Papers by Ásta Hjartardóttir
Jökull
The Kerlingar fault is a ∼30 km long fault located at the boundary between the Northern Volcanic ... more The Kerlingar fault is a ∼30 km long fault located at the boundary between the Northern Volcanic Rift Zone, and the Tertiary Eastern Fjords Block in Iceland. The fault has a throw of 2–9 m down to the east and is most likely a normal fault. It probably ruptured in several earthquakes over extended time, but assuming it ruptured in one event it would have a magnitude of about $M_w$ = 6.7. The Kerlingar fault forms a sharp offset in a flat moraine, showing that the fault was active in the Holocene. Several characteristics of the fault are different from that of the presently active fissure swarms of the NVZ. It is unusually long, straight and continuous, and it is parallel with the boundary between the NVZ and EFB not perpendicular to the plate spreading. We consider three possible explanations for the existence of the fault. It may be formed in a rifting event, by stress transfer in relation to the Húsavík transform, or by a stress field caused by rapid crustal unloading during the l...
EGU General Assembly Conference Abstracts, Apr 1, 2018
<p>Using ground deformation measurements of high spatial and temporal resol... more <p>Using ground deformation measurements of high spatial and temporal resolution SAR, the understanding of new vents created during volcanic eruptions can be improved with 3D mapping of the activated shallow magma plumbing system. Interferometric analysis of radar data from ICEYE X-band satellites with daily coherent ground track repeat (GTR) provides unprecedented time series of deformation in relation to the opening of 6 eruptive vents over 26 days in 2021, at Fagradalsfjall, Iceland. Unrest started in this location at the end of February and tens of thousands of earthquakes were recorded during the following four weeks. The seismicity was linked to gradual formation of a magma-filled dike in the crust and triggered seismicity along the plate boundary. On 19 March, an eruptive fissure opened near the center of the dyke. New vents and eruptive fissures opened on the 5th, 7th, 10th, and 13th April. The daily acquisition rate of the ICEYE satellite facilitated the observation of the ground openings associated with each new vents. Each event can be observed individually and with minimal loss of signal caused by new lava emplacement, which would occur if images were acquired at a slower rate. Being able to retrieve deformation near the edge of the fissure ensures that we have the optimal constraints needed for modelling the subsurface magma path. The ICEYE dataset consists of Stripmap acquisitions (30x50km) in the period 3-21 March, and Spotlight acquisitions (5x5 km) from 22 March and onward. Images have a resolution of about 2 m x 3 m, and 0.5 m x 0.25 m, respectively. The descending 1-day interferogram covering each individual event is used to invert for the distributed opening along the dike plane. We find that each fissure was associated with opening of up to 0.5 meters in the topmost 200 m of crust. The conduits propagated vertically at least 50–80 m/h. The new fissure locations were influenced by local conditions and induced stress changes within the shallow crust.</p>
After more than a year of unrest, a small effusive eruption commenced in Fagradalsfjall on 19 Mar... more After more than a year of unrest, a small effusive eruption commenced in Fagradalsfjall on 19 March 2021. The eruption lasted six months with multiple fissure openings characterizing the first six weeks. During the eruption lava and low-level gases propagated over the complex terrain: a hyaloclastite massif with mountain peaks up to about 350 m asl with valleys in the between. It is uninhabited, but easily accessible at about 30 km distance from the Reykjavík capital area. While the eruption was on-going, more than 356,000 accesses were counted. Monitoring the onset, the eruption in real-time, forecasting the transport of gas and emplacements of lava flows, and assessing the hazards were instrumental in maintaining safe access to the area. In addition to data accessibility and interpretation, managing this volcanic crisis was possible thanks to a strong collaboration between the scientific institutions and civil protection agencies. The eruption presented an opportunity to tune, tes...
<p>A large deep seated gravitational slope deformation has been detected in... more <p>A large deep seated gravitational slope deformation has been detected in a mountain slope north of the Tungnakvíslarjökull outlet glacier, in the western part of the Mýrdalsjökull ice cap in South Iceland. Mýrdalsjökull also hosts the Katla central volcano, which erupted spectacularly last in 1918. Based on comparison of Digital Elevation Models (DEMs) obtained from aerial photographs, lidar and Pléiades stereoimages, the slope has been showing slow gravitational slope deformation since 1945 to present. The total vertical displacement in 1945-2020 is around 200 m. The deformation rate has not been constant over this time period and the maximum deformation occurred between 1999 and 2004 of total of 94 m or about 19 m/year.</p><p>The mountain slope north of the Tungnakvíslarjökull outlet glacier reaches up to around 1100 m height. The head scarp of the slide, which is almost vertical, is around 2 km wide rising from about 400-500 m in the western part up to the Mýrdalsjökull glacier at 1100 m in the east. The area of deformation, from the head scarp down to the present-day ice margin is around 1 km<sup>2</sup>. The total volume of the moving mass is not known as the depth of the sliding plane is not known, but the minimum mobile rock volume is between 100 to 200 million m<sup>3</sup>. The entire slope shows signs of displacement and is heavily fractured. Continuous GNSS stations which were installed in the uppermost part of the slope in August 2019 and in the lower part of the slope in 2020 provide real-time displacements. The GNSS time series show evidence of seasonal motion of the landslide, with highest deformation rates occurring in late summer or fall. Historically, seismicity in the area has been at maximum in the fall, although little seismicity has been observed since the GNSS stations were installed.</p><p>There are two main ideas of the causes for this deformation. One is the consequences of slope steepening by glacial erosion, followed by unloading and de-buttressing due to glacial retreat. Another proposed cause for the deformation is related to its location on the western flank of the Katla volcano. Persistent seismic activity in this area for decades may be explained by a slowly rising cryptodome into the base of the slope, which may also explain the slope failure.</p>
In the Northern Volcanic Zone of Iceland, the geometry, kinematics and offset amount of the struc... more In the Northern Volcanic Zone of Iceland, the geometry, kinematics and offset amount of the structures that form the active Krafla Rift were studied. This rift is composed of a central volcano and a swarm of extension fractures, normal faults and eruptive fissures, which were mapped and analysed through remote sensing and field techniques. In three areas, across the northern, central and southern part of the rift, detailed measurements were collected by extensive field surveys along the post-Late Glacial Maximum (LGM) extension fractures and normal faults, to reconstruct their strike, opening direction and dilation amount. The geometry and the distribution of all the studied structures suggest a northward propagation of the rift, and an interaction with the Husavik–Flatey Fault. Although the opening direction at the extension fractures is mostly normal to the general N–S rift orientation (average value N99.5° E), a systematic occurrence of subordinate transcurrent components of moti...
Analysis of a time series of ground deformation measurements at active volcanoes can provide an i... more Analysis of a time series of ground deformation measurements at active volcanoes can provide an improved understanding of sub-volcanic and sub-aerial processes; including those related to magmatic, hydrothermal and structural development. Interpreting a long time series may also help determine background behavior, and identify any deviations from this, including the migration of new melt. We use Persistent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) techniques to generate a time series of high-resolution deformation measurements, in the vicinity of the most active volcanoes in Iceland: Bárðarbunga, Askja, Hekla, Katla and Eyjafjallajökull and compare these to other geodetic measurements. A comprehensive network of continuous GPS stations is already deployed at these volcanoes and a series of campaign GPS measurements are routinely undertaken each summer. InSAR observations are complementary to these field based measurements and their high spatial resolution assists...
Nature, 2014
Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like ... more Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of km long 1. Previous models of rifting events indicate either a lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens 2,3,4 , or magma flowing vertically into dykes from an underlying source 5,6 , with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system, grew laterally for over 45 km at a variable rate, with an influence of topography on the direction of propagation. Barriers at the ends of each segment were overcome by the buildup of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar
Bulletin of Volcanology, 2012
Fissure swarms at divergent plate boundaries are activated in rifting events, during which intens... more Fissure swarms at divergent plate boundaries are activated in rifting events, during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions. To study the evolution of fissure swarms and the behaviour of rifting events, detailed mapping was carried out on fractures and eruptive fissures within the Krafla fissure swarm (KFS). Fracture densities of dated lava flows ranging from 10,000 years BP to~30 years old were studied, and the fracture pattern was compared with data on the historical Mývatn rifting episode (1724-1729) and the instrumentally recorded Krafla rifting episode (1975-1984). Additionally, the interaction of transform faults and fissure swarms was studied by analysing the influence of the Húsavík transform faults on the KFS. During the historical rifting episodes, eruptions on the fissure swarm occurred within~7 km from the Krafla central volcano, although faults and fractures were formed or activated at up to 60-70 km distance. This is consistent with earlier rifting patterns, as Holocene eruptive fissures within the KFS are most common closer to the central volcano. Most fractures within the central Krafla caldera are parallel to the overall orientation of the fissure swarm. This suggests that the regional stress field is governing in the Krafla central volcano, while the local stress field of the volcano is generally weak. A sudden widening of the graben in the northern KFS and a local maximum of fracture density at the junction of the KFS and the extrapolation of the Húsavík transform fault zone indicates possible buried continuation of the Húsavík transform fault zone which extends to the KFS. Eruptive fissures are found farther away from the Krafla central volcano in the southern KFS than in the northern KFS. This is either due to an additional magma source in the southern KFS (the Heiðarsporður volcanic system) or caused by the Húsavík transform faults, transferring some of the plate extension in the northern part. Fracture density within particular lava flow fields increases with field age, indicating that repeated rifting events have occurred in the fissure swarm during the last 10,000 years BP. The fracture density in the KFS is also generally higher closer to the Krafla central volcano than at the ends of the fissure swarm. This suggests that rifting events are more common in the parts of the fissure swarm closer to the Krafla central volcano.
Jökull
Hekla volcano is known to have erupted at least 23 times in historical time (last 1100 years); of... more Hekla volcano is known to have erupted at least 23 times in historical time (last 1100 years); often producing mixed eruptions of tephra and lava. The lava flow volumes from the 20th century have amounted 80\% to almost 100\% of the entire erupted volume. Therefore, evaluating the extent and volume of individual lava flows is very important when assessing the historical productivity of Hekla volcano. Here we present new maps of the historical lava flow fields at Hekla in a digital format. The maps were produced at a scale of 1:2000–10000 using a catalogue of orthophotos since 1945, acquired before and after each of the last five eruptions, combined with field observation of stratigraphy, soil profiles, tephra layers and vegetation cover. The new lava flow maps significantly improve the historical eruptive history of Hekla, prior to the 1947 eruption. The historical lava flow fields from Hekla cover ∼233 km² and the lavas reach up to 16 km from Hekla volcano. Flow lengths up to 20 km...
Icelandic Journal of Engineering, 2018
Straddling the boundary between two of the major tectonic plates on Earth, Iceland offers unique ... more 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 a...
Bulletin of Volcanology, 2009
... (1992) used this method to determine the relative age of the post-glacial lava flows in and c... more ... (1992) used this method to determine the relative age of the post-glacial lava flows in and close to Askja central volcano (Fig. 2). They used the marker ash layers H1, H3 and H4 from Hekla, dated at 1158 AD, 2900 BP and 4500 BP respectively (Kjartansson et al. ...
Bulletin of Volcanology, 2011
... shields. Information on the age of these pre-Holocene formations is not available, although h... more ... shields. Information on the age of these pre-Holocene formations is not available, although hyaloclastite formations indicate formation beneath a glacier (Kjartansson 1943), while lava shields are formed subaerially. Holocene ...
On 19 March 2021, a 6-month subaerial, effusive, basaltic eruption started at Mt. Fagradalsfjall ... more On 19 March 2021, a 6-month subaerial, effusive, basaltic eruption started at Mt. Fagradalsfjall ending a 781-year dormancy on the Reykjanes Peninsula, Iceland (Figure 1). Initially, the eruption started in the valley Geldingardalir, with the lava infilling that valley. Following a phase of active vent migration, the lava flow field expanded into neighboring valleys in the Fagradalsfjall area, which is a complex of intergrown tuyas and tindars formed during subglacial eruptions (e.g., Jones, 1969; Pedersen & Grosse, 2014). Reykjanes Peninsula is an onshore continuation of the Mid-Atlantic plate boundary and is, together with the Grímsey rift, atypical compared to other Icelandic volcanic zones by being a highly oblique spreading zone. It has N-S trending strike slip faults and volcanic systems consisting of 10-40 km long NE-SW-trending fissure swarms and geothermal areas. Fagradalsfjall volcanic system is an exception displaying none of these characteristics and is the least active volcanic system of the Peninsula (e.g.
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Papers by Ásta Hjartardóttir