Fagradalsfjall Lava Fountain Acoustics the lava fountain sequence. Finally, we propose that higher acoustic amplitudes, in addition to a wider conduit in late May, indicates higher gas flux through the conduit culminating in shorter lava fountain events. This study highlights the value of deploying acoustic sensors for providing additional constraints on eruption dynamics and source parameters during effusive fissure eruptions in Iceland and elsewhere.

The massive worldwide deglaciation leads to more frequent slope instabilities in mountainous terrains. The physical processes leading to such destabilizations are poorly constrained due to little monitoring of dynamic parameters at the local scale. Here we study a very large slow‐moving landslide (∼0.8 km2), on the flank of Tungnakvíslarjökull glacier in Iceland. Based on a combination of remote sensing images, we monitor the landslide and glacier kinematics over 75 years, with a focus over the period 1999–2019 when rapid glacier wastage has been observed. The landslide accelerates from 2 to 45 m/yr in the 6 years following a sudden increase in glacier mass loss. This acceleration coincides with intense quake activity (Mℓ < 2.8), recorded by a regional seismic network. We show that this seismicity is caused by the landslide sliding on a rough surface. The evolution of the quake magnitudes suggests a progressive segmentation of the landslide mass during its acceleration.

The Reykjanes peninsula (RP) in SW Iceland is an oblique spreading rift marking the plate boundary between the North American and the Eurasian plates. The complex trans-tensional regime leads to intense episodes of seismicity (Einarsson, 1991). Several volcanic systems make up the RP, that is, Reykjanes (RVS) which last erupted in 1240 CE and Krýsuvík (KVS) (Clifton & Kattenhorn, 2006; Larsen & Guðmundsson, 2016;Saemundsson et al., 2020). The fissure swarms, associated with the volcanic systems, are characterized by normal faults, eruptive fissures, crater rows, and high-temperature geothermal fields (Figure 1). Mt. Þorbjörn is situated within the fissure swarm of Svartsengi, which is a geothermal area, and a part of RVS. Recent seismic tomography of Martins et al. ( 2020) also indicated possible presence of partially molten magma beneath Svartsengi at 3-6 km depth. In January 2020, surface deformation was detected by GPS and InSAR observations with the center of uplift located ∼2 km west of Mt. Þorbjörn, indicating magmatic inflation (Geirsson et al., 2020). This is the first time such a signal has been recorded in the Reykjanes volcanic system. The inflation continued until February 6, marking the end of this first intrusive event. Two

After more than a year of unrest, a small effusive eruption commenced in Fagradalsfjall, Iceland, on 19 March 2021. The eruption lasted six months. The first six weeks were characterized by multiple fissure openings, and the remainder was dominated by effusive activity from a single crater. During the eruption, lava and low-level gases propagated over the complex terrain: a hyaloclastite massif with mountain peaks up to about 350 m asl with valleys in between. The area is uninhabited, but easily accessible at about 30 km distance from Reykjavík. While the eruption was ongoing, more than 356,000 tourists visited the eruptive site. To maintain low risk access to the area, it was critical to monitor the eruption (including opening of new fissures) in real-time, forecast the transport of gas and lava flow emplacement, and assess the evolving hazards. In addition to data accessibility and interpretation, managing this volcanic crisis was possible thanks to strong collaboration between the scientific institutions and civil protection agencies. The eruption presented an opportunity to tune, test and validate a variety of numerical models for hazard assessment as well as to refine and improve the delivery of information to the general public, communities living near the eruption site and decision makers. The monitoring team worked long hours during both the pre- and syn-eruptive phases for identifying low risk access areas to the eruption site and to provide a regular flow of information. This paper reviews the eruption and its associated hazards. It also provides an overview of the monitoring setup, the adopted numerical tools and communication materials disseminated to the general public regarding current exclusion zones, hazards and possible future eruptive scenarios.

Entirely covered by the Vatnajökull ice cap, Grímsvötn is among Iceland’s largest and most hazardous volcanoes. Here we demonstrate that fiber-optic sensing technology deployed on a natural floating ice resonator can detect volcanic tremor at the level of few nanostrain/s, thereby enabling a new mode of subglacial volcano monitoring under harsh conditions. A 12.5 km long fiber-optic cable deployed on Grímsvötn in May 2021 revealed a high level of local earthquake activity, superimposed onto nearly monochromatic oscillations of the caldera. High data quality combined with dense spatial sampling identify these oscillations as flexural gravity wave resonance of the ice sheet that floats atop a subglacial lake. Although being affected by the ambient wavefield, the time–frequency characteristics of observed caldera resonance require the presence of an additional persistent driving force with temporal variations over several days, that is most plausibly explained in terms of low-frequency volcanic tremor. In addition to demonstrating the logistical feasibility of installing a large, high-quality fiber-optic sensing network in a subarctic environment, our experiment shows that ice sheet resonance may act as a natural amplifier of otherwise undetectable (volcanic) signals. This suggests that similar resonators might be used in a targeted fashion to improve monitoring of ice-covered volcanic systems.