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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. [ABSTRACT FROM AUTHOR]

On 2008 May 29 an earthquake doublet shook the southwestern part of Iceland. The first main shock originated beneath Mt Ingólfsfjall, located near the western margin of the South Iceland Seismic Zone (SISZ) approximately 40 km east of the capital Reykjavík. Immediate aftershock activity was recorded by the SIL seismic network, operated by the Icelandic Meteorological Office (IMO), with both N–S and E–W structures illuminated over a broad area. A continuous GPS (CGPS) network, also operated by the IMO, recorded coseismic offsets with up to 200 mm of horizontal motion at the closest stations. We estimate the coseismic surface deformation observed by campaign and continuous GPS and satellite radar data (InSAR). We invert the geodetic data to find the optimal geometry, location and slip on the main faults, accounting for variation in the elastic parameters of the crust with depth. Our models indicate that most of the slip occurred on two N–S structures spaced ∼5 km apart. From a joint inversion of GPS and InSAR data for variable slip models we find that most of the slip for the first (Ingólfsfjall) event was concentrated at 2–4 km depth with a maximum of 1.9 m, whereas the slip on the second (Kross) fault was located deeper, at 3–6 km depth with up to 1.4 m of motion. The models give similar geodetic moments for the two main events, equivalent to a moment magnitude of and for the first and second event, respectively. Our estimated composite moment therefore equals a for the doublet, smaller than the estimated from teleseismic data (e.g. NEIC and Harvard). The geodetic data support rupture on two main faults and analysis of high-rate (1 Hz) CGPS data suggests that slip on the second fault initiated within 3 s of the first main shock. Static Coulomb failure stress calculations indicate that the first event caused a stress increase in the area of the main asperity (i.e. at the location of the largest slip patch) on the second fault. However, we cannot rule out dynamic stress triggering due to the short time between the two main events. The 2008 May 29 earthquake doublet appears to be a continuation of the earthquake sequence that started in 2000 June, when two events struck the eastern and central part of the South Iceland Seismic Zone, in the span of 81 hr. The 2000 June–2008 May sequence has released about half of the moment accumulated by plate motion since the previous earthquake sequence in 1896–1912. Therefore, continued earthquake activity with moderate size events rupturing N–S faults in the SISZ in the coming decades is likely. [ABSTRACT FROM AUTHOR]

We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion and opening of below a locking depth of . The resulting deep motion of in the direction of agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula. [ABSTRACT FROM AUTHOR]

Surface displacements induced by ice load variation through time are calculated by spatial integration of Green's function for both end-members: an elastic half-space and a thick elastic plate lying over an inviscid mantle. The elastic half-space model allows the consideration of displacements caused by short-term (seasonal) variations. The thick plate model describes the final relaxed state. The transition between these two stages is dominated by an effective relaxation time which depends on mantle viscosity. This behaviour is considered to estimate displacements induced by long-term load changes (ice retreat over decades). We apply these methods to the Mýrdalsjökull ice cap, Iceland, where an annual cycle in ice load occurs as well as a gradual ice retreat as a consequence of climate warming. Seasonal vertical displacements measured from 2000 to 2006 at two continuous GPS stations located near the edge of Mýrdalsjökull ice cap fit well to a model of an elastic response to the annual variation in ice load. A comparison of model displacements and observations provides a minimum value of for the effective static local value of the Young's modulus. We infer long-term displacements induced by ice retreat over the last 115 yr using a combination of the instantaneous elastic response and the final relaxed state. Results are compared to GPS measurements used to monitor the Katla volcano lying beneath the Mýrdalsjökull ice cap. A forward model considering an elastic thickness of 5 km can explain a fraction of the uplift recorded from 1999 to 2004, but it cannot account for the observed horizontal velocities. The study confirms that magma inflow is required to explain observed inflation of the Katla volcano 1999–2004. [ABSTRACT FROM AUTHOR]

Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation. • Measurements of surface deformation can contribute to the understanding of episodes of induced seismicity. • Initial pressure increase at the Húsmúli site due to fluid injection caused both deformation and seismicity. • Deformation indicates northward flow from the injection site, whereas the intended flow direction was northeastward. [ABSTRACT FROM AUTHOR]