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Maintenance of infrastructure is costly and difficult to implement systematically when it spreads over wide areas, such as road or pipeline networks. In the monitoring of road ways, conventional methods to control the road integrity rely on discrete measurements in space and time. There is a large demand for innovative technologies that are able to assess the structural integrity as a whole and in regular intervals or even continuously. Distributed fiber-optic sensing opens the opportunity to measure numerous physical quantities such as temperature and strain with high spatial and temporal resolution over tens of kilometers. In addition, it is easily deployable at reasonable cost.In order to address the issue of asphalt aging due to exposure to heavy traffic loads, we installed a fiber-optic cable into a reworked road interval and recorded fiber-optic data in a controlled experiment with numerous test vehicles of different sizes and weights. The recorded data suggests that elastic properties of the asphalt can be retrieved from the bypassing traffic. Vehicles can be characterized by the number of axes and load on the asphalt composite. In the next phase, we will monitor the aging of the test field to deduce how varying matrial properties can be better identified for geotechnical and geoscience applications.

Volcano monitoring has been experiencing significant improvements in recent years, yet eruption forecasting and scenarios have still lack of understanding, due to the poor observations in low amplitude events and hindered by surface external noise of similar amplitudes. Volcanic events have been shown to be accurately recorded with fiber optic techniques at the surface. In this study, we present preliminary results of fibre optic cable deployed in a new 200 m deep borehole on the southern flank of Etna at about 6 km away from the summit crater. This borehole has been designed primarily for the future deployment of a new strain sensor type. We benefited from the drilling of this new borehole to deploy a single-mode fibre optic cable. We connected an interrogator and recorded dynamic strain rate during several periods: first, in 2020 for several days during the completion of the borehole and the final stage of the drilling; second, in 2021 for several weeks during an active volcanic period; and in December 2022 during a quiet activity period of several months. We present a selection of records of noise while drilling, local volcano-tectonic earthquakes and volcanic events and tremor that occurred during those periods. These examples show the benefit of deploying a fibre in a borehole far from the active area and demonstrate the great variety of signals fibre optic can record is such configuration.

The timing of the recordings of ocean-bottom seismometers (OBSs) is critical for accurate earthquake location and Earth model studies. GNSS signals, however, cannot reach OBSs deployed at the ocean bottom. This prevents their clocks from being synchronized with a known reference time. To overcome this, we developed OCloC, a Python package that uses time-lapse cross-correlations of ambient seismic noise to synchronize the recordings of large-scale OBS deployments. By simultaneously quantifying deviations from symmetry of a set of lapse cross-correlations, OCloC recovers the incurred clock errors by means of a least-squares inversion. In fact, because non-uniform noise illumination patterns also break the symmetry of (lapse) cross-correlations, we introduce a distance-based weighted least-squares inversion. This mitigates the adverse effect of the noise illumination on the recovered clock errors. Using noise recordings from the IMAGE project in Reykjanes, Iceland, we demonstrate that OCloC significantly reduces the time and effort needed to detect and correct timing errors in large-scale OBS deployments. In addition, our methodology allows one to evaluate potential timing errors at the time of OBS deployment. These might be caused by incorrect initial synchronization, or by rapidly changing temperature conditions while the OBS is sunk to the sea bottom. Our work advances the use of OBSs for earthquake studies and other applications.

The sustainable exploitation of a geothermal reservoir is usually assessed through continuous field monitoring and structural exploration of the hydrothermal reservoir. However, accurate subsurface mass and fluid displacement as well as energy transfer model of the hydrothermal reservoir is most of the time not resolved enough both in space and time. Since 2017, both at Krafla and Theistareykir powerplants (northern Iceland), we use several multi-parameter stations each equipped with a gravity meter (superconducting or spring relative meter), a broad band seismometer, a GNSS receiver and other meteorological and hydrological sensors. With this set-up, we aim to model mass and stress transfer through the combination of absolute and micro gravity measurements, continuous signals measured at the multi-parameter stations and seismic measurements.We present results from the 2022 micro gravity and absolute gravity campaigns conducted at Theistareykir geothermal field. Through inversion and interpretation of such results, as well as the analysis of the continuous measurements, and injection and production data, we aim to assess the anthropogenic contribution in the mass and energy transfer models of the investigated area. Furthermore, we show the first continuous measurements and accurate Earth tide model for the Krafla area. The final goal of this study is to verify the conditions of sustainable exploitation of the reservoirs, and establish reservoir parameters, such as permeability, that regulate the response of the geothermal system to changes in production and injection rates.

Distributed Dynamic Strain Sensing (DDSS), also known as Distributed Acoustic Sensing (DAS), is becoming a popular tool for volcano monitoring. The sensing method relies on sending coherent light pulses into an optical fibre and measuring the phase-shift of Rayleigh back-scattered light due to strain on the fibre. This provides distributed strain rate measurements at high temporal and spatial sampling rates. Standard telecom fibres have been conventionally used for this purpose, however engineered fibres are being developed to enhance the back-scattered light, providing up to 100 times improved sensitivity in contrast to the conventional standard fibre. Despite the technical advantages of engineered fibres, standard fibres already have extensive coverage around the Earth surface, and so there is an interest in using the existing telecommunication infrastructure. In this study we compare stack DDSS data from a fibre loops made of several fibres within the same optical fibre cable, with DDSS data measured on an engineered fibre. We analyse how stacking can improve the signal quality of the recorded DDSS data. In an area located 2.5 km NE from the craters of Mt. Etna, we spliced 9 standard fibres together from a 1.5 km long cable to create a single optical path and interrogated using an iDAS unit. At the same time, we interrogated with a Carina unit a 0.5 km engineered fibre installed parallel to the standard multi-fibre cable. Both fibres were interrogated in a common period of 5 days. We use a spatial cross-correlation function to find the channel equivalences between each fibre and then stack them to evaluate the changes in the DDSS data and compare with the engineered fibre data. Our results show that, despite engineered fibres have lower noise, a stack of 5 fibres can achieve a maximum noise reduction of 20% outside of the optical noise band, in comparison to the engineered fibre. We achieved this noise reduction for our specific configuration, and so we show how the stack improvement is dependent on the type of configuration in terms of fibres stacked and length of the fibres. Our findings motivate the exploitation of multi-fibre cables in existing infrastructures, so-called dark fibres, for monitoring volcano and applications to other environments.

Rotational sensors have been deployed on several volcanoes worldwide including Kilauea, Stromboli, Etna and a few volcanoes in Iceland. Within this presentation we focus on our first experiment using a rotational sensor on Etna in Italy. We recorded the volcanic activity including degassing and vigorous strombolian activity in August to September 2019. We compare our results using a rotational sensor with a normal Trillium Compact seismometer and the seismic network maintained by the INGV. We detect LP events, VT events and volcanic tremor, study the wavefield and back azimuths of the events and derive phase velocities of the ground. Luckily, we were able to assess the quality of our results using the detailed earthquake catalogues and locations derived at the INGV. We can easily detect changes in the wavefield e. g. when strong strombolian activity kicks in and are looking forward to applications on other volcanoes where details of the volcanic activity or changes might go unnoticed if no rotational sensor is present.

Since September 2021, signs of unrest at Vulcano Island have been noticed after four years of quiescence, along with CO2 degassing and the occurrence of long-period and very long-period events. With the intention of improving the monitoring activities, a submarine fiber optic telecommunications cable linking Vulcano Island to Sicily was interrogated from 15 January to 14 February 2022. Of particular interest has been the recording of 1488 events with wide range of waveforms made up of two main frequency bands (from 3 to 5 Hz and from 0.1 to 0.2 Hz).With the aim of the automatic detection of seismic-volcanic events, different approaches were explored, particularly investigating whether the application of machine learning could provide the same performance as conventional techniques. Unlike many traditional algorithms, deep learning manages to guarantee a generalized approach by automatically and hierarchically extracting the relevant features from the raw data. Due to their spatio-temporal density, the data acquired by the DAS can be assimilated to a sequence of images; this property has been exploited by re-designing deep learning techniques for image processing, specifically employing Convolutional Neural Networks.The results demonstrate that deep learning not only achives good performance but that it even outperforms classical algorithms. Despite providing a generalized approach, Convolutional Neural Networks have been shown to be more effective than traditional tecniques in expoiting the high spatial and temporal sampling of the acquired data.

Driven by the need to understand magmatic systems, to improve volcano monitoring strategy, and to develop next-generation, high-enthalpy, geothermal energy, we introduce the Krafla Magma Testbed (KMT) – located in Northeast Iceland. KMT aims to establish the first magma observatory – an international, open access, scientific platform to advance ductile zone to magma research via drilling and novel sensor systems. This frontier undertaking will enable direct, in situ sampling, instrumentation and manipulation, and monitoring of magma and its interface with solid Earth’s crust, vastly advancing models of high-temperature crustal processes. This initiative is enabled by past geothermal drilling at Krafla volcano that was serendipitously intersected and thus determined the exact location of magma for the first time. This unprecedented experience, including safe control of the wells, provides the basis for KMT, which stands to transform modern volcanology and geothermic disciplines. KMT will develop a long-term infrastructure (>25 years) for the conduct of interdisciplinary scientific, engineering, technological, and educational activities. The Krafla volcano has the advantage of a long history of geological study, volcano monitoring, and drilling as well as supporting surface facilities combining to produce the safest and most efficient base from which to explore Earth beyond the solidus. KMT will be the place to develop (1) our science of hot and molten Earth; (2) new ways of understanding and monitoring volcanoes; (3) our ability to extract and exploit geothermal energy sources; and (4) new technology and materials that function in the most extreme conditions in planetary systems. The value of potential gains in fundamental understanding of crustal processes is beyond our possibility to estimate. There is the prospect of an order of magnitude gain in geothermal energy productivity. The need to improve understanding of the source of catastrophic eruptions and to better forecast them is a compelling humanitarian one.

As part of a global distributed acoustic sensing (DAS) campaign, multiple DAS interrogators (from academia and industry) recorded simultaneously from 1st till 28th February 2023 in different regions of the globe. The objective is to define if and how a global monitoring system based on DAS could perform for teleseismic event record and analysis. Each participant uploaded triggered data window from earthquakes with magnitude larger than 5, as defined by global seismological networks, to a central storage location. Data was pre-processed following common filtering parameters (spatial and temporal sampling). Bottle-necks in data format, storage, and legal issues are identified and reviewed to pose the basis for a common DAS data archive strategy. In this study, we present a selection of DAS records of the Turkey earthquake sequence, from borehole, surface, on-land, submarine telecommunication or dedicated cables all over the globe. They comprise a few kilometers long railroad track (Switzerland), an 0.8 km long deployed cable in the Limmat river, near Zürich (Switzerland), a 1 km deployed cable at Mt. Zugspitze in the Alps (Germany/Austria), a 21 km telecom cable in the forest around Potsdam (Germany), a 17 km telecom cable surface geothermal field (north Iceland), a 0.2 km borehole at Etna volcano (Italy), a telecom cable in the city of Istanbul (Turkey), a 25 km telecom cable in Melbourne (Australia), in the inner city line in Graz (Austria), in the city of Seattle, WA (USA), a submarine cable in the North Sea, a submarine cable connecting Ny Ålesund and Longyearbyen at Svalbard (Norway), a 0.8 km dedicated fibre in a quick clay area in Norway, amongst many others. We show that signals from the two destructive earthquakes in Turkey were recorded all over the globe. We discuss the signal quality and their potential use to study teleseism signals. We analyze recorded strain amplitudes according to the different array geometries and the differing sensitivities to wave types (body, surface waves, possibly others) and deployment conditions. When available, comparison with other sensors located in the same place is performed. Finally, we analyze the influence of local geological conditions due to the passing large amplitudes waves. With the increasing availability, reduced cost and improved simplicity of DAS systems and the wide spread existing fibre optic networks, we believe fibre-optic sensing will play an ever-increasing role in the global seismic monitoring.

A common challenge in acoustic meteoroid signal analyses is to discriminate whether the observed wavefield can be better described by line-source or point-source models. This challenge typically arises from a sparse availability of observations. In this work, we present an outstanding record of ground-coupled waves from local large-N seismic and distributed acoustic sensing (DAS) observations of a meteoroid in Iceland. Our complete data set includes additional regional stations located within 300 km of the meteoroid’s trajectory. The dense large-N and DAS data allow identification of acoustic phases that are almost impossible to discriminate on sparser networks, including a weak late arrival resolved mostly only by DAS. Using this data set with a new Bayesian inversion model, we estimate the trajectory parameters of one fragment from the meteoroid. With these results we investigate its orbit in the solar system and propose a classification of the Icelandic event as a slow meteoroid of asteroidal origin with an energy on the order of 4–40 GJ, a probable size on the order of centimeters, and an orbit range consistent with the main asteroid belt.

The excitation of long-period signals has been reported for large eruptions such as El Chichon (1982) and Mount Pinatubo (1991).We report on series of pressure and gravity signals associated with the series of violent explosions at the Hunga-Tunga eruption, on 14-15th January 2022 at different locations and set-up configurations.We present records from various stations all over the world. Those stations have different set up and local configurations.At most stations, gravity signals records reveal clearly surface waves associated to the main eruption at ~4:30, 15th January and pressure wave up to Iceland.We analyze similarity and difference in the signals in association with the location and the station configurations

Slow slip plays an important role in accommodating plate motion along plate boundaries throughout the world. Further understanding of the interplay between aseismic and seismic slip has gained particular attention as it is crucial for the assessment of seismic risk. A wide range of instruments and acquisition techniques exist to quantify tectonic deformation which spans multiple orders of magnitude in duration as well as spatial extend. For example, seismometers acquire dense temporal data, however are sparsely deployed, leading to spatial aliasing. As opposite, remote sensing techniques have wide aperture but rather crude temporal resolution and accuracy (mm-range). In selected areas, strain is continuously measured with laser or borehole strainmeters.In this contribution, we investigate the distribution of permanent strain along a telecommunication optic fibre on the Reykjanes Peninsula, South West Iceland. Continuous strain-rate was recorded via DAS (Distributed Acoustic Sensing) over a period of six months during the recent unrest of the Svartsengi volcano which began in January 2020. The interrogated fibre connects the town of Gridavik with the Svartsengi geothermal power plant and was patched to a second fibre leading to the western most tip of the Reykjanes Peninsula. It is approximately between 10 and 20km west of the active volcanic area which produced abundant local seismicity as well as surface uplift and subsidence in areas crossed with the optical fiber. The fibre was installed in a trench at less than one meter depth and consists of two roughly straight segments of 7 and 14km length. Whereas the longer segment trends WSW parallel to the strike of the Mid-Atlantic Ridge at this geographic height, the shorter segment trends NEN and thus almost coincides with the maximum compressive stress axis of the region.Inspection of the spatio-temporal strain-rate records after the occurrence of local earthquakes indicates the accumulation of compressive as well as extensive strain in short fibre sections of a few dozen meters which could correlate with local geologic features like faults or dykes. This holds for events of M~2.5 and fibre segments in epicentral distances of more than 20km. Preliminary results regarding the total deformation of the fibre as response to an individual seismic event show a distinct behaviour for differently oriented fibre segments correlating with the overall stress regime, i.e. shortening in the order of some dozen nanometers in the direction of SHmax. Unfortunately, recordings of the two largest intermediate M>=4.8 events indicate saturation of the recording system or loss of ground coupling thus preventing a meaningful interpretation of their effect on permanent surface motion. Perspectively, our efforts aim at investigating the feasibility of distributed optical strain-rate measurements along telecommunication infrastructure to track locally accumulated strain.

On the 13th December 2020, a Strombolian eruption occurred on Mount Etna. We present a study of the temporal and spatial variation of strain measured at the underwater base of volcano during this event. As part of the FOCUS project, a BOTDR (Brillouin Optical Time Domain Reflectometry) interrogator has been connected to the INFN-LNS ( Istituto Nazionale di Fisica Nucleare - Laboratori Nazionali del Sud) fibre optic cable that extends from the port of Catania 25km offshore to TTS (Test Site South) in a water depth of 2km. This interrogator has been continuously recording the relative strain changes at 2m spacing along the length of the cable every 2 hrs since May 2020. On preliminary analysis, a change in strain is observed at the around the time of the eruption, however this variation occurs close to the shore where seasonal variations in water temperatures are in the order of 5°C. As Brillouin frequency shifts are caused by both temperature and strain variations, it is necessary to remove this effect. To do so, numerical simulations of seasonal sea temperature specific to offshore Catania have used to estimate the change in temperature along the cable. This temperature change is then converted to a Brillouin frequency shift and removed from the frequency shift recorded by the interrogator before being converted to relative strain measurements. This processing produces a strain signature that is consistent with deformation observed by nearby geodetic stations on land.

On July 2, 2021, around 22:44 CET, a meteoroid was observed crossing the sky near Lake Thingvallavatn east of Reykjavik in Iceland. During this event, a large-N seismic network consisting of 500, 3-component geophones was monitoring local seismicity associated with the Hengill geothermal field southwest of the lake. In addition to the large-N network, two fiber optic telecommunication cables, covering a total length of more than 40 km, were connected to distributed acoustic sensing (DAS) interrogation units. The systems were deployed under the frame of the DEEPEN collaboration project between the Eidgenössische Technische Hochschule Zürich (ETHZ), the German Research Centre for Geosciences (GFZ), NORSAR, and Iceland Geo-survey (ISOR). Both the large-N and the DAS recordings display multiple trains of infrasound arrivals from the meteoroid that coupled to the surface of the earth at the locations of the sensors. In particular, three strong phases and multiple other weaker arrivals can be identified in the DAS data.Fitting each of the strong phases assuming point sources (i.e., fragmentations) generates travel time residuals on the order of several seconds, resulting in an unsatisfactory explanation of the observations. On the other hand, inverting the arrival times for three independent hypersonic-trajectories generating Mach cone waves reduces travel time residuals to well under 0.5 s for each arrival. However, whereas the 1st arrival is well constrained by more than 900 travel times from the large-N, DAS and additional seismic stations distributed over the Reykjanes peninsula, the 2nd and 3rd arrivals are mainly constrained by DAS observations near Lake Thingvallavatn. The less well-constrained, latter trajectories show a weak agreement with the trajectory of the first arrival. Currently, neither the multi-Mach-cone model nor the multi-fragmentation model explain all our observations satisfactorily. Thus, traditional models for interpreting meteoroid observations appear unsuitable to explain the combination of phase arrivals in the large-N network and DAS data consistently.

The goal of the ERC (European Research Council) funded project - FOCUS is to apply laser reflectometry on submarine fiber optic cables to detect deformation at the seafloor in real time using BOTDR (Brillouin Optical Time Domain Reflectometry). This technique is commonly used monitoring large-scale engineering infrastructures (e.g. - bridges, dams, pipelines, etc.) and can measure very small strains (<< 1 mm/m) at very large distances (10 - 200 km), but until now has never been used to study tectonic faults and deformation on the seafloor.Here, we report that BOTDR measurements detected movement at the seafloor consistent with ≥1 cm dextral strike-slip on the North Alfeo fault, 25 km offshore Catania, Sicily over the past 10 months. In Oct. 2020 a dedicated 6-km long fiber-optic strain cable was connected to the INFN-LNS (Catania physics institute) cabled seafloor observatory at 2060 m depth and deployed across this submarine fault, thus providing continuous monitoring of seafloor deformation at a spatial resolution of 2 m. The laser observations indicate significant elongation (20 - 40 microstrain) at two fault crossings, with most of the movement occurring between 19 and 21 Nov. 2020. A network of 8 seafloor geodetic stations for direct path measurements was also deployed in Oct. 2020, on both sides of the fault to provide an independent measure of relative seafloor movements. These positioning data are being downloaded during ongoing oceanographic expeditions to the working area (Aug. 2021 R/V Tethys; Jan. 2022 R/V PourquoiPas) using an acoustic modem to communicate with the stations on the seafloor. An additional experiment was performed in Sept. 2021 using an ROV on the Fugro vessel Handin Tide, by weighing down unburied portions of the submarine cable with pellet bags and sandbags (~25kg each) spaced every 5m. The response was observed simultaneously by DAS (Distributed Acoustic Sensing) recordings using two DAS interrogators (a Febus and a Silixa). The strain caused by the bag deployments was observed using BOTDR and typically produced a 50 - 100 microstrain signal across the 120 meter-long segments which were weighed down. In Jan. 2022 during the FocusX2 marine expedition, 21 ocean bottom seismometers were deployed for 12-14 months, which together with 15 temporary land-stations as well as the existing network of permanent stations (both operated by INGV) will allow us to perform a regional land-sea passive seismological monitoring experiment.

Seismic monitoring refers to the measurement of time-lapse changes of seismic wave velocities and is a frequently used technique to detect dynamic changes in the Earth‘s crust. Its applications include a broad range of topics, such as natural hazard assessment and structural health monitoring. To obtain reliable measurements, results are usually stacked over time. Thereby, temporal resolution is lost, which makes the measurement less sensitive to short-term environmental processes. Another problem is that conventional datasets often lack spatial density and velocity changes can only be attributed to large areas. Recently, distributed acoustic sensing (DAS) has gained a lot of attention as a way to achieve high spatial resolution at low cost. DAS is based on Rayleigh-scattering of photons within an optical fibre. Because measurements can be taken every few meters along the cable, the fibre is turned into a large seismic array that provides information about the Earth’s crust at unprecedented resolution. In our study, we explore the potential of DAS for monitoring studies. Specifically, we investigate how spatial stacking of DAS traces affects the measurements of velocity variations. We use data recorded by a 21-km-long dark fibre located on Reykjanes Pensinsula, Iceland. The cable is sampled with a channel spacing of 4 meters. We analyze the energy of the oceans microseism continuously recorded between March and September 2020. At first, we stack adjacent traces on the fibre in space. We then cross correlate the stacks to obtain approximations of the Green’s functions between different DAS-channels. By measuring changes in the coda waveform of the extracted seismograms, velocity variations can be inferred. Our analysis shows that spatial stacking improves the reliability of our measurements considerably. Because of that, less temporal stacking is required and the time resolution of our measurements can be increased. In addition, the enhancement of the data quality helps resolve velocity variations in space, allowing us to observe variations propagating along the cable over time. These velocity changes are likely linked to magmatic intrusions associated with a series of repeated uplifts on the Peninsula. Our results highlight the potential of DAS for improving the localization capabilities and accuracy of seismic monitoring studies.

A period of intense seismicity started more than a year prior to the 2021 Fagradalsfjall eruption in Iceland. During the same period, repeated cycles of surface uplift and subsidence were observed in the Svartsengi and Krýsuvík high-temperature (HT) fields, about 8-10 km west and east of the eruption site in Fagradalsfjall, respectively. Such an uplift has never been observed during 40 years of surface deformation monitoring of the exploited Svartsengi HT field. However, cycles of uplift followed by subsidence have been observed earlier at the unexploited Krýsuvík HT field.Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and ÍSOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.The data was used for multidisciplinary modelling of the pre-eruption processes (see Flóvenz et al, 2022. Cyclical geothermal unrest as a precursor to Iceland's 2021 Fagradalsfjall eruption. Nature Geoscience (in revision)). It included a poroelastic model that explains the repeated uplift and subsidence cycles at the Svartsengi HT field, by cyclic fluid intrusions into a permeable aquifer at 4 km depth at the observed brittle-ductile transition (BDT). The model gives a total injected volume of 0.11±0.05 km3. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850±350 kg/m3. A high-resolution seismic catalogue of 39,500 events using the optical cable recordings was created, and the poroelastic model explains very well the observed spatiotemporal seismicity.The geodetic, gravity, and seismic data are explained by ingression of magmatic CO2 into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.The poroelastic model and the feeder-channel model are combined into a conceptual model that is consistent with the geochemical signature of the erupted magma. It explains the pre-eruption processes and gives estimates of the amount of magma involved.The conceptual model incorporates a magmatic reservoir at 15-20 km depth, fed by slowly upwelling currents of mantle derived magma. Volatiles released from inflowing enriched magma into the sub-Moho reservoir migrated upwards. The volatiles were possibly trapped for weeks or months at the BDT at ~7 km depth beneath Fagradalsfjall, generating overpressure, but not high enough to lift the overburden (~220 MPa) and cause surface deformation. After reaching a certain limiting overpressure, or when a certain volume had accumulated, the magmatic volatiles were diverted upwards, just below the BDT towards the hydrostatic pressurized aquifer (~ 40 MPa) at 4 km depth at the bottom of the convective HT fields. They passed through the BDT and increased the pressure sufficiently (>110 MPa) to cause the uplift.The lessons learned enlighten the most important factors to help detect precursory volcanic processes on the Reykjanes Peninsula; including detailed monitoring of seismicity, surface deformation, gravity changes and gas content in geothermal fluids. Furthermore, geophysical exploration of the deeper crust by seismic and resistivity measurements are crucial to map possible melt and possible pathways towards the surface.

Etna volcano in Italy is one of the most active volcanoes in Europe. We recorded the volcanic activity including degassing and vigorous strombolian activity using a seismometer and a rotational sensor in August to September 2019. We test the newly developed rotational sensor in the field in comparison to the broadband seismometer and seismic-network-based locations using the INGV network. We demonstrate that a single rotational sensor co-located with a seismometer can be used to identify specific seismic wave types, to estimate the back azimuth of wave arrivals and the local seismic phase velocities.Using the rotational sensor, we easily detected the dominant SH-type waves composing volcanic tremor during weak volcanic activity and the recorded VLP/ LP events. Changes in the composition of the tremor wavefield caused by the onset of vigorous volcanic activity are obvious and can be detected in near real-time if data is streamed. We discuss the changes in the wavefield composition from SH-type waves to a mixed wavefield in the context of the volcanic activity, the back azimuth of the signals and associated phase velocities. Our findings are consistent with observations by INGV and hence the rotational sensor reliably enlarges our sensor portfolio in volcanic environments. In fact, wavefield and ground properties can be derived using just one sensor instead of a sensor network, which makes experiments in remote areas cheaper and easier to maintain. In addition, you can observe phenomena that otherwise go unnoticed, like near vent block rotation.

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.

From June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high-temperature zones at high resolution. The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlíð geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short-period and broadband stations. In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators. During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant. Here, we describe the network and the recorded datasets. Furthermore, we show some initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high-resolution event location to delineate faults and body- and surface-wave tomographies to image the subsurface velocity structure in great detail.

Since the nineteen-eighties, the depth, number, position and shape of the heat sources feeding the super-hot geothermal system of the Los Humeros caldera (Mexico) is under debate. With the aim of providing a seismic constraint of this region, we present a high resolution three-dimensional anisotropic shear wave velocity model. We used the continuous records of ambient seismic noise at 45 seismic stations deployed for more than one year. From the data, we obtained a set of about 1444 phase and 1534 group velocity dispersion curves, for Love and Rayleigh waves, respectively. By using the dispersion data, we obtained three dimensional models for shear wave velocities and radial anisotropy down to 17 km of depth. These models allowed to image three low velocity bodies at different depths: 1) a deep storage of potential molten material rising from the depth and stopping at ~11 km of depth, 2) a second storage region from 3 to 8 km of depth, related with the Los Humeros caldera collapse, and that feeds the 3) shallow low-velocity bodies between 1.3 and 2.2 km, representing the expression of the geothermal reservoir currently exploited. All these low velocity bodies are interconnected and represent part of the remnant magmatic plumbing system beneath the Los Humeros caldera. These seismic images provide insights to the deep upward pathways of fluids feeding the geothermal reservoir from the heat source(s) and its relationship with the structures controlling the hydrothermal system. Previous article in issue

Volcanic explosions produce energy that propagates both in the subsurface as seismic waves and in the atmosphere as acoustic waves. We analyse thousands of explosions which occurred at different craters at Etna volcano (Italy) in 2018 and 2019. We recorded signals from infrasound sensors, geophones (GPH), broadband seismometers (BB) and Distributed Acoustic Sensing (DAS) with fibre optic cable. The instruments were deployed at Piano delle Concazze at about 2 to 2.5 km from the active craters, within (or onto) a ~300,000 m2 scoria layer deposited by recent volcanic eruptions. The DAS interrogator was setup inside the Pizzi Deneri Volcanic Observatory (~2800 m elevation). Infrasonic explosion records span over a large range of pressure amplitudes with the largest one reaching 130 Pa (peak to peak), with an energy of ca. 2.5x1011 J. In the DAS and the BB records, we find a 4-s long seismic “low frequency” signal (1-2 Hz) corresponding to the seismic waves, followed by a 2-s long “high-frequency” signal (16-21 Hz), induced by the infrasound pressure pulse. The infrasound sensors contain a 1-2 Hz infrasound pulse, but surprisingly no high frequency signal. At locations where the scoria layer is very thin or even non-existent, this high frequency signal is absent from both DAS strain-rate records and BB/GPH velocity seismograms. These observations suggest that the scoria layer is excited by the infrasound pressure pulse, leading to the resonance of lose material above more competent substratum. We relate the high frequency resonance to the layer thickness. Multichannel Analysis of Surface Wave from jumps performed along the fibre optic cable provide the structure of the subsurface, and confirm thicknesses derived from the explosion analysis. As not all captured explosions led to the observation of these high frequency resonance, we systematically analyze the amplitudes of the incident pressure wave versus the recorded strain and find a non-linear relationship between the two. This non-linear behaviour is likely to be found at other explosive volcanoes. Furthermore, our observations suggest it might also be triggered by other atmospheric pressure sources, like thunderstorms. This analysis can lead to a better understanding of acoustic-to-seismic ground coupling and near-surface rock response from natural, but also anthropogenic sources, such as fireworks and gas explosions.

The Theistareykir geothermal field is located in North Iceland on the Mid-Atlantic ridge. A power plant produces 90 MWe using two 45 MWe turbines in operation since autumn 2017 and spring 2018, respectively. We performed hybrid microgravity measurements from 2017 to 2019 to monitor the short-term mass redistribution induced by geothermal production. Time-lapse microgravity surveys conducted each summer with a Scintrex CG5 gravimeter reveal the spatial gravity variations with respect to a reference, where the temporal gravity changes are monitored by absolute gravity measurements done with FG5#206 from Micro-g Solutions. In parallel, continuous gravity changes are recorded by a network of several GWR Instruments iGrav superconducting gravimeters and spring gravimeter, located in the injection and production areas. A height correction is applied to the gravity data using InSAR and GNSS measurements. We notice a regular residual gravity decrease in the production area versus a stable behaviour in the injection area. Time-lapse gravity measurements reveal a minimum residual decrease of − 38 ± 10 µGal (1 µGal = 10–8 m s−2) in 2019 with respect to 2017. Simplistic forward modelling of the produced geothermal fluid using a multiple Mogi sphere model can partly explain the residual gravity decrease. This suggest that a significant part of the injected geothermal fluid flows away, maybe drained by the Tjarnarás fault to the South where an increase of the water table level is observed. However, further modelling work is needed to confirm this.

One year of deformation and seismicity prior to a volcanic eruption in March 2021 at an oblique plate boundary in Iceland created a unique opportunity to study the interaction between upwelling magma and geothermal processes. We apply poroelastic modelling to explain satellite geodetic data showing three uplift and subsidence cycles at the Svartsengi geothermal field and use gravity data to constrain the density of intruded material. We use recordings on optical cable to generate a high-resolution earthquake catalogue and developed new waveform stacking and migration methods to detect and locate 39,500 earthquakes. The resulting model explains the geodetic, gravity, and seismic data by magmatic derived gas intruded into a horizontal sealed aquifer at 4 km depth in the roots of the geothermal field at the top of up-doming brittle-ductile boundary. The total injected volume is estimated 9.5·107m3 with optimal density of 840 kg/m3. Our results suggest upward migration of three packages of volcanic gas along the brittle-ductile boundary from a subcrustal magmatic source 8–10 km east of the geothermal field, with important implications for the dynamics leading to the eruption.

Continuous high-resolution gravimetry is increasingly used to monitor mass distribution changes in volcanic, hydrothermal or other complex geosystems. To quantify the often small target signals, gravity contributions from, e.g. atmospheric mass changes, global and local hydrology should be accounted for. We set up three iGrav superconducting gravity meters for continuous monitoring of the Þeistareykir geothermal field in North Island. Additionally, we installed a set of hydrometeorological sensors at each station for continuous observation of local pressure changes, soil moisture, snow and vertical surface displacement. We show that the contribution of these environmental parameters to the gravity signal does not exceed 10 µGal (1 µGal = 10–8 m s−2), mainly resulting from vertical displacement and snow accumulation. The seasonal gravity contributions (global atmosphere, local and global hydrology) are in the order of ± 2 µGal at each station. Using the environmental observations together with standard gravity corrections for instrumental drift and tidal effects, we comprehensively reduced the iGrav time-series. The gravity residuals were compared to groundwater level changes and geothermal mass flow rates (extraction and injection) of the Þeistareykir power plant. The direct response of the groundwater levels and a time-delayed response of the gravity signal to changes in extraction and injection suggest that the geothermal system is subject to a partially confined aquifer. Our observations indicate that a sustainable “equilibrium” state of the reservoir is reached at extraction flow rates below 240 kg s−1 and injection flow rates below 160 kg s−1. For a first-order approximation of the gravity contributions from extracted and injected masses, we applied a simplified forward gravity model. Comparison to the observed gravity signals suggest that most of the reinjected fluid is drained off through the nearby fracture system.

Understanding physical processes prior to and during volcanic eruptions has improved significantly in recent years. However, uncertainties about subsurface structures distorting observed signals and undetected processes within the volcano prevent volcanologists to infer subtle triggering mechanisms of volcanic phenomena. Here, we demonstrate that distributed acoustic sensing (DAS) with optical fibres allows us to identify volcanic events remotely and image hidden near-surface volcanic structural features. We detect and characterize strain signals associated with explosions and locate their origin using a 2D-template matching between picked and theoretical wave arrival times. We find evidence for non-linear grain interactions in a scoria layer of spatially variable thickness. We demonstrate that wavefield separation allows us to incrementally investigate the ground response to various excitation mechanisms. We identify very small volcanic events, which we relate to fluid migration and degassing. Those results provide the basis for improved volcano monitoring and hazard assessment using DAS.

Accurate timing of seismic records is essential for almost all applications in seismology. Wrong timing of the waveforms may result in incorrect Earth models and/or inaccurate earthquake locations. As such, it may render interpretations of underground processes incorrect. Ocean bottom seismometers (OBSs) experience clock drifts due to their inability to synchronize with a GNSS signal (with the correct reference time), since electromagnetic signals are unable to propagate efficiently in water. As OBSs generally operate in relatively stable ambient temperature, the timing deviation is usually assumed to be linear. Therefore, the time corrections can be estimated through GPS synchronization before deployment and after recovery of the instrument. However, if the instrument has run out of power prior to recovery (i.e., due to the battery being dead at the time of recovery), the timing error at the end of the deployment cannot be determined. In addition, the drift may not be linear, e.g., due to rapid temperature drop while the OBS sinks to the seabed. Here we present an algorithm that recovers the linear clock drift, as well as a potential timing error at the onset.The algorithm presented in this study exploits seismic interferometry (SI). Specifically, time-lapse (averaged) cross-correlations of ambient seismic noise are computed. As such, virtual-source responses, which are generally dominated by the recorded surface waves, are retrieved. These interferometric responses generate two virtual sources: a causal wave (arriving at a positive time) and an acausal wave (arriving at a negative time). Under favorable conditions, both interferometric responses approach the surface-wave part of the medium's Green's function. Therefore, it is possible to calculate the clock drift for each station by exploiting the time-symmetry between the causal and acausal waves. For this purpose, the clock drift is calculated by measuring the differential arrival times of the causal and acausal waves for a large number of receiver-receiver pairs and computing the drift by carrying-out a least-squares inversion. The methodology described is applied to time-lapse cross-correlations of ambient seismic noise recorded on and around the Reykjanes peninsula, SW Iceland. The stations used for the analysis were deployed in the context of IMAGE (Integrated Methods for Advanced Geothermal Exploration) and consisted of 30 on-land stations and 24 ocean bottom seismometers (OBSs). The seismic activity was recorded from spring 2014 until August 2015 on an area of around 100 km in diameter (from the tip of the Reykjanes peninsula).

The highly productive high temperature geothermal fields in Iceland are located within active volcanic systems on the plate boundaries. When an earthquake swarm or an unusual surface uplift or subsidence occur, it is important to assess the hazards and whether the unrest is triggered or controlled by volcanic or anthropogenic processes, or a combination of both.On January 22nd, 2020, a rapid, large-scale uplift (14 km x 12 km) started at the Svartsengi geothermal field on the plate boundary of the Reykjanes Peninsula, along with an intense earthquake swarm that began simultaneously about 3 km east of the centre of uplift. The centre of uplift was located about 1 km west of Mt. Thorbjörn, in the middle of the Svartsengi geothermal field, close to the reinjection wells. Over a period of 6 months, three such uplift cycles occurred, each lasting for several weeks and followed by periods of relatively rapid subsidence. The duration and timing of the uplift-subsidence cycles appears to follow a clear trend where the successive inflation episodes lasted longer but with lower inflation rate.The centres of uplift and the deflation cycles are the same and remained stationary. The accompanied intense earthquake swarms migrated along the 40 km long oblique plate boundary of the Reykjanes Peninsula, demonstrating a major plate tectonic event. The maximum depth of earthquakes was close to 4.5 km directly above the centre of uplift but extending to 6-7 km in the surroundings where the maximum magnitudes reached MW 4.8.A few weeks after the onset of the unrest, nine additional seismic stations were deployed to densify the local seismic network in place. In addition, complimentary data from an existing 21 km long fibre optics cable were used to monitor high-frequency linear strain rates. Both measures led to a significant improvement in the earthquake detection and location which predominantly occurred in swarms. Likewise, InSAR data analysis of temporal uplift cycles was performed, repeated gravity measurements at permanent sites were performed, and resistivity was remeasured at chosen sites. Multiple different elementary models were developed and tested to explain the cyclic excitation of the uplift, subsidence, and seismicity. While the individual unrest episodes might be controlled by possible magma intrusions into the lower crust, our favoured model explains the spatio-temporal pattern of ground uplift by the rise and diffusion of pore pressure in a 4-5 km deep geothermal aquifer. To distinguish between different models, we use multi-disciplinary geophysical datasets, such as deformation, seismicity, and gravity.

The Theistareykir geothermal field is located at the intersection between the active Northern Rift Zone and the active Tjörnes Fracture Zone in NE Iceland, and its study is of vital importance for further development of local and regional geothermal resources. Since autumn 2017, a seismic network consisting of 21 stations was deployed to monitor the high temperature Theistareykir geothermal field (Iceland). This seismic network belongs to a set of multiparameter networks installed to better understand the underlying structure and behavior of the geothermal reservoir under exploitation.In this framework, we use the continuous ambient noise seismic records between October 2017 and October 2019 to compute a 3D shear wave velocity model of the geothermal field and to detect possible stress changes due to the injection and production activities. We compute the phase auto- and cross-correlations of the vertical component recordings, measure the Rayleigh wave group velocity dispersion curves, and obtain 2D group velocity maps between 1 and 5 s. The 2D group-velocity maps are used to construct regionalized dispersion curves which are then inverted using a Neighborhood Algorithm to retrieve the 3D Vs model of Theistareykir. We observe various underground structures and identify the locations of possible magmatic or hydrothermal bodies in light of available and newly acquired geological and geophysical data. In addition, we analyze the short and long term temporal evolution of the phase auto-correlations using coda wave interferometry and discuss their relationship to the geothermal field operations. We notice a slightly stronger velocity reduction around the production site in comparison to the surrounding regions.

Fiber-optic cables form an integral part of modern telecommunications infrastructure and are ubiquitous in particular in regions where dedicated seismic instrumentation is traditionally sparse or lacking entirely. Fiber-optic seismology promises to enable affordable and time-extended observations of earth and environmental processes at an unprecedented temporal and spatial resolution. The method’s unique potential for combined large-N and large-T observations implies intriguing opportunities but also significant challenges in terms of data storage, data handling and computation.Our goal is to enable real-time data enhancement, rapid signal detection and wave field characterization without the need for time-demanding user interaction. We therefore combine coherent wave field analysis, an optics-inspired processing framework developed in controlled-source seismology, with state-of-the-art deep convolutional neural network (CNN) architectures commonly used in visual perception. While conventional deep learning strategies have to rely on manually labeled or purely synthetic training datasets, coherent wave field analysis labels field data based on physical principles and enables large-scale and purely data-driven training of the CNN models. The shear amount of data already recorded in various settings makes artificial data generation by numerical modeling superfluous – a task that is often constrained by incomplete knowledge of the embedding medium and an insufficient description of processes at or close to the surface, which are challenging to capture in integrated simulations.Applications to extensive field datasets acquired with dark-fiber infrastructure at a geothermal field in SW Iceland and in a town at the flank of Mt Etna, Italy, reveal that the suggested framework generalizes well across different observational scales and environments, and sheds new light on the origin of a broad range of physically distinct wave fields that can be sensed with fiber-optic technology. Owing to the real-time applicability with affordable computing infrastructure, our analysis lends itself well to rapid on-the-fly data enhancement, wave field separation and compression strategies, thereby promising to have a positive impact on the full processing chain currently in use in fiber-optic seismology.

The field of rotational seismology has only recently emerged. Portable 3 component rotational sensors are commercially available since a few years which opens the pathway for a first use in volcano-seismology. The combination of rotational and translational components of the wavefield allows identifying and filtering for specific seismic wave types, estimating the back azimuth of an earthquake, and calculating local seismic phase velocities.Our work focuses on back-azimuth calculations of volcano-tectonic and long-period events detected at Etna volcano in Italy. Therefore, a continuous full seismic wavefield of 30 days was recorded by a BlueSeis-3A, the first portable rotational sensor, and a broadband Trillium Compact seismometer located next to each other at Mount Etna in August and September of 2019. In this study, we applied two methods for back-azimuth calculations. The first one is based on the similarity of the vertical rotation rate to the horizontal acceleration and the second one uses a polarization analysis from the two horizontal components of the rotation rate. The estimated back-azimuths for volcano-tectonic events were compared to theoretical back-azimuths based on the INGV event catalog and the long-period event back-azimuths were analyzed for their dominant directions. We discuss the quality of our back azimuths with respect to event locations and evaluate the sensitivity and benefits of the rotational sensor focusing on volcano-seismic events on Etna regarding the signal to noise ratios, locations, distances, and magnitudes.

For the first time, a 6-km long fiber-optic strain cable was deployed across an active fault on the seafloor with the aim to monitor possible tectonic movement using laser reflectometry, 25 km offshore Catania Sicily (an urban area of 1 million people). Brillouin Optical Time Domain Reflectometry (BOTDR) is commonly used for structural health monitoring (bridges, dams, etc.) and under ideal conditions, can measure small strains (10-6) along a fiber-optic cable, across very large distances (10 - 200 km), with a spatial resolution of 10 - 50 m. The FocusX1 expedition, (6-21 October 2020) onboard the R/V Pourquoi Pas? was the first experiment of the European funded FOCUS project (ERC Advanced Grant). We first performed micro-bathymetric mapping and a video camera survey using the ROV Victor6000 to select the best path for the cable track and for deployment sites for eight seafloor geodetic stations. Next we connected a custom designed 6-km long fiber-optic cable (manufactured by Nexans Norway) to the TSS (Test Site South) seafloor observatory in 2100 m water depth operated by INFN-LNS (Italian National Physics Institute) via a new Y-junction frame and cable-end module. Cable deployment was performed by means of a deep-water cable-laying system with an integrated plow (updated Deep Sea Net design Ifremer, Toulon) to bury the cable 20 cm in the soft sediments in order to increase coupling between the cable and the seafloor. The cable track crosses the North Alfeo Fault at four locations. Laser reflectometry measurements began on 18 October 2020 and are being calibrated by a 3 - 4 year deployment of eight seafloor geodetic instruments (Canopus acoustic beacons manufactured by iXblue) deployed on 15 October 2020. During a future marine expedition, tentatively scheduled for early 2022 (FocusX2) a passive seismological experiment is planned to record regional seismicity. This will involve deployment of a temporary network of Ocean Bottom Seismometers (OBS) on the seafloor and seismic stations on land, supplemented by INGV permanent land stations. The simultaneous use of laser reflectometry, seafloor geodetic stations as well as seismological land and sea stations will provide an integrated system for monitoring a wide range of slipping event types along the North Alfeo Fault (e.g. - creep, slow-slip, rupture). A long-term goal of the project is the development of dual-use telecom cables with industry partners.

Geysers are characterized by regular eruptions of hot water fountains. Their internal system consists of a heat source at depth, an often complex crack system and a conduit linking it to the surface. The conduit and crack system is filled with water, steam and gases similar to a volcano. Bubble traps are sometimes and rarely mapped and alternative heat-driven models for geyser eruptions exist.Using a multidisciplinary, dense and close network of video cameras, seismometers, water pressure sensors and a tiltmeter we studied pool geyser Strokkur in June 2018. These multidisciplinary observations and particle-motion based tremor locations enabled us to derive a schematic cross section describing the driving mechanisms and the fluid dynamic processes within the bubble trap, crack system and conduit. We imaged a bubble trap at 23.7+-4.4 m depth, 13 to 23 m west of the conduit. We divide the eruptive cycle into eruption, refilling of the conduit, gas accumulation in the bubble trap and a trail of bubbles from the bubble trap into the conduit where they collapse at depth and have gained novel insights in understanding the gas accumulation, migration and collapse in such hot geyser systems in different phases of the eruptive cycle.The dataset of this experiment can be accessed here:- Eibl, E. P. S., Müller, D., Allahbakhshi, M., Walter, T. R., Jousset, P., Hersir, G. P., Dahm, T., (2020) ' Multidisciplinary dataset at the Strokkur Geyser, Iceland, allows to study internal processes and to image a bubble trap.' GFZ Data Services. DOI: 10.5880/GFZ.2.1.2020.007- Eibl, E. P. S.; Walter, T.; Jousset, P.; Dahm, T.; Allahbakhshi, M.; Müller, D.; Hersir, G.P. (2020): 1 year seismological experiment at Strokkur in 2017/18. GFZ Data Services. Other/Seismic Network. DOI:10.14470/2Y7562610816

We demonstrate the capability of Distributed Acoustic Sensing (DAS) in recording volcano related dynamic strain at Etna (Italy). In summer 2019, we gathered DAS measurements from a 1.5 km long fibre in a shallow trench and seismic records from a conventional dense array comprising 26 broadband sensors deployed in Piano delle Concazze close to the summit area. The multifaceted style of Etna activity during the acquisition period gives the extraordinary opportunity to record and detect tiny strain changes (few 10−8 strain) in correspondence with volcanic events. To validate the DAS strain measures, we explored array-derived methods to estimate strain changes from the seismic signals and to compare with strain DAS signals. A general good agreement was found between array-derived strain and DAS measures along the fibre optic cable. Short wavelength discrepancies correspond with fault zones, showing the potential of DAS in mapping local perturbations of the strain field, and thus site effect, due to small-scale heterogeneities in volcanic settings.

The plate boundary between the American and Eurasian plates runs in southwest Iceland along a 5-10 km wide seismicity zone on the Reykjanes Peninsula. There, tectonic spreading events take place as continuous seismic release and seismic episodes (swarms and individual large events) with recurrence interval of about 40 years and volcanic episodes at intervals of 800-1000 years. The crust in Reykjanes is, therefore, particularly thin and hot and geothermal energy is currently harnesses in two areas on the western part of the peninsula in Reykjanes and Svartsengi. Since January 2020, earthquake swarms with larger events up to M5.6 have been occurring frequently over the entire Reykjanes Peninsula, accompanied by unusual uplift (up to 12 cm) and subsidence cycles in the Svartsengi-Eldvörp fissure swarm. This raises the question whether we might be at the beginning of a new volcanic episode. In order to classify such processes at an early stage, multidisciplinary geophysical measurements are particularly valuable. The Icelandic Meteorological Office (IMO), University of Iceland as well as ISOR and several partners responded immediately after the unrest began. As soon as January 2020, GFZ proposed a rapid response field campaign (MAGIC: MultidisciplinAry imaGIng and Characterization of the magma/fluid reservoir beneath Svartsengi). Only one week after the uplift start and first earthquake swarm, we connected a Distributed Acoustic Sensing interrogator to a 21 km long telecommunication fibre optic cable which crosses the uplift and swarm area. In addition, while we complied to strict constraints due to the Covid-19 pandemic, the rapid response activities comprised deployment of several additional sensors including broadband seismology, rotational seismology and we performed repeated surveys including gas-, gravity-, electromagnetic-, airborne and ground magnetic- measurements. We present preliminary results from various techniques and discuss their role in discriminating different scenarios aiming at explaining the magma-tectonic unrest phase. In particular, we analyze how the combination of airborne snapshots of ground morphology can be combined with the high temporal and spatial resolution deformation fields along the fibre optic cable., EGUsphere

To determine crustal properties distribution, seismic source processes, and wave propagation mechanisms, the acquisition of dense seismic and ground motion datasets is required (e.g., Jousset et al. 2018). Seismic and ground-motion datasets are typically acquired measuring acceleration, velocity (e.g., geophones and broadband sensors), or position (e.g., GNSS sensors) with individual sensors located in a favorable geometry. A high-resolution image of the subsurface, thereby, requires a dense spatial coverage of sensors. In recent years, fiber optic sensor technologies have been increasingly used for seismic applications. Different acquisition strategies have been applied to measure the subsurface deformation: measurements at a single point using specially engineered fiber optic point sensors, quasi-distributed measurements along an optical fiber with an array of point sensors, or fully distributed, where deformation is detected using interaction of light with the molecular structure of the fiber itself. We will consider fully distributed technologies in the following, only.