the March 2021 Fagradalsfjall eruption. We describe three preexisting N-S fault zones, with fault segments that were reactivated up to three times during the pre-eruptive seismic activity, while the magma intrusion did not trigger graben-related ground fractures typically observed during magmatic injections. Our depiction of a system dominated by strike-slip tectonic features helps in understanding the geometry and bookshelf-mode of tectonic activity along a diffuse and highly oblique extensional plate boundary. Evidence of transient fracturing is typically quickly lost because of erosion or lava flow burial, highlighting a potential under-representation of diffuse fracturing when studying old tectonic and volcanic systems.

Feeders vs arrested dikes: A case study from the Younger Stampar eruption in Iceland

Understanding the mechanical conditions for dike arrest and associated surface deformation or, alternatively, dike propagation to the surface to supply magma to an eruption, is of fundamental importance for volcanology in general and for volcanic hazards in particular. Here we present the results of a study of an outcrop located in the Reykjanes Peninsula, SW Iceland, where one dike became arrested only 5 m below the surface of an active volcanic system, without inducing any brittle deformation at the surface. In the same outcrop, at a distance of 30 m, a feeder dike is exposed. Both dikes are associated with the Younger Stampar eruption (1210–1240 CE). We reconstructed a high-resolution 3D model, through drone surveys and Structure from Motion (SfM) techniques, on which we collected detailed structural data combined with field surveys. These data, integrated with petrographic and geochemical analyses, became inputs to Finite Element Method (FEM) numerical models, made using the COMSOL Multiphysics® software. Our results indicate that compression exerted by the intrusion of the feeder dike (inferred to have been emplaced first) can explain the arrest of the second dike and the absence of induced brittle deformation even if the dike tip is only 5 m below the surface. Furthermore, the contrasting mechanical properties of the layers that constitute the outcrop, with alternating stiff lavas and compliant tuffs, raise (concentrate) the compressive stresses in the lava flows ahead of the second dike, thereby encouraging its arrest. Both the dikes are basaltic, but the earlier emplaced feeder dike is crystal poor and slightly more evolved than the later emplaced arrested dike. The results throw a new light on the conditions for dike arrest and (the lack of) dike-induced brittle deformation at very shallow depths in active rift zones, with important implications for volcanic-hazard assessments.

Magma plumbing architectures and timescales of magmatic processes during historical magmatism on the Reykjanes Peninsula, Iceland

In the past 4000 years, three rifting episodes have occurred on the Reykjanes Peninsula (RP, SW Iceland) at time intervals of about 800-1000 years, with the activation of four of the five volcanic systems on the peninsula: Reykjanes, Svartsengi, Krýsuvík and Brennisteinsfjöll. In 2021 AD, 2022 AD and 2023 AD, three eruptions occurred in the Fagradalsfjall volcanic system, the only volcanic system that remained inactive during these past rifting episodes. Considering the eruptive history of the peninsula, this could mark the onset of a new eruptive episode. Although extensive petrochemical datasets have emerged from past rifting episodes, little is known about magma plumbing configuration and timescales of pre-eruptive magmatic processes. In this work, we focus on 16 basaltic lava units erupted across RP during a well-established rifting episode: the 800-1240 AD Fires. We analysed major and minor element contents of glasses, mineral phases and melt inclusions, and we reconstruct plumbing system architecture and constrain mixing-to-eruption timescales, with the goal of assessing likely eruption scenarios of a possible future eruptive episode. Mineral-melt equilibria suggests that most of crystal cores are antecrysts that did not crystallize from the magmas that carried them to the surface, but instead were scavenged from disaggregating mush layers genetically related to the erupted magmas. Magmas erupted in the easternmost part of the RP reveal larger contributions from geochemically enriched mantle-derived melts. However, MIs that record various extents of mixing of geochemically enriched and depleted components throughout the entire peninsula appear devoid of any spatial control of the underlying mantle. Independent clinopyroxene-melt and melt-based barometry, after correction for microlite crystallization, show consistent results and suggest that magmas from the western part of the peninsula were extracted from reservoirs located at about 7-10 km depth. In contrast, and similar to observations from recent eruptions at Fagradalsfjall, the easternmost system, Brennisteinsfjöll, was fed from deeper crustal reservoirs, at about 14-21 km depth. Fe-Mg diffusion chronometry applied to olivine rims suggests short mixing-to-eruption timescales on the order of days to weeks and likely faster ascent rates of magmas underneath Brennisteinsfjöll. Compared to 2021 AD Fagradalsfjall, mixing-to-eruption timescales were considerably shorter during the 800-1240 AD Fires. With inferred eruption durations on the order of months, the short olivine rim diffusion records are consistent with several discrete eruptive phases, which eventually assembled the individual magmatic units. In the light of possible future eruptive episodes on the RP, our work suggests that magmas are likely to accumulate at persistent depths across the RP, with key implication for interpreting pre-eruptive signals and eruption mitigation.

Tectonic stress changes related to plate spreading prior to the 2021 Fagradalsfjall eruption in SW Iceland

The Reykjanes Peninsula in SW Iceland is a part of the Mid-Atlantic plate boundary. It forms its transtensional segment with several volcanic and faulting systems. We focus on the 2017 seismicity that occurred in the central part of Reykjanes at the place of the Fagradalsfjall volcano prior to its eruption on March 19, 2021. We invert well-determined focal mechanisms of the 2017 seismicity and provide mapping of tectonic stress in space and time. Our results disclose heterogeneous stress field manifested by mix of shear, tensile and compressive fracturing. The prominent stress direction was in the azimuth of 120° ± 8°, which represents the overall extension related to rifting in the Reykjanes Peninsula. The activity initiated on the transform fault segment with predominantly shear strike-slip events. The non-shear fractures occurred later being associated with normal dip-slips and corresponding to the opening of volcanic fissures trending in the azimuth of 30–35°, perpendicular to the extension. The dip-slips were mainly located above an aseismic dike detected in the centre of the 2017 swarm. This dike represents a zone of crustal weakening during a preparatory phase of the future 2021 Fagradalsfjall volcanic eruption located at the same place. Moreover, we detected local variation of stress when the stress axes abruptly interchanged their directions in the individual stress domains. These stress changes are interpreted in a consequence of plate spreading and upcoming fluid flow during a preparatory phase of a rifting episode.

Alkenone Uk’37 index differs between thermally separated populations of fin whales and krill

Although habitat temperature is an important determinant of species distributions, its measurement is difficult in mobile marine organisms. Chemical tracers such as alkenones constitute an alternative tool for gauging marine habitat temperature, something particularly relevant in the current scenario of global warming. The unsaturation index of alkenones, the Uk’37 index, can be used to estimate sea surface temperature (SST) in living organisms. Here, we analyse alkenone concentrations in a predator species, the fin whale, and its prey, the northern krill, both sampled in 2 areas with different SSTs: SW Iceland and NW Spain. In NW Spain (but not in SW Iceland), alkenone concentrations were higher in krill than in fin whale blubber, suggesting that they may biodilute along the trophic transfer. Consistently with local SST, the Uk’37 index was lower in the Icelandic samples of both fin whales and krill than in those from NW Spain, confirming the ability of this index to distinguish populations that are thermally separated. Whales are migratory animals; thus, the alkenones present in their blubber are a mix of those ingested locally and in recently visited regions. Consistently, the Uk’37 index showed a higher correlation with the local SST in krill than in fin whales. Alkenone turnover rates should be considered when using this proxy to assess the thermal range of the feeding habitat of a species. We conclude that the Uk’37 index is a potentially efficient tool to study the behaviour of whales, thus providing an alternative perspective to other chemical tracers.

Cliff-top boulder morphodynamics on the high–energy volcanic rocky coast of the Reykjanes Peninsula (SW Iceland)

Cliff-top boulder deposits (CBDs) are morphological indicators of high-energy conditions. Since 2014, a monitoring of CBDs dynamics has been undertaken on the south-western coast of Iceland (Reykjanes Peninsula) to monitor their long-term activation (quarrying, transport and deposition) as a proxy of the inter-annual winter storminess variations and basaltic cliff erosion processes in a context of rocky coast progradation. Annual topomorphological surveys of four study sites were conducted and Structure-from-Motion photogrammetry was performed to quantify CBDs displacements. Hydrodynamic conditions were analyzed based on offshore waves and water level. Results show that CBDs activation occurs every winter, regardless of the variability of hydrodynamic conditions. Depending on the site and the year, >2% and 17% of the CBDs accumulated above 8 m to 10 m asl at the top of the cliffs are regularly mobilized. While inland movements represent the main mode of transport of blocks (between 50% to 60%), seaward and longshore movements are also well represented (10% to 20%). Longshore displacement is favored by the wide tabular morpho-structural setting of the wave-scour cliff-top platforms, which is explained by the structure of pāhoehoe lava flows. The activation of CBDs –measured from the volumes of displaced boulders–, shows a good correspondence with the frequency and duration of storms. However, as was the case during the winter of 2018–2019, it was rather the intensity of two highly morphogenic episodes combining storm waves and especially very high spring tide water levels, that generated the largest boulders displacements. Substantial interannual activation of the CBDs confirms that they constitute an important and still understudied proxy of the morphogenic system of high-energy rocky coasts, whose the dynamic in terms of carrying, transport, and deposition, could significantly increase with rising sea level.

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

We image shallow crustal structures and analyze seismicity patterns in the Hengill high-enthalpy geothermal area in SW Iceland, exploiting a temporary densification of the seismic network 2018 to 2020. Using a subset of 6,300 high-quality manually picked P- and S-phases, we compute a minimum 1-D model for the region. Our results suggest that the most consistent and accurate hypocenter locations are derived from a joint inversion of P and S arrival times for the Hengill area. We demonstrate that this minimum 1-D model in combination with SeisComP detection and location algorithms can be used to produce fully-automated yet high-quality earthquake catalogs. Our analysis established that both the induced and natural seismicity in the Hengill area occurs in several distinct, spatially constrained clusters. In production and injection areas, the depth of the clusters is at about 2 km, near the bottom of the production and injection wells. These are most likely triggered by the injection and induced by the production, respectively. Outside of these clusters, the seismicity is generally deeper, with the depth of the deepest seismicity indicating the brittle-ductile transition zone. This zone is encountered at about 4 km near the center of the Hengill volcanic area and deepens with increasing distance from its volcanic center, to about 7 km in the southernmost region. A spatial analysis of b-values shows slightly increased values in areas with numerous injection wells and slightly decreased values in production areas. Three-dimensional crustal imaging of VP, VS, VP/VSshows a SE-NW trending fast velocity that extends, at 1–3 km depth between the extinct Grensdalur volcanic center and the presently active Hengill volcanic center. The fastest velocities are found in the NW corner of the Grensdalur volcanic center coinciding with a gravity high and probably reflecting dense solidified magmatic intrusion(s). This trend coincides with traces of geothermal surface manifestations, a shallow lying low resistivity anomaly and an aero-magnetic low. All these anomalies are caused by high temperature at some point in the geological history of the area and are most likely due to migration of the crustal accretion and volcanic activity between the two volcanic centers. Below-average VP/VSratios at similar depth, coincide with the main production field. We suggest that this anomaly is caused by the extensive fluid extraction, which lowers the pore-pressure in the field and consequently increases the steam dominated zone, leading to lower Vp/Vs ratios. Most of the earthquakes are within the Vp/Vs low and at the boundary of the high and low Vp/Vs anomalies, which might indicate a region of good permeability.

The behaviour of metals in deep fluids of NE Iceland

In this contribution, we present some of the first data on the elemental signature of deep crustal fluids in a basalt-hosted, low-chloride magmatic-hydrothermal system. Down-hole fluid samples (850–1600 m) from wells in the Theistareykir and Krafla geothermal fields in the Northern Volcanic Zone of Iceland were combined with well-head samples of condensed vapor, cuttings of altered rock, and fresh basalt (being some of the first concentration data for volatile and semi-volatile elements (Sb, Tl, Bi, Cd and As) for this area of Iceland). Results show that the deep fluids are relatively enriched in base metals and (semi)-volatile metals (in particular Te, Hg, Re and Tl) compared to local basalt. We interpret this enrichment in volatile metals to reflect a significant element input from magma degassing. Boiling of this deep fluid results in a well-head fluid composition that is significantly depleted in most elements. This well-head fluid has a distinct elemental signature, including a depletion in Sb that is mirrored in the altered rocks, and a depletion in the base metals that shows their selective sequestration in scale minerals, likely sulphides. As expected, the element content and patterns in surface fluids can thus not be interpreted to directly reflect that of the deep reservoir fluid. The behaviour of elements in Theistareykir and Krafla fluids is consistent, and largely agrees with similar data obtained for the Reykjanes geothermal system in SW Iceland. We therefore posit that our results are representative for this geological setting and indicate a significant magmatic degassing cation input to deep fluids, variably modified by water–rock interaction.

Intertidal Ostracoda from Fossvogur and Kópavogur bays (SW Iceland): Diversity and distribution

Ostracods have been used in coastal monitoring as indicators of recent and past environmental changes due to their relatively high diversity, abundance and sensitivity. To assess the influence of salinity and potential human impacts on their diversity and distribution in two neighbouring bays in a growing urban area in SW Iceland, a set of 28 surface-sediment samples was collected at low tide in Fossvogur and Kópavogur. This first systematic record of intertidal ostracods in Iceland yielded a total of 16 taxa belonging to 10 families common to the Arctic and Temperate Northern Atlantic marine realms. The assemblages are dominated by Leptocythere castanea, Hemicythere villosa, Semicytherura nigrescens, Leptocythere lacertosa and Hirschmannia viridis. In general, lower diversity and abundance were recorded at engineered rocky shores as a result of coarser sediment at these locations, whilst L. lacertosa and Semicytherura sella show a marked preference for finer-grained substrates associated with more natural shores. Although inflows with lower salinity were not identified as a controlling factor of diversity and abundance in general, 85% of specimens of S. sella were found at locations influenced by inflows. Higher overall diversity was recorded at locations less exposed to the open sea where wave energy and currents are probably less significant. This study represents a missing reference for the spatial and temporal ostracod distribution in coastal ecosystems of Iceland in times of global change.

Tectonic Stress Changes Related to Plate Spreading During a Preparatory Phase of the 2021 Fagradalsfjall Eruption in Sw Iceland

Tectonic Stress Changes Related to Plate Spreading During a Preparatory Phase of the 2021 Fagradalsfjall Eruption in Sw Iceland

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

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.

Imaging high-temperature geothermal reservoirs with ambient seismic noise tomography, a case study of the Hengill geothermal field, SW Iceland

The Hengill volcanic system is located in the Reykjanes Peninsula (Iceland) and hosts two large geothermal power plants: Nesjavellir and Hellisheiði. This area is the next target for the Iceland Deep Drilling Project (IDDP) to search for super-critical fluids. Here, we investigate the potential for ambient seismic noise tomography to be used as a geothermal exploration tool. The results are compared with different geophysical observables, of which resistivity modeling shows an excellent correlation with our results. We also observe a prominent seismic velocity reduction to the south of Hverahlíð, where the most powerful boreholes in the region are located.

Seismic Anisotropy in the Rift of the Reykjanes Peninsula, SW Iceland, Calculated Using a New Tomographic Method

Seismic Anisotropy in the Rift of the Reykjanes Peninsula, SW Iceland, Calculated Using a New Tomographic Method

Non-double-couple earthquakes in 2017 swarm in Reykjanes Peninsula, SW Iceland: Sensitive indicator of volcano-tectonic movements at slow-spreading rift

The analysis of the 2017 earthquake swarm along the obliquely divergent Reykjanes Peninsula plate boundary revealed the most frequent focal mechanisms corresponding to main activated fault, which relates to transform faulting of the North Atlantic Rift in Iceland. Detailed double-difference locations, focal mechanisms and non-double-couple (non-DC) volumetric components of seismic moment tensors indicate an activation of three fault segments suggesting continuous interactions between tectonic and magmatic processes. They are related to inflation/deflation of a vertical magmatic dike and comprise: (1) shearing at strike-slip transform fault with left lateral motion; (2) collapses at normal faulting with negative volumetric components due to magma/fluid escape, and (3) shear-tensile opening at oblique strike-slip faulting with positive volumetric components connected to flow of trapped over-pressurized fluids. The identification of three regimes of complex volcano-tectonic evolution in divergent plate movement proves an enormous capability of the non-DC volumetric components to map tectonic processes in such settings.