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Cyclic behaviour is observed in volcanic phenomena ranging from caldera collapses to explosions, spattering or lava fountaining. The repeating processes can define irregular, regular or systematically changing patterns. These patterns yield information about the subsurface structure, which often is not considered in detail. We analyse the pattern of 7058 lava fountaining episodes that occur between 2 May and 14 June 2021 during the Geldingadalir eruption, Iceland. Our seismometer records the lava fountaining episodes as tremor episodes. We analyse the seismic tremor amplitude, the episode duration, the repose time and the sum of episode duration and repose time (cycle duration). We define six periods characterised by different patterns: Three periods feature long episodes that exponentially shorten with time. One period features coexisting long and short episodes in a haphazard sequence. One period shows a stable pulsing duration but increasing repose time, and one period has stable, short episodes and repose times. We conclude that the episodic fountaining starts because a shallow-conduit container forms on 2 May shifting the magma degassing from sustained continuous to an episodic state. This situation evolves until 11 May when a semi-stable state is reached. The length of the repose times is most likely influenced by the amount of outgassed magma present in the uppermost part of the shallow conduit. Finally, we suggest that the vent is mechanically eroded and widens with time causing increasing seismic tremor amplitudes. However, the trends are frequently punctuated by partial crater wall collapses that temporarily disrupt the system. [ABSTRACT FROM AUTHOR]

The eruption frequency of geysers can be studied easily on the surface. However, details of the internal structure including possible water and gas filled chambers feeding eruptions and the driving mechanisms often remain elusive. We used a multidisciplinary network of seismometers, video cameras, water pressure sensors and one tiltmeter to study the eruptive cycle, internal structure, and mechanisms driving the eruptive cycle of Strokkur geyser in June 2018. An eruptive cycle at Strokkur always consists of four phases: (1) Eruption, (2) post‐eruptive conduit refilling, (3) gas filling of the bubble trap, and (4) regular bubble collapse at shallow depth in the conduit. For a typical single eruption 19 ± 4 bubble collapses occur in Phase 3 and 8 ± 2 collapses in Phase 4 at a mean spacing of 1.52 ± 0.29 and 24.5 ± 5.9 s, respectively. These collapses release latent heat to the fluid in the bubble trap (Phase 3) and later to the fluid in the conduit (Phase 4). The latter eventually reaches thermodynamic conditions for an eruption. Single to sextuple eruptions have similar spacings between bubble collapses and are likely fed from the same bubble trap at 23.7 ± 4.4 m depth, 13–23 m west of the conduit. However, the duration of the eruption and recharging phase linearly increases likely due to a larger water, gas and heat loss from the system. Our tremor data provides documented evidence for a bubble trap beneath a pool geyser. Plain Language Summary: It is easy to study the eruptions of a geyser on the surface. It is however difficult to study the shape of the geyser at depth and the processes that cause eruptions since we cannot observe them directly. Here, we use seismometers, cameras, pressure sensors and one tiltmeter to study the behavior and area beneath pool geyser Strokkur, Iceland, in June 2018. We find that the geyser always passes through four phases: (i) eruption, (ii) refilling of the conduit with water, (iii) gas accumulation in a bubble trap, and (iv) bubbles leaving the bubble trap regularly to collapse in the conduit at shallow depth. Bubble collapses, heating and gas accumulation in the bubble trap takes place at a mean depth of 23.7 ± 4.4 m, 13–23 m west of the conduit (Phase 3). Phase 4 is characterized by regular bubble collapses at shallow depth in the conduit heating the fluids in the conduit to prepare for an eruption. Single to sextuple eruptions behave similar and are likely fed from the same bubble trap offset from the conduit. However, the eruption and recharging phase persists for longer most likely caused by a larger water, gas and heat loss from the system. Key Points: Eruptive cycle of Strokkur consists of eruption, conduit refilling, bubble trap gas accumulation and bubble collapses at depth in conduitDuration of phases in the eruptive cycle linearly increases from single to sextuple eruptions, except for the conduit refilling phaseWe infer a bubble trap at a pool geyser at 23.7 ± 4.4 m depth, 13–23 m west of the conduit feeding single to sextuple eruptions [ABSTRACT FROM AUTHOR]

The article informs on that following the discovery of magma at modest depth beneath Krafla Caldera, Iceland, an international coalition of scientists and engineers has been assembled to establish a long-term natural laboratory or testbed to take full advantage of this important discovery the Krafla Magma Testbed. It mentions that volcanologists rely on surface measurements from seismometers, Global Positioning System sensors, and radar satellites to guess its movements.

Abstract  The lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity. The Icelandic mid-Atlantic rift is characterized by high heat flow, abundant magmatism generating up to 25–30 km thick crust, and seismicity within the upper 8 km of the crust. In a 20-seismometer survey in July-August 2006 within the northern rift zone around the Askja volcano we recorded ~1700 upper-crustal earthquakes cutting off at 7–8 km depth, marking the brittle-ductile boundary. Unexpectedly, we discovered 100 small-magnitude (ML [ABSTRACT FROM AUTHOR]

The subglacial Grímsvötn central volcano, lying within a volcanic zone directly above the core of the Iceland mantle plume, is one of the most active in Iceland. Local, regional and teleseismic earthquake data recorded on a temporary seismometer array across western Vatnajökull icecap during the summer of 1998 have provided a three-dimensional image of the shallow crustal structure of the volcano. Microearthquake activity at depths of 1–4 km along the Grímsvötn caldera rim coincided with inflation of a shallow magma chamber beneath the caldera, which culminated in a 0.1 km3 eruption in December 1998. Tomographic inversion of these earthquakes define the extent of a low-velocity body beneath Grímsvötn with a volume of ∼20 km3 extending to ∼3 km below the surface. This low-velocity body is flanked by high velocities under the caldera rim. Delays in the P-wave arrival times through the Grímsvötn caldera from regional and teleseismic earthquakes and from two detonations ∼150 km east of Grímsvötn are 0.10–0.15 s greater than the delays through the uppermost 3–4 km of crust shown by local earthquake arrivals. This suggests the presence of a further low-velocity body at depths greater than 3–4 km beneath Grímsvötn, presumed to be due to the presence of melt. Using the distribution of local seismicity and shear wave attenuation we estimate the maximum lateral extent of the region containing partial melt to be 7–8 km E–W and 4–5 km N–S. P-wave delays require a thickness of less than 1 km of pure/high percentage partial melt, assuming a sill-like magma chamber. [ABSTRACT FROM AUTHOR]

The temperature of mantle plumes may vary on geologic time scales, from a few million years to tens of millions of years. In special circumstances such as near Iceland in the North Atlantic, where the plume underlies an oceanic spreading center, temporal variations in the oceanic crustal thickness provide a sensitive proxy for the mantle temperature if, as is likely, the crustal thickness is controlled primarily by passive decompression of mantle rising beneath the spreading center. We show from both seismic reflection imaging and wide-angle ocean bottom seismometer data from the Norwegian Sea that the temperature of the Iceland mantle plume decreased by ∼50 °C over the first 5 m.y. following continental breakup and then oscillated by ∼25 °C over an ∼3 m.y. period. Similar temperature variations on a 3–6 m.y. time scale, creating strong lineations in the gravity field, are inferred from the regional North Atlantic. They occur both in the period immediately following breakup and at the present-day Reykjanes Ridge south of Iceland, where they create V-shaped ridges as the mantle thermal anomalies propagate away from the center of the plume beneath Iceland. We propose that mantle plume temperature variations of∼25 °C have occurred in the Iceland plume with a similar amplitude and frequency since at least 49 Ma, and are likely to be a feature of all mantle plumes. [ABSTRACT FROM AUTHOR]

Describes the radial receiver functions calculated from teleseisms recorded by broad-band seismometers distributed in Iceland. Factors influencing the arrival time of direct P phase on radial receiver functions; Increase of seismic velocities of the crust; Nature and depth of crust-mantle transition.

Investigates the influence of Iceland hotspot on the adjacent mid-Atlantic spreading center. Causes of volcanic formation; Implication of seismograph inspections; Description of the polarization anisotropy.

A research project with an exciting field component presents a unique opportunity for broad public engagement. Outreach with schools is especially valuable in that it can have a profound, lasting impact. It shows the students how science works, encourages them to study science, and broadens their career perspectives – to quote from the feedback survey for the outreach programme that we present here as an example. The project SEA-SEIS (Structure, Evolution and Seismicity of the Irish Offshore) kicked off in 2018 with a 3-week expedition on the research ship Celtic Explorer. For the first time, broadband ocean-bottom seismometers were deployed across the vast Irish offshore, with a few instruments also in the UK and Iceland waters. Prior to the expedition, we asked schools across Ireland to help the scientists to give each seismometer a name. Participating in a real, exciting research project got the students enthusiastically engaged. Many of the classes participating in the naming competition then took part in our live, ship-to-class video link-ups. After a brief introduction to the project and meeting the team, the students asked numerous questions about the research, life on the ship, and how one becomes a scientist. The follow-up survey responses from the teachers confirmed that the engagement was not only exciting ("it was like speaking to Indiana Jones!") but had a lasting impact. With most of our scientists on the ship being female, the female students in the classrooms were particularly encouraged. The SEA-SEIS outreach programme is sustainable and now continues with follow-up national competitions (a seismology song-and-rap one for secondary schools and a drawing one for primary schools – we shall present some remarkable entries). The programme's success in engaging students across the country and sharing with them the excitement of research demonstrates the value of active research projects as a means of broad outreach and education. Obviously, the outreach programme could not succeed without the scientists on the project putting in the work. Fortunately, researchers see quickly the very real impact of the outreach and get involved with enthusiasm and commitment. The programme's outcomes thus include not only school students getting more interested in science but also young researchers getting experience and appreciation of the education and outreach work. Website: www.sea-seis.ie [ABSTRACT FROM AUTHOR]

The history of seismography in Iceland began in 1909 with the installation of one horizontal Mainka seismograph in Reykjavík. Following a period of intermittent operation regular operation was initiated in1925 with the establishment of the Icelandic Meteorological Office. The number of stations increased gradually over the following decades, and in the sixties four stations were in operation. The number of permanent stations proliferated following the Heimaey eruption in 1973, and during most of the eighties, the number of stations was 40-50. The first digital seismograph stations were installed in 1990 and the analog seismic network was gradually replaced by digital stations over the next two decades. The number of analog seismograms in the Icelandic archives is estimated to be around 300 000 covering the period 1910-2010. A four-year project to make this record collection accessible on the internet has been initiated and funded. So far around 100 000 seismograms have been scanned and made available for download on the open website https://seismis.hi.is.This database will eventually be merged with a general Icelandic science data repository currently being established by the University of Iceland and various research institutes.The seismograms are scanned with a resolution of 300 dpi and presented for examination on the website as low resolution.png files but high-resolution jpg files can also be selected for download. The high-resolution files are on the order of 4-8 Mb each. Digitization of the seismic traces has not been attempted, since most of the seismograms are from short-period instruments and the waveforms are already lost.In addition to numerous teleseismic body wave phases the record collection contains primary data from various tectonic and magmatic events in Iceland during the last century. This includes eruptions of Hekla in 1947, 1970, 1980-81, 1991 and 2000, Surtsey in 1963-67, Heimaey in 1973, Askja in 1961, Grímsvötn in 1934, 1983, 1998, and 2004, Gjálp in 1996, rifting episode at Krafla in 1975-1984, persistent seismic activity of the Bárðarbunga and Katla volcanoes, numerous suspected subglacial magmatic events, earthquake swarms on the Reykjanes Peninsula Oblique Rift and within the Tjörnes Fracture Zone, and earthquake sequences in the transform zones of South and North Iceland. [ABSTRACT FROM AUTHOR]

The North Atlantic Ocean and its margins present important, unanswered geodynamics questions, both in the continental and oceanic domains. To better understand the structure and evolution of this area, a set of 18 state-of-the-art ocean-bottom seismometers (OBSs) have been deployed in the North Atlantic Ocean in September 2018 as part of the project SEA-SEIS (Structure, Evolution And Seismicity of the Irish offshore). The network covers the entire Irish offshore and parts of the UK and Iceland waters. Over a period of 15 months, they will record seismic data at 1-4 km depths on the ocean bottom, with the retrieval planned for April, 2020. With this new broadband data, together with all other globally available broadband data, we will improve the tomographic resolution of the North Atlantic region, including the Iceland hotspot. To image the lithosphere and mantle, structural information will be extracted from the large amount of vertical- and transverse-component waveform fits, including the unique new OBS data, using the Automated Multimode Inversion (AMI) of surface, S- and multiple S-waves. Realistic body-wave sensitivity kernels will be developed and integrated into the inversion. The improved tomographic models of the North Atlantic region should yield new insights into the origins of the enigmatic North Atlantic Igneous Province volcanism, the evolution and deep structure of the Iceland Hotspot and its effect on the evolution of the region. To minimize the effect of data errors on the model, extensive quality control has been applied to the available global and regional network data, including extensive checks of the correctness of the instrument response information, which can influence the accuracy of the structural information yielded by seismograms. The quality control revealed a number of problems with the response files at stations from various of networks. The problems are being fixed in joint efforts with the data centers and network operators. [ABSTRACT FROM AUTHOR]

Natural hazard prediction and efficient crust exploration require dense seismic observationsboth in time and space. Seismological techniques provide ground-motion data, whoseaccuracy depends on sensor characteristics and spatial distribution. Here we demonstratethat dynamic strain determination is possible with conventional fibre-optic cables deployedfor telecommunication. Extending recently distributed acoustic sensing (DAS) studies, wepresent high resolution spatially un-aliased broadband strain data. We recorded seismicsignals from natural and man-made sources with 4-m spacing along a 15-km-long fibre-opticcable layout on Reykjanes Peninsula, SW-Iceland. We identify with unprecedented resolutionstructural features such as normal faults and volcanic dykes in the Reykjanes Oblique Rift,allowing us to infer new dynamic fault processes. Conventional seismometer recordings,acquired simultaneously, validate the spectral amplitude DAS response between 0.1 and100 Hz bandwidth. We suggest that the networks of fibre-optic telecommunication linesworldwide could be used as seismometers opening a new window for Earth hazard assess-ment and exploration. [ABSTRACT FROM AUTHOR]

The formation of the oceanic island Surtsey in the shallow ocean off the south coast of Iceland in 1963-1967 remains one of the best-studied examples of basaltic emergent volcanism to date. Cycles of the phreatomagmatic explosive activity occurred several times during the eruptions: (1) At the onset of the eruption - eruption of Surtur crater, November 1963-January 1964; (2) The submarine eruption of Surtla - December 1963; (3) the formation of the parasitic cone of Surtungur - January-April 1964; (4) formation of the temporary island of Syrtlingur - May-October 1965; and (5) the formation of the temporary island of Jólnir - December 1965-August 1966. Explosive activities at these vents are well-documented and preserved in photographic records and reports. The first submarine volcanic activity was detected on 14th of November 1963 where ocean depth was 130 m prior to the eruption. The onset time of the submarine eruption was not resolved in contemporary records. Birth of the submarine eruption at the seafloor can be detected by investigation of tremor signals or any unusual 0seismic activity at nearby seismograms. Seismic records from the time of the Surtsey eruption are available from two permanent seismic stations in south Iceland: Reykjavík in southwest Iceland and Kirkjubæjarklaustur in southeast Iceland. These records have now been re-analyzed. Any potential eruption signals are drowned by microseisms at the Reykjavík station while at the Kirkjubæjarklaustur station eruption tremor signals appeared before the first explosive eruption had broken through the sea floor. Earthquake activity related to the beginning of the eruption was weak, hardly detectable by the existing seismograph stations.These observations indicate that the actual start of the submarine eruption occurred no later than on 13th of November, with eruption tremors recorded 18-24 hours before the first visual observation of eruption from a fishing boat in the early morning of the 14th of November. Similar tremors were also detected during other four explosive phases. The potential of seismic records to detect the onset of eruptions is demonstrated through these results. Based on the shreds of evidence for the onset time and the visual observations, maps and descriptions, we derive the early growth history of Surtsey. When combined with bathymetric data prior to the eruption, we obtain constraints on the rate of submarine cone buildup. This places constraints on the eruption rate in these early submarine-to-emergent phases of the eruption. Early results indicate that the maximum mass eruption rate in the beginning phase was relatively modest, below 1,000,000 kg/s. [ABSTRACT FROM AUTHOR]

A large landslide fell from the eastern side of the Fagraskógarfjall Mountain in the Hítardalur valley, W-Iceland, on the morning of July 7th, 2018. The landslide crossed the salmon fishing river Hítará along 1.6 km of the river path, blocked the river and led to the formation of a ~47 ha lake upstream of the slide. The Hítará River was subsequently diverted to the east into the Stekká River, a riverbed that reconnects with Hítará River a few km farther downstream. The Hítardalur landslide is considered among the largest landslides in historical time in Iceland.The landslide was detected on a seismograph at 05:17 in the morning of July 7th, but signs of instability in the mountain were detected 6 hours prior to the slide, when a foxhunter noticed a small landslide at the location where the large landslide was subsequently released. The landslide fell from an area of mountainside with evidence of earlier displacements. The release was at around 630 m a.s.l. in the eastern side of the mountain 3.5 km inside the valley mouth and had a vertical drop of around 450 m. The width of the debris tongue is approximately 1.5 km and the run-out length approximately 2.3 km. The run-out angle is 12–13° which indicates high mobility of the moving material. The debris covers approximately 1.5 km2 of the valley floor. The maximum thickness of the tongue above the original valley bottom is ca. 30 m and the average thickness is around 7 m. Volume calculations were made by comparing DEMs from before and after the event. The results show that approximately 7 million m3 of material where released from the source area. The landslide entrained a substantial amount of material on the way down the mountainside. The surface elevation changes in the bottom of the valley indicate a debris volume of ca. 10 million m3 above the former terrain. Field observations show that the landslide ploughed through sediments at the foot of the mountain and on the valley bottom and, therefore, the debris tongue probably reaches several meters below the former terrain surface. It is hard to estimate the volume of displaced material below the former terrain, but the total slide volume might be 10–20 million m3. InSAR analysis of Sentinel-1 satellite radar data shows major displacements in the source area weeks prior to the event with displacements in the last days before the landslide of tens of cm. The source area also appears to have been moving in 2015, 2016 and 2017 but at a slower rate. The detection of movement before the Hítardalur landslide indicates that large landslides may be associated with precursors that can be detected by satellite data or other measurements. It is planned to analyze slopes above settlements and frequently travelled areas in Iceland where instability is suspected, and detect areas where displacement is occurring. Such areas could then be monitored in near real-time with the same technology. [ABSTRACT FROM AUTHOR]

The article discusses the environmental aspects of the eruption resulting from the Bárðarbunga volcano near Reykjavik in Iceland. Topics include how the pollution has surprised volcanologists who are part of a project funded by the European Union, systems used by researchers to monitor the volcano like seismometers and global-positioning system (GPS) stations and the seismic activity and earthquakes leading up to the eruption.