<p>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 Kr&#253;suv&#237;k 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 Kr&#253;suv&#237;k HT field.</p><p>Shortly after the start of the unrest, a group of scientists from GFZ-Potsdam and &#205;SOR installed additional seismometers, used an optical telecommunication cable to monitor the seismicity and performed gravity measurements in the unrest area.</p><p>The data was used for multidisciplinary modelling of the pre-eruption processes (see Fl&#243;venz 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&#177;0.05 km<sup>3</sup>. Constraining the intruded material jointly by the deformation and gravity data results in a density of 850&#177;350 kg/m<sup>3</sup>. 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.</p><p>The geodetic, gravity, and seismic data are explained by ingression of magmatic CO<sub>2</sub> into the aquifer. To explain the behaviour of cyclic fluid injections, a physical feeder-channel model is proposed.</p><p>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.</p><p>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.</p><p>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.</p>
The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was followed by effusive lava outflow lasting six months. It was preceded by an intensive earthquake swarm lasting one month with the largest earthquake exceeding ML 5. We analyze seismic data recorded by the Reykjanet local seismic network to trace the processes leading to the eruption to understand the relation between seismic activity and magma accumulation. Precise relocations show two hypocenter clusters of the 2021 swarm in the depth range of 1-6 km; a NE-SW trending cluster that maps the dyke propagation, and a WSW-ENE trending cluster that follows the axis of the oblique plate boundary. Additionally, we relocated the preceding earthquake swarms of 2017, 2019 and 2020 and found that they form two branches along the oblique plate boundary, which coincide with the WSW-ENE trending cluster of the 2021 swarm. These branches form a stepover of ∼1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the place where the eruption occurred, suggesting that magma erupted at the place of crustal weakening. The strong seismic activity started with a ML 5.3 earthquake of 24 February 2021, which triggered the aftershocks on the oblique plate boundary and in the area of magmatic dyke, both in an area of elevated Coulomb stress. The seismicity shows a complex propagation of the dyke, which started at its northern end, migrated southwestward and then jumped back to the central part where the effusive eruption took place. The observed N-S striking focal mechanisms are interpreted as right-lateral antithetic Riedel shears that accommodate the left lateral slip along the oblique plate boundary. The co-existence of seismic and magmatic activity suggests that the past seismic activity weakened the crust in the eruption site area, where magma accumulated. The following ML 5.3 earthquake of 24 February 2021 triggered the seismic swarm and likely perturbed the magma pocket which led to the six-months lasting eruption that started on 19 March.
The geothermal system related to the Ngozi and Rungwe volcanoes, SW Tanzania, lies at the intersection of the west and east branches of the East African Rift System and has been investigated by many geoscientists for decades. Here we present a 3D electrical resistivity model based on 190 magnetotelluric resistivity soundings that have been integrated with geochemical and geological results to support the development of the geothermal resource conceptual model presented here. The model includes two separate reservoirs, a larger system located beneath the Rungwe volcano and a smaller chloride water reservoir located under the Ngozi caldera, which contains a neutral chloride hot spring with geothermometry >230°C. An extensive conductive clay cap with variable thickness extends along the 30 km long NW-SE trending Ngozi-Rungwe Fault Zone from the Kiejo area SE of the Rungwe volcano to the Ngozi caldera. The absence of geothermal surface manifestations directly over the inferred Rungwe upflow zone is consistent with effective sealing of the proposed underlying geothermal reservoir by the clay cap. The scarcity of thermal manifestations on the up-dip margins of the low-resistivity clay cap can be explained by coincidence of the base of the clay cap with impermeable Precambrian formations and by structural boundaries. This interpretation implies that the area with the highest geothermal resource potential is the Rungwe volcano where proposed drilling sites might intersect the proposed high-temperature reservoir.
<p>The Nesjavellir geothermal field in the Northeastern part of the Hengill central volcano, South West Iceland, has been exploited since 1990. Geothermal energy is currently produced by Reykjav&#237;k Energy (OR) at two power plants around Hengill, at Nesjavellir to the northeast and at Hellishei&#240;i to the southwest. Part of the surplus geothermal water from both plants goes into injection wells, and in analogy with the nearby Hellishei&#240;i power plant the re-injection of geothermal gases into basaltic formations is planned in Nesjavellir. Currently, a test of deep fluid injection is conducted in preparation of the experimental re-injection of carbon dioxide and hydrogen sulphide. The seismicity recorded in the study area is due to volcano-tectonic processes, natural geothermal activity as well as induced seismicity due to production and injection.</p><p>The aim of this work is to seismically image the production area of the Nesjavellir geothermal plant. Where the elastic properties of the propagation medium are investigated through the 3D and 4D seismic tomography and the b-value.</p><p>The available dataset in Nesjavellir consists of 6906 seismic events extracted from &#205;SOR&#8217;s catalogue, with local magnitude -0.8&#8804;M<sub>L</sub>&#8804;3.8 recorded between October 2016 and June 2020. The earthquakes were relocated in a 1D velocity model optimized for the area. We used tomographic method in which the P- and S-arrival times are simultaneously inverted for earthquakes location and velocity parameters estimation. Re-located earthquakes are further analysed to image the b-value in the investigated volume. Time variations of the seismic properties of the medium are observed in terms of V<sub>P</sub>, Vs and V<sub>P</sub>/Vs ratio obtained from the 4D tomography.</p><p>The results indicate that seismicity in Nesjavellir is mainly concentrated in three different clusters: two are located at shallow depths (1-2 km) while the third reaches down to 6 km depth. The three clusters of earthquakes are striking SW-NE and are all dipping to the west. Both the P- and S-velocity obtained models show lateral variation in E-W direction. A high V<sub>P</sub>/Vs ratio value is observed at shallow depths (due to low Vs values) and high V<sub>P</sub>/V<sub>S</sub> ratio is observed between 3.5 and 6 km depth (due to high V<sub>P</sub> and low Vs values). From the b-value mapping we observe low values (less than 1) at shallow depths and high values where the rate of small magnitude events is considerably higher. For each timestep we observe variations in V<sub>P</sub> and V<sub>S</sub> velocities that seem to be correlated with the fluids involved in field operation.</p><p>This work has been supported by the S4CE ("Science for Clean Energy") project, funded by the European Union&#8217;s Horizon 2020 - R&I Framework Programme, under grant agreement No 764810 and by PRIN-2017 MATISSE project, No 20177EPPN2, funded by the Italian Ministry of Education and Research.</p>
Timing errors are a notorious problem in seismic data acquisition and processing. We present a technique that allows such time shifts to be detected and corrected in a systematic fashion. The technique relies on virtual-source surface-wave responses retrieved through the crosscorrelation of ambient seismic noise. In particular, it relies on the theoretical time-symmetry of these time-averaged receiver-receiver crosscorrelations. By comparing the arrival time of the surface waves at positive time to the arrival time of the surface waves at negative time for a large a number of receiver-receiver pairs, relative timing errors can be determined in a least-squared sense. The time-symmetry of the receiver-receiver crosscorrelations, however, is contingent on a uniform surface-wave (noise) illumination pattern. In practice, the illumination pattern is often not uniform. We therefore show that weighting different receiver-receiver pairs differently in the inversion allows timing errors to be determined more accurately. The weights are based on the susceptibility of different receiver pairs to illumination-related travel-time errors. The proposed methodology is validated using both synthetic data and field data. The field data consists of recordings of ambient seismic noise by an array of stations centered around the tip of the Reykjanes peninsula, southwest Iceland (some of these stations exhibit time shifts of an unknown nature).
SUMMARY Ambient noise seismic tomography has proven to be an effective tool for subsurface imaging, particularly in volcanic regions such as the Reykjanes Peninsula (RP), SW Iceland, where ambient seismic noise is ideal with isotropic illumination. The primary purpose of this study is to obtain a reliable shear wave velocity model of the RP, to get a better understanding of the subsurface structure of the RP and how it relates to other geoscientific results. This is the first tomographic model of the RP which is based on both on- and off-shore seismic stations. We use the ambient seismic noise data and apply a novel algorithm called one-step 3-D transdimensional tomography. The main geological structures in the study area (i.e. covered by seismic stations) are the four NE–SW trending volcanic systems, orientated highly oblique to the plate spreading on the RP. These are from west to east; Reykjanes, Eldvörp-Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. Using surface waves retrieved from ambient noise recordings, we recovered a 3-D model of shear wave velocity. We observe low-velocity anomalies below these known high-temperature fields. The observed low-velocity anomalies below Reykjanes and Eldvörp-Svartsengi are significant but relatively small. The low-velocity anomaly observed below Krýsuvík is both larger and stronger, oriented near-perpendicular to the volcanic system, and coinciding well with a previously found low-resistivity anomaly. A low-velocity anomaly in the depth range of 5–8 km extends horizontally along the whole RP, but below the high-temperature fields, the onset of the velocity decrease is shallower, at around 3 km depth. This is in good agreement with the brittle–ductile transition zone on the RP. In considerably greater detail, our results confirm previous tomographic models obtained in the area. This study demonstrates the potential of the entirely data-driven, one-step 3-D transdimensional ambient noise tomography as a routine tomography tool and a complementary seismological tool for geothermal exploration, providing an enhanced understanding of the upper crustal structure of the RP.
<p>The Fagradalsfjall eruption on the Reykjanes peninsula, Iceland, lasted from 19 March to 18 September 2021. While it continuously effused lava at the beginning, it opened up 7 further vents in April and focused the activity from late April on Vent 5. Surprisingly the continuous effusion changed to pulses of lava effusion (as lava fountains or vigorous overflow) between 2 May and 14 June that was seismically recorded as tremor pulses. We examined the frequency of 6939 lava fountaining pulses based on seismological data recorded at NUPH at the SE corner of N&#250;pshl&#237;&#240;arh&#225;ls 5.5 km southeast of the active vent.</p><p>We subdivide the time period into 6 episodes based on sudden changes in the pattern. In this presentation we present the different fountaining patterns and systematic changes and discuss their origin. Our comparison with vent height, vent stability and lava effusion style, led us to conclude that the changes in the pulsing behaviour might be caused by collapses from the crater walls. The system is clearly unstable and evolving with time.</p>
<p>Since late 2019, the Reykjanes Peninsula in Iceland has experienced elevated seismic activity, which culminated in a dyke intrusion beneath Fagradalsfjall on 24<sup>th</sup> February 2021, and an eruption on 19<sup>th</sup> March. Seismic anisotropy &#8211; the directional dependence of seismic wave speed &#8211; can be used to study structural properties of the crust, which may be controlled by the state of stress through preferential closure of micro-cracks. This provides an opportunity to investigate changes in crustal stress regime caused by a dyke intrusion, with potential applications in eruption monitoring and forecasting.</p><p>&#160;</p><p>A dense seismic network spanning Fagradalsfjall recorded more than 130,000 earthquakes between June 2020 and August 2021; detected and located using QuakeMigrate<sup>1</sup>. From this dataset, we calculate the seismic anisotropy of the upper crust through shear-wave splitting analysis. Exceptional ray-path coverage allows for imaging at high spatial and temporal resolution. We present these results in relation to the regional stress regime and tectonic structure, and search for changes in anisotropy before, during, and after the dyke intrusion and eruption.</p><p>&#160;</p><p>1: Winder, T., Bacon, C., Smith, J., Hudson, T., Greenfield, T. and White, R., 2020. QuakeMigrate: a Modular, Open-Source Python Package for Automatic Earthquake Detection and Location. https://doi.org/10.1002/essoar.10505850.1</p>
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