Seismic imaging in the Krafla high-temperature geothermal field, NE Iceland, using zero- and far-offset vertical seismic profiling (VSP) data

The research leading to these results has received funding from the European Community's Seventh Framework Program under grant agreement No. 608553 (Project IMAGE). We thank Landsvirkjun, the operator of the Krafla geothermal field, for technical and logistical support during the survey. We also thank the Operational Support Group of the International Continental Scientific Drilling Program (ICDP) for their technical support. We further acknowledge the support from the Research Council of Norway through its Centres of Excellence funding scheme, project 22372 (SP).Peer reviewedPostprin

Tomographic image of the Mid-Atlantic Plate Boundary in southwestern Iceland

Publisher's version (útgefin grein)The 170 km South Iceland Seismic Tomography (SIST) profile extends from the west and across the Mid‐Atlantic Ridge spreading center in the Western Volcanic Zone and continues obliquely through the transform zone (the South Iceland Seismic Zone) to the western edge of the Eastern Volcanic Zone. A total of 11 shot points and 210 receiver points were used, allowing precise travel times to be determined for 1050 crustal P wave rays and 180 wide‐angle reflections. The large amplitudes of the wide‐angle reflections and an apparent refractor velocity of 7.7 km/s are interpreted to be from a relatively sharp Moho at a depth of 20–24 km. This interpretation differs from the earlier models (based on data gathered in the 1960s and 1970s), of a 10–15 km thick crust underlain by a upper mantle with very slow velocity of 7.0–7.4 km/s. Nevertheless, these older data do not contradict our new interpretation. Implication of the new interpretation is that the lower crust and the crust‐mantle boundary are colder than previously assumed. A two‐dimensional tomographic inversion of the compressional travel times reveals the following structures in the crust: (1) a sharp increase in thickness of the upper crust (“layer 2A”) from northwest to southeast and (2) broad updoming of high velocity in the lower crust in the Western Volcanic Zone, (3) depth to the lower crust (“layer 3”) increases gradually from 3 km at the northwestern end of the profile to 7 km at the southeastern end of the profile, (4) a low‐velocity perturbation extends throughout the upper crust and midcrust into the lower crust in the area of the transform in south Iceland (South Iceland Seismic Zone), and (5) an upper crustal high‐velocity anomaly is associated with extinct central volcanos northwest of the Western Volcanic Zone. The travel time data do not support the existence of a large (> 0.5 km thick) crustal magma chamber in this part of the Western Volcanic Zone but do not exclude the possibility of a smaller one.This research was supported by the U.S. National Science Foundation, the Iceland National Science Foundation, the National Energy Authority of Iceland, the Incorporated Research Institutions for Seismology, IcelandAir, and the Lamont-Doherty Geological Observatory of Columbia University.Peer Reviewe

Sub-surface geology and velocity structure of the Krafla high temperature geothermal field, Iceland : Integrated ditch cuttings, wireline and zero offset vertical seismic profile analysis

The research leading to these results has received funding from the European Community's Seventh Framework Programme under grant agreement No. 608553 (Project IMAGE). The VMAPP project run by VBPR, DougalEARTH Ltd. and TGS also contributed funding to the borehole characterization of the K-18 borehole. Landsvirkun is acknowledged for their effort and assistance in this work and in particular for allowing the use of the data from well K-18. We further acknowledge the support from the Research Council of Norway through its Centres of Excellence funding scheme, project 22372 (SP and DAJ).Peer reviewedPostprin

Cyclical geothermal unrest as a precursor to Iceland’s 2021 Fagradalsfjall eruption

Understanding and constraining the source of geodetic deformation in volcanic areas is an important component of hazard assessment. Here, we analyse deformation and seismicity for one year before the March 2021 Fagradalsfjall eruption in Iceland. We generate a high-resolution catalogue of 39,500 earthquakes using optical cable recordings and develop a poroelastic model to describe three pre-eruptional uplift and subsidence cycles at the Svartsengi geothermal field, 8 km west of the eruption site. We find the observed deformation is best explained by cyclic intrusions into a permeable aquifer by a fluid injected at 4 km depth below the geothermal field, with a total volume of 0.11 ± 0.05 km3 and a density of 850 ± 350 kg m–3. We therefore suggest that ingression of magmatic CO2 can explain the geodetic, gravity and seismic data, although some contribution of magma cannot be excluded

Reply

Publisher's version (útgefin grein)The central question discussed by Gudmundsson [this issue] can be succinctly stated: "Is the temperature of the shallowest upper mantle of Iceland at the peridotite solidus" (nominally, 1200°C). The traditional view, as developed by numerous authors during the 1970s and early 1980s (reviewed by Palmason [1986]) and to which Gudmundsson ascribes, is that it is supersolidus and partially molten

Cation-Exchange Capacity Distribution within Hydrothermal Systems and Its Relation to the Alteration Mineralogy and Electrical Resistivity

Cation-exchange capacity (CEC) measurements are widely used to quantify the smectite content in altered rocks. Within this study, we measure the CEC of drill cuttings in four wells from three different high-temperature geothermal areas in Iceland. The CEC measurements in all four wells show similar depth/temperature related pattern, and when comparing the CEC with electrical resistivity logs, we could show that the low resistivity zone coincides with CEC values >5 meq/100 g. The measurements show, in general, an exponential decrease of the CEC with increasing depth. At the facies boundary between the mixed-layer clay and epidote-chlorite zone, the CEC reaches a steady state at about 5 meq/100 g and below that it only decreases slightly within a linear trend with increasing depth. The facies boundary overlaps with the transition where the electrical resistivity logs show an increase in resistivity. It is shown that the measured CEC can be related to the clay mineral alteration within the geothermal system and the CEC reflects the smectite component within the interstratified chlorite/smectite minerals for similar alteration degree. Furthermore, CEC was measured in seven core samples from different alteration zones that had previously been studied in detail with respect to petrophysical and conductivity properties. The results show a clear correlation between CEC and the iso-electrical point, which describes the value of the pore fluid conductivity where transition from surface conductivity to pore fluid conductivity occurs. The presented study shows that the CEC within hydrothermal altered basaltic systems mimics the expandable clay mineral alteration zones and coincides with electrical logs. The presented method can, therefore, be an easy tool to quantify alteration facies within geothermal exploration and drilling projects

Crustal structure above the Iceland mantle plume imaged by the ICEMELT refraction profile

The crustal structure of central Iceland is modelled using data from a 310 km long refraction profile shot during summer 1995. The profile traversed Iceland from the Skagi Peninsula on the north coast (surface rocks of age 8.5–0.8 Myr) to the southeast coast (surface rocks of age 8.5–3.3 Myr), crossing central Iceland (surface rocks of age 3.3–0 Myr) over the glacier Vatnajökull, below which the locus of the Iceland mantle plume is currently centred. The crustal thickness is 25 km at the north end of the profile, increasing to 38–40 km beneath southern central Iceland. The upper crust is characterized by seismic P-wave velocities from 3.2 to approximately 6.4 km s−1. At the extreme ends of the profile, the upper crust can be modelled by a two-layered structure, within which seismic velocity increases with depth, with a total thickness of 5–6 km. The central highlands of Iceland have a single unit of upper crust, with seismic velocity increasing continuously with depth to almost 10 km below the surface. Below the central volcanoes of northern Vatnajökull, the upper crust is only 3 km thick. The lower-crustal velocity structure is determined from rays that turn at a maximum depth of 24 km below central Iceland, where the seismic velocity is 7.2 km s−1. Below 24 km depth there are no first-arriving turning rays. The Moho is defined by P-and S-wave reflections observed from the shots at the extreme ends of the profile.P- to S-wave velocity ratios give a Poisson's:of 0.26 in the upper crust and 0.27 in the lower crust, indicating that, even directly above the centre of the mantle plume, the crust is well below the solidus temperature.We thank Rob Staples for assistance and many helpful discussions. This work is funded by the Natural Environment Research Council (NERC) and National Science Foundation research grants; FD is supported by NERC. Instruments were loaned by the University of Cambridge, the University of Iceland, the NERC Geophysical Equipment Pool, the PASSCAL instrument pool, the Lamont-Doherty Earth Observatory, the Nordic Volcanological Institute, the University of Oregon and St Louis University. We thank Jósef Hólmjárn (shotmaster) and all who assisted in the field: Rob Staples and Mark Muller (Cambridge); Rob Dunn (Oregon); Yang Shen (Woods Hole); Arnar Hjartarson and Ólafur Rognvaldsson (Orkustofnun); Hrappur Magnusson (independent), Randy and Adriana Kuehnel (The Carnegie Institute of Washington, DTM); also Einar Kjartansson (independent) and Bob Busby (PASSCAL). Thanks to the wardens of the mountain huts at Nýidalur and Laugafell, and special thanks to Magnús Óskarsson and his family at the farm Sölvanes for looking after us and allowing us to store equipment at the farm. We thank Clare Enright for invaluable assistance with code and with data processing, and Helgi Torfason for supplying unpublished geological maps. Department of Earth Sciences, Cambridge, contribution number 5208.Peer Reviewe

Author Correction: Hydrothermal fluid flow triggered by an earthquake in Iceland

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