Fissure Swarm Research Articles

Crustal Characterization of the Hengill Geothermal Fields: Insights From Isotropic and Anisotropic Seismic Noise Imaging Using a 500‐Node Array

AbstractThe Hengill volcano and its associated geothermal fields represent Iceland's most productive harnessed high‐temperature geothermal fields, where resources are fueled by cooling magmatic intrusions connected to three volcanic systems. The crustal structure in this area is highly heterogeneous and shaped by the intricate interplay between tectonic forces and magmatic/hydrothermal activities. This complexity makes detailed subsurface characterization challenging. In this study, we aim to push the current resolution limits using a 500‐node temporary seismic array and perform an isotropic and, for the first time, radially‐anisotropic velocity model of the area. The high‐resolution isotropic velocity model reveals the characteristic N30ºE fissure swarm that crosses the area within the top 500 m and outlines a deep‐seated low‐velocity body composed of cooling magmatic intrusions at 5 km depth. This deeper body is located near the eastern part of the three volcanic centers and connected to a shallower body at 2–3 km depth that strikes westward toward Hengill volcano. Additionally, our study discovered that non‐induced earthquakes deeper than 2 km align with velocity contrasts that reflect structural variability, indicating the potential to identify deep permeable pathways using dense array imaging. The anisotropic model indicates that the shallow crust of Hengill within the top 2 km is dominated by vertical fractures or cracks, likely attributed to overall divergent deformation from rifting in the study area. This characteristic is diminished at depths greater than 2–3 km, replaced by a layering pattern where the lava flows and/or subhorizontal intrusions become the primary factors influencing the observed anisotropy.

Soil CO2 emission and stable isotopes (δ13C, δ18O) of CO2 and calcites reveal the fluid origin and thermal energy in the supercritical geothermal system of Krafla, Iceland

The Krafla geothermal system is located within a volcanic center that periodically erupts basaltic lavas, and has recently attracted an economic interest due to supercritical fluids forming near a shallow magma intrusion (∼ 2 km depth). Here, we discuss new soil CO2 flux and stable isotope data of the CO2 efflux (δ13C) and hydrothermal calcites (δ13C, δ18O) of drill cuttings to estimate both the current magmatic outgassing from soils and the thermal flows in the geothermal system. Soil CO2 emission is controlled by tectonics, following the NNE-SSW fissure swarm direction and a WSW-ENE trend, and accounts for ∼62.5 t d−1. While the δ18O of the H2O in equilibrium with deep calcites is predominantly meteoric, both the δ13C of the soil CO2 efflux and of the fluids from which calcite precipitated have a clear magmatic origin, overlapping the δ13C estimated for the Icelandic mantle (−2.5 ± 1.1 ‰). Estimates based on the soil CO2 emission from the southern part of the system show that these fluxes might be sustained by the ascent and depressurization of supercritical fluids with a thermal energy of ∼800 MW. Such significant amount of energy might reach 1.5 GW if supercritical conditions extended below the whole investigated area. Finally, we report an increase in the soil CO2 emission of about 3 times with respect to 14 years ago, likely due to recent changes in the fluid extracted for power production or magmatic activity. Pairing the soil CO2 emission with stable isotopes of the efflux and calcite samples has important implications for both volcano monitoring and geothermal exploration, as it can help us to track magmatic fluid upflows and the associated thermal energy.

A step forward to understanding the development of volcanotectonic rifts: the structure of the Fremrinamar Fissure Swarm (Iceland)

We analysed all the Holocene structures defining the Fremrinamar Fissure Swarm (FFS), in the Northern Volcanic Zone of Iceland, through the interpretation of aerial photos, orthomosaics and Digital Surface Models (DSMs), and field surveys. We measured the strike, dip, length and kinematics of 761 normal faults and reconstructed the slip profile of 76 main faults (length >2 km), with the purpose of evaluating the overall direction of along-axis rift propagation. We also measured the strike of 146 eruptive fissures and 1,128 extension fractures. A total of 421 faults dip towards the east and 340 dip towards the west, mainly striking N0°-10°E. Maximum fault length is 14.2 km, and W-dipping faults are longer than E-dipping faults. The majority of eruptive fissures strike N10°-20°E, and are concentrated in the southern part of the FFS, around the Fremrinamar central volcano. Extension fractures mainly strike N0°-10°E, with a maximum length of 2,508 m. We evaluated the variation of strike, fracture density and spacing along the FFS, and observed a change of its trend from NNE-SSW in the central-southern part, to NNW-SSE in the northern part. We interpret this evidence as the effect of the intersection with the Grimsey Lineament. The tapering of fault slip profiles indicates a main northward propagation of the rift, and thus of the deformation, interpreted as the effect of lateral propagation of dykes from the magma chamber below the central volcano towards the north. Such interpretation is also supported by the distribution of normal faults, vertical offset and dilation values, and also by the rift width, which tend to decrease towards the north.

The Role of On‐ and Off‐Axis Faults and Fissures During Eruption Cycles and Crustal Accretion at 9°50′N, East Pacific Rise

AbstractFissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate.

Tectonic position, structure, and Holocene activity of the Hofsjökull volcanic system, central Iceland

The Hofsjökull volcanic system is located at the northern border of the Hreppar microplate on the Mid-Atlantic plate boundary in Iceland, between the more active Western and Eastern volcanic zones. In this study, fractures, and faults within the fissure swarms of the volcanic system were mapped and the throw and orientation of faults measured. This was done using both aerial photographs and ArcticDEM digital elevation models, as well as during fieldwork. The Hofsjökull volcanic system contains 3–4 fissure swarms extending northwards or southwards from the glacially covered Hofsjökull central volcano. Although these fissure swarms have been active during the Holocene, no clear sink holes were found along the faults, suggesting that they have been filled by sediments. This indicates that the fissure swarms have been less active than fissure swarms in other branches of the plate boundary in Iceland where GPS geodetic measurements show current spreading. Unbroken hyaloclastite covering a fault in the northern Hofsjökull fissure swarm suggests that this part of the northern Hofsjökull fissure swarm has not been active since the earliest part of Holocene, or during the latest stage of glaciation in the area. Still, the fault scarps in the northern Hofsjökull fissure swarm are rather sharp, indicating little erosion by glaciers. This may suggest increased activity in the Hofsjökull fissure swarm during the end of the last glaciation or at the beginning of the Holocene, which is in line with other studies showing increased magmatic activity in Iceland during that period. Fractured Holocene lava flows in the southern and western Hofsjökull fissure swarms indicate that they have been active during the Holocene. The Kerlingarfjöll rhyolitic massif is located south of the Hofsjökull central volcano. The southern fissure swarms are located both east and west of Kerlingarfjöll, but no clear indications are found of the fissure swarm in the rhyolitic massif itself. This may occur as dike intrusions (which likely form the faults) are prevented from penetrating the rhyolite due to density differences, and/or due to the topographic high of Kerlingarfjöll.

Seismicity of the Northern Volcanic Zone of Iceland

A half century of monitoring of the Northern Volcanic Zone of Iceland, a branch of the North America—Eurasia plate boundary, shows that the seismicity is very unevenly distributed, both in time and space. The four central volcanoes at the boundary, Þeistareykir, Krafla, Fremrinámar, and Askja, show persistent but very low-level seismicity, spatially coinciding with their high-temperature geothermal systems. On their rift structures, on the other hand, seismicity is almost absent, except during rifting episodes. Krafla went through a rifting episode in 1975–1984 with inflation, interrupted by 20 diking events with extensive rifting, eruptive activity, and intense seismicity along an 80 km long section of the rift. During inflation periods, the seismicity was contained within the caldera of the volcano, reflecting the inflation level of the magma chamber. Diking events were marked by seismicity propagating away from the volcano into the fissure swarms to the south or north of the volcano, accompanied by rapid deflation of the caldera magma chamber. These events lasted from 1 day to 3 months, and the dike length varied between 1 and 60 km. The area around the Askja volcano is the only section of the Northern Volcanic Zone that shows persistent moderate seismicity. The largest events are located between fissure swarms of adjacent volcanic systems. Detailed relative locations of hypocenters reveal a system of vertical strike-slip faults, forming a conjugate system consistent with minimum principal stress in the direction of spreading across the plate boundary. A diking event into the lower crust was identified in the adjacent fissure swarm at Upptyppingar in 2007–2008. Four nests of anomalously deep earthquakes (10–34 km) have been identified in the Askja region, apparently associated with the movements of magma well below the brittle-ductile transition. Several processes have been pointed out as possible causes of earthquakes in the deformation zone around the plate boundary. These include inflation and deflation of central volcanoes, intrusion of propagating dikes, both laterally and vertically, strike-slip faulting on conjugate fault systems between overlapping fissure swarms, migration of magma in the lower, ductile crust, and geothermal heat mining.

A new model for the growth of normal faults developed above pre-existing structures

AbstractConstraining the mechanisms of normal fault growth is essential for understanding extensional tectonics. Fault growth kinematics remain debated, mainly because the very earliest phase of deformation through recent syn-kinematic deposits is rarely documented. To understand how underlying structures influence surface faulting, we examined fault growth in a 10 ka magmatically resurfaced region of the Krafla fissure swarm, Iceland. We used a high-resolution (0.5 m) digital elevation model derived from airborne lidar to measure 775 fault profiles with lengths ranging from 0.015 to 2 km. For each fault, we measured the ratio of maximum vertical displacement to length (Dmax/L) and any nondisplaced portions of the fault. We observe that many shorter faults (<200 m) retain fissure-like features, with no vertical displacement for substantial parts of their displacement profiles. Typically, longer faults (>200 m) are vertically displaced along most of their surface length and have Dmax/L at the upper end of the global population for comparable lengths. We hypothesize that faults initiate at the surface as fissure-like fractures in resurfaced material as a result of flexural stresses caused by displacements on underlying faults. Faults then accrue vertical displacement following a constant-length model, and grow by dip and strike linkage or lengthening when they reach a bell-shaped displacement-length profile. This hybrid growth mechanism is repeated with deposition of each subsequent syn-kinematic layer, resulting in a remarkably wide distribution of Dmax/L. Our results capture a specific early period in the fault slip-deposition cycle in a volcanic setting that may be applicable to fault growth in sedimentary basins.

Field evidence of the interplay between rift and transform structures in the Krafla geothermal area, N-Iceland

The Krafla volcano-geothermal area is located in Northeast Iceland, within the active Northern Volcanic Zone sector of the axial rift and has experienced two eruptions in historical times. The Krafla system displays NNE-SSW oriented fissure swarm, a central volcano and associated geothermal manifestations (i.e. steam and gas emissions, hydrothermal alterations). However, during its exploration and exploitation, several geophysical indications have suggested the occurrence of WNW-ESE trending structures, i.e. near orthogonal with respect to the faults delimiting the axial rift. This paper describes the results of a structural survey carried out in a wide area in the NE-sector of Iceland along the Húsavík-Flatey shear zone and in the Krafla area, measuring fractures and alignments of geothermal manifestations and fissure swarms. The dataset highlights the occurrence of structures oriented in two main trends, i.e. NNW-SSE to NNE-SSW and WNW-ESE. At their intersection, geothermal manifestations and silica sinter deposits commonly occur. Both of these trends have been documented in the Krafla geothermal area and compared with borehole and geophysical data. Our conclusions support that the location of the high-temperature Krafla geothermal area is mainly controlled by the enhanced permeability at the intersection between structures of the axial rift zone and incipient transform faults crossing the Krafla area.

Reconstruction of Late Pleistocene-Holocene Deformation through Massive Data Collection at Krafla Rift (NE Iceland) Owing to Drone-Based Structure-from-Motion Photogrammetry

In the present work, we demonstrate how drone surveys coupled with structure-from-motion (SfM) photogrammetry can help to collect huge amounts of very detailed data even in rough terrains where logistics can affect classical field surveys. The area of study is located in the NW part of the Krafla Fissure Swarm (NE Iceland), a volcanotectonic rift composed of eruptive centres, extension fractures, and normal faults. The surveyed sector is characterized by the presence of a hyaloclastite ridge composed of deposits dated, on a stratigraphic basis, to the Weichselian High Glacial (29.1–12.1 ka BP), and a series of lava flows mostly dating back to 11–12 ka BP. The integration of remotely sensed surveys and field inspections enabled us to recognize that this segment of the Krafla rift is made of grabens arranged en-échelon with a left-stepping geometry. A major graben increases in width in correspondence of the hyaloclastite cone; we interpret this geometry as resulting from the mechanical contrast between the stiffer lava succession and the softer hyaloclastites, which favours the development of concentric faults. We also measured a total extension of 16.6 m and 11.2 m along the fractures affecting the lava units, and a total extension in the hyaloclastites of 29.3 m. This produces an extension rate of 1.4 mm/yr in the Holocene lavas and 1.7 ± 0.7 mm/yr in the Weichselian hyaloclastite deposits. The spreading direction we obtained for this area is N97.7° E, resulting from the av. of 568 opening direction values.

Fractional degassing of S, Cl and F from basalt magma in the Bárðarbunga rift zone, Iceland

The composition of gas emitted from a volcano producing basalt magma can vary during an eruption and according to the volcano-tectonic setting of the degassing vents. Post-eruptive filter-pack gas samples from the 2014–2015 Holuhraun crater in the Barðarbunga rift zone have lower ratios of S over halogens (Cl and F) and elevated F/Cl (~ 50 times lower S/Cl and ~ 5 times higher F/Cl; mass ratios) compared with samples of the syn-eruptive gas plume. The compositional changes are readily explained by Rayleigh distillation with decreasing sulphur concentrations and increasing concentrations of halogens and F relative to Cl in the final gas phase. For Cl, the vapour-melt partition coefficient (DV/M) decreased from 13–85 to 2.2 during residual degassing, whereas that of F remained uniform at approximately 1.8. Distinctly different degassing behaviour is observed for Cl and F. High D for Cl may indicate an important influence of sulphur and water on Cl volatility in basaltic melt, whereas that of F remains unaffected. The primary gas of the Holuhraun magma had similar ratios of S over Cl and F as observed at the Kilauea rift zone which, together with lower S/halogens in the residual gas in both cases, suggests similar degassing mechanism. By inference, initial CO2 degassing is likely to have occurred subglacially close to the Barðarbunga central volcano before and during the 2014–2015 eruption on the rift-related fissure swarm.

Rifting Kinematics Produced by Magmatic and Tectonic Stresses in the North Volcanic Zone of Iceland

In the North Volcanic Zone of Iceland, we studied with the greatest possible detail the complete structural architecture and kinematics of the whole Theistareykir Fissure Swarm (ThFS), an N-S-trending, 70-km-long active rift. We made about 7500 measurements along 6124 post-Late Glacial Maximum (LGM) extension fractures and faults, and 685 pre-LGM structures. We have collected the data over the last six years, through extensive field surveys and with the aid of drone mapping with centimetric resolution. In the southern sector of the study area, extension fractures and faults strike mainly N10°-20°, the opening direction is about N110°, and the dilation amount is in the range 0.1-10 m. In the central sector, faults and extension fractures strike mainly N00-10°, the opening direction is N90-100°, and the dilation amount is 0.1-9 m. In the northern sector, extension fractures and faults strike N30-40°, the opening direction is about N125°, and the dilation amount is 0.1-8 m. The variations in strike are attributable to two processes: the interaction with the WNW-ESE-striking Husavik-Flatey transform fault and Grímsey Oblique Rift (Grímsey lineament), and the structural inheritance of older NNE- to NE-striking normal faults. Most extension fractures show a minor strike-slip component: a systematic right-lateral component can be accounted for by the interaction with the WNW-ESE-striking fault zones and the regional, oblique opening of the rift. We regard dyke propagation as a possible cause for the more complex strike-slip components measured at several other fractures. Cumulated dilation and fracture frequency decrease along the rift with distance away from the Theistareykir volcano, situated in the central sector of the ThFS. This is interpreted as a decrease in the number of dykes that are capable of reaching great distances after being injected from the magma chamber.

Surface deformation and strike-slip faulting controlled by dyking and host rock lithology: A compendium from the Krafla Rift, Iceland

In the present work, we have obtained a detailed, complete view of surface deformation at a volcano-tectonic structure belonging to the Krafla Fissure Swarm (NE Iceland), through the integration of field mapping and the Aerial Structure from Motion technique (ASfM), as well as through orthomosaics and Digital Surface Models (DSMs). The documented, 2.5-km-long deformation zone is marked by a range of features, such as topographic bulging, parallel extension fractures, and narrow grabens affected by local floor uplift, which we have explained as the effect of the northward, shallow propagation of a dyke from the Krafla magma chamber. Our lithostratigraphic field survey of the area, at a 1: 5000 scale, indicates that major changes in the pattern of surface deformation take place across the contacts between deposits with different rheological properties. The transition from very stiff lavas to softer hyaloclastites produces changes from extensional fracturing to normal faulting, though the width of the deformation zone does not change. We have also documented a number of NE-SW, left-lateral and NNW-SSE, right-lateral strike-slip fault zones that are rotated in clockwise and anticlockwise sense to the main NNE-SSW graben trend, and extend outwards as much as about 17 m. The graben zone terminates to the north by splitting into two strike-slip fault zones. These were likely generated at the front of the dyke tip during its propagation, and were later on bypassed by the advancement of the dyke. As the dyke entered the soft hyaloclastites, propagation was slowed down and the dyke became thicker.

Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing

The spatial distribution of seismicity in the Hengill region, SW Iceland, is analyzed by relocation and collapsing. The Hengill region is a diffuse triple junction with volcano-tectonic activity associated with rifting, tectonic activity on a transecting transform and induced seismicity due to drilling and injection of fluid into geothermal fields. The Icelandic Meteorological Office has compiled 114,000 events over a 20-year period within an area of approximately 600 km2. The events in their catalog are relocated by application of empirical travel-time tables using a non-linear location strategy. The relocations are then redone applying a Bayesian inversion using the catalog event density as a prior. Finally, they are collapsed using the same catalog density as an attractor. We show that this catalog processing reproduces details of the spatial pattern of seismicity that independently emerges from relative relocations of a small subset of the catalog events (swarm activity). In particular, the predominant faulting orientations are reproduced in different parts of the region and the depth distribution of events resembles that obtained by dense deployments in the area. Its depth extent varies between 5 and 7 km in the northern part of the region, where volcanic processes dominate, and between 7 and 8 km in the southern part, where tectonic deformation is predominant. Induced seismicity is shallower than adjacent natural seismicity. An intriguing lineation emerges in the lateral distribution of inferred depth to the brittle-ductile transition in the northern volcanic part of the region, which is parallel to the strike of the fissure swarms in the area. Associating this transition with an isotherm (650 °C), the Hengill volcanic system and its fissure swarm appear to be considerably cooler than the Hrómundartindur system. This may relate to a recent intrusion into the latter or more efficient cooling in the Hengill fissure swarm due to deeper penetrating permeability. In both cases this has potential consequences for geothermal exploitation in the area.

Imaging Torfajökull's Magmatic Plumbing System With Seismic Interferometry and Phase Velocity Surface Wave Tomography

AbstractTorfajökull volcano, Iceland, has not erupted since 1477. However, intense geothermal activity, deformation, and seismicity suggest a long‐lasting magmatic system. In this paper, we use ambient noise tomography to image the magmatic system beneath Torfajökull volcano. One hundred days of ambient noise data from 23 broadband seismometers show the consistent presence of double‐frequency microseism noise with significant power between ∼0.1 and 0.5 Hz. Beamforming results indicate microseism noise with persistent higher energy propagating from west and SE directions and apparent velocities below 3 km/s. We use ambient noise seismic interferometry to retrieve Rayleigh waves, and we introduce a method to estimate the reliability of the retrieved surface waves. We find stable estimation of surface wave phase velocities between 0.16 and 0.38 Hz. Azimuthal velocity variations show a trend of higher velocities in the NE/SW direction, the strike of the rift zone intersecting Torfajökull, and orientation of erupted lavas on a NE‐SW fissure swarm. Tomographic results indicate low‐velocity anomalies beneath the volcano caldera (between −5% and −10%) and even lower velocity variations in the southeast and southwest study area (below −10%), outside the volcano caldera. Low anomalies may indicate the existence of hot material, more prominent outside the caldera outskirts. High‐velocity variations (between 5% and 10%) outline the volcano caldera between 4‐ and 5‐km depth and more pronounced velocities (between 10% and 15%) up to 5‐km depth in the north of the volcano caldera. We interpret the former as possible caldera collapse structure and the latest as solidified intrusive magma from the old preferred magma paths.

Geometry, oblique kinematics and extensional strain variation along a diverging plate boundary: The example of the northern Theistareykir Fissure Swarm, NE Iceland

The boundary between the American and European plates emerges in Iceland, an outstanding natural laboratory where it is possible to analyse ongoing rifting processes. In the North Volcanic Zone, we studied with unprecedented detail an active rift, known as the Theistareykir Fissure Swarm (ThFS). We surveyed an area of 85 km2 with 694 measurement sites along 1537 post-Late Glacial Maximum extension fractures. In the southern sector of the study area, fractures strike N30–40° with opening directions about N120°. Fractures in the central sector strike about N00° and opening directions are N90–100°. In the northern sector, fractures strike about N30° with opening directions about N125°. Through a comparison with older faults cropping out in the substrate at the shoulder of the ThFS, we are able to suggest that variations in fracture strike are the effect of substrate structural inheritance as well as the possible interaction with the Tjörnes Fracture Zone. With regard to kinematics, we highlight that most fractures show a small, but systematic, strike-slip component (a more frequent right-lateral component and a less common, left-lateral one). This cannot be explained as the result of fracture strike rotation relative to the regional, tectonic least principal stress. We conclude that the net opening directions can result from the combination of tectonic offsets and events caused by shallow magma chamber inflation and/or dyke intrusions. The latter can produce transcurrent components of displacement along new or already existing fractures.

UAV-based surveying in volcano-tectonics: An example from the Iceland rift

In the present work, we applied the use of an Unmanned Aerial Vehicle (UAV) - a quadcopter - and the Aerial Structure from Motion digital photogrammetry image processing technique (ASfM) to study volcano-tectonics and tectonic features in an active Icelandic rift. Data have been collected in order to evaluate the Holocene deformation in the Northern Volcanic Zone of Iceland. We mapped 397 structures, mainly related to extension fractures and subordinately normal faults in the Theistareykir Fissure Swarm, obtaining 1098 and 21 structural data, respectively. This allowed to reconstruct an overall spreading direction of N108° during Holocene times, and to calculate a stretch of 1.013 regarding 8–10 ka old lava units. Deformation in the area is related to both dyke intrusions and extensional tectonics. Furthermore, detailed geological-structural field and UAV surveys were also performed in two test areas in order to determine data accuracy and the associated reliability of this approach. In addition to the above, different flight heights were tested, suggesting that photo collection with a 12.4 MPx camera at 100 m is efficient to study fracture dilation and kinematics.

Krýsuvík high temperature geothermal area in SW Iceland: Geological setting and 3D inversion of magnetotelluric (MT) resistivity data

The Krýsuvík area in SW Iceland is characterized by a 40 km long and around 6–8 km wide fissure swarm and two NE-SW trending tindars complexes. Associated with the fissure swarms are fumarole fields located primarily on parallel postglacial eruptive fissure segments. There are indications that Krýsuvík hosts a buried caldera and an intrusion complex in its roots. The geothermal surface manifestations cover an area of around 50–60 km2 and has potential for future exploitation. Faulting accompanies the ridges and postglacial volcanic fissures, with normal faults and open fissures with a maximum throw of 10 m.A 3D inversion was performed of the static shift corrected off-diagonal impedance tensor elements of 102 MagnetoTelluric (MT) resistivity soundings from the Greater Krýsuvík geothermal area. In the inversion, 21 periods were used, evenly distributed from 0.01 to 100 s on a logarithmic scale. The robustness of the inversion was tested by using three different initial models: A model compiled from a joint 1D inversion of TEM and MT soundings, with a homogeneous Earth of resistivity 100 Ω m and a homogeneous Earth of resistivity 10 Ω m. A flat topographic surface was assumed in the 3D inversion. The resulting models were later elevation corrected. The results of the 1D inversion and the 3D inversion are strikingly similar, where the 1D inversion reproduces the main resistivity structures while the 3D inversion shows considerably more details. An analysis of the electrical strike directions based on the vertical magnetic field data is in fairly good agreement with the final resistivity model.The subsurface resistivity structure in Krýsuvík has the same main features as other high temperature areas in Iceland and in general where the host rocks are basaltic. Above a shallow conductive cap, a high resistivity zone is seen, reflecting unaltered rock. The conductive cap reflects smectite hydrothermal mineral alteration. Below the low-resistivity cap, a resistive core is found, reflecting chlorite-epidote alteration. Good correlation is observed in the Krýsuvík field between the subsurface resistivity structure and the hydrothermal alteration revealed by boreholes cuttings. Parts of the field have cooled down and therefore the resistivity structure indicates alteration mineralogy but not necessarily the present rock temperature. The geothermal up-flow zones are therefore most likely where the hydrothermal alteration and the resistive core reach the highest elevation.The 3D inversion shows an indication of a relatively deep-seated conductive body below the central part of the high temperature area. This body coincides horizontally with the centre of the 2010–2011 inflation source at 4–5 km depth. It has been suggested that the 2007–2016 inflation/deflation periods are linked to gas flux, as no signs of S-wave attenuation have been found. It is therefore unlikely that the deep-seated conductive body resolved by our resistivity models comprises partial melt. The body is probably connected to the heat source of the geothermal field and could be due to emission of gas or dehydration.

Geology and structure of the Reykjanes volcanic system, Iceland

The Reykjanes Peninsula is a trans-tensional plate boundary with several volcanic systems from the centers of which fissure swarms extend to the NE and SW. The fissure swarms accommodate the extensional component, whereas north-south trending strike-slip faults accomodate the transform component. The fissure swarms release stress during volcano-tectonic episodes that occur at intervals of several hundred years. Stress is released by the strike-slip faults by microearthquake swarms at intervals of a few decades.The fissure swarms are segmented along their trend. The segments include a volcanic center with a clustering of fissure eruptions and extensional faults. A proximal zone of 20–40 km follows were faults become dominant with distance from the center. Linear anomalies of a high geothermal gradient extend further still, into the marginal area of Early Quaternary to Miocene rocks indicating dyke propagation for another 30–40 km beyond visible faults.The Reykjanes volcanic center is nested in a 5–6 km wide rift zone with boundary faults of just over 20 m visible throw. Volcanic production keeps pace with extension and subsidence to fill the rift. The last three volcano-tectonic episodes occurred at Reykjanes in the 13th century, about 2000 years ago and about 3200 years ago. The lavas from these three fissure eruptions cover >2/3 of Reykjanes by area, largely smoothing over faults of the rift floor. During the Weichselian glacial maximum the ice margin may have reached 75–100 km beyond Reykjanes. It had become ice free 14,500 years ago. There is evidence of at least 10 eruptions on the Reykjanes volcanic system since, of which the first ones were of lava shield type. A similar eruption frequency may have prevailed at Reykjanes during its postglacial time interval.There are three volcanic fissure zones within the Reykjanes center. The middle zone of 1.5 km2 hosts the main geothermal resource of the system. Recently the western zone proved to host an exploitable resource also. It may be restricted to a narrow zone of dykes. The reservoir temperatures of these two lie in the range 280–310 °C. The eastern zone has not proved productive so far. It has erupted olivine rich shield lavas on most of its trace whereas the others have erupted slightly evolved tholeiite. Feed zones in the main production zone have been correlated partly with the axis of the eastern of two tindars. They cluster also in near horizontal intervals which may suggest that density controlled offshots also play a role.

Bergsprungur og byggingar á höfuðborgarsvæðinu

Straddling the boundary between two of the major tectonic plates on Earth, Iceland offers unique conditions for engineering structures that require special attention. Urban areas are rapidly expanding into areas where the bedrock is cut by numerous active fractures and faults. The fissure swarm of the Krísuvík volcanic system runs through the outskirts of Reykjavík and other towns of the metropolitan area. Activity of its fractures mostly occurs during magmatic events along the Reykjanes Peninsula oblique rift on a thousand years timescale. Hazard caused by the fractures is mostly twofold: Relative displacement of the walls of the fracture during magmatic intrusion and small relative displacements during the passage of seismic waves from distant earthquakes may damage structures built across them. The risk of structural damage may most likely be reduced considerably by avoiding building structures across the fractures. We suggest a change in building practice in fractures areas to achieve that.

Deformation due to geothermal exploitation at Reykjanes, Iceland

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.