Long-period seismicity reveals magma pathways above a laterally propagating dyke during the 2014–15 Bárðarbunga rifting event, Iceland

The 2014-15 Bárðarbunga-Holuhraun rifting event comprised the best-monitored dyke intrusion to date and the largest eruption in Iceland in 230 years. A huge variety of seismicity was produced, including over 30,000 volcano-tectonic earthquakes (VTs) associated with the dyke propagation at ∼ 6 km depth below sea level, and large-magnitude earthquakes accompanying the collapse of Bárðarbunga caldera. We here study the long-period seismicity associated with the rifting event. We systematically detect and locate both long-period events (LPs) and tremor during the dyke propagation phase and the first week of the eruption. We identify clusters of highly similar, repetitive LPs, which have a peak frequency of ∼ 1 Hz and clear P and S phases followed by a long-duration coda. The source mechanisms are remarkably consistent between clusters and also fundamentally different to those of the VTs. We accurately locate LP clusters near each of three ice cauldrons (depressions formed by basal melting) that were observed on the surface of Dyngjujökull glacier above the path of the dyke. Most events are in the vicinity of the northernmost cauldron, at shallower depth than the VTs associated with lateral dyke propagation. At the two northerly cauldrons, periods of shallow seismic tremor following the clusters of LPs are also observed. Given that the LPs occur at ∼ 4 km depth and in swarms during times of dyke-stalling, we infer that they result from excitation of magmatic fluid-filled cavities and indicate magma ascent. We suggest that the tremor is the climax of the vertical melt movement, arising from either rapid, repeated excitation of the same LP cavities, or sub-glacial eruption processes. This long-period seismicity therefore represents magma pathways between the depth of the dyke-VT earthquakes and the surface. Notably, we do not detect tremor associated with each cauldron, despite melt reaching the base of the overlying ice cap, a concern for hazard forecasting

Melt movement through the Icelandic crust

We use both seismology and geobarometry to investigate the movement of melt through the volcanic crust of Iceland. We have captured melt in the act of moving within or through a series of sills ranging from the upper mantle to the shallow crust by the clusters of small earthquakes it produces as it forces it way upward. The melt is injected not just beneath the central volcanoes, but also at discrete locations along the rift zones and above the centre of the underlying mantle plume. We suggest that the high strain rates required to produce seismicity at depths of 10 − 25 km in a normally ductile part of the Icelandic crust are linked to the exsolution of carbon dioxide from the basaltic melts. The seismicity and geobarometry provide complementary information on the way that the melt moves through the crust, stalling and fractionating, and often freezing in one or more melt lenses on its way upwards: the seismicity shows what is happening instantaneously today, while the geobarometry gives constraints averaged over longer timescales on the depths of residence in the crust of melts prior to their eruption

The effect of wind and plume height reconstruction methods on the accuracy of simple plume models — a second look at the 2010 Eyjafjallajökull eruption

Real-time monitoring of volcanic ash plumes with the aim to estimate the mass eruption rate is crucial for predicting atmospheric ash concentration. Mass eruption rates are usually assessed by 0D and 1D plume models, which are fast and require only a few observational input parameters, often only the plume height. A model’s output, however, depends also on the plume height data handling strategy (sampling rate, gap reconstruction methods and statistical treatment), especially in long-term eruptions with incomplete plume height records. Representing such an eruption, we used Eyjafjallajökull 2010 to test the sensitivity of six simple and two explicitly wind-affected plume models against 22 data handling strategies. Based on photogrammetric measurements, the wind deflection of the plume was determined and used to re-calibrate radar height data. The resulting data was then subjected to different data handling strategies, before being used as input for the plume models. The model results were compared to the erupted mass measured on the ground, allowing us to assess the prediction accuracy of each combination of data handling strategy and model. Combinations that provide highest prediction accuracies vary, depending on data coverage, eruptive strength, and fragmentation style. However, for this type of moderate to weak eruption, the most important factor was found to be the prevailing windspeed. When windspeeds exceed 20 m/s, most combinations of strategies and models provide predictions that underestimate the erupted mass by more than 40%. Under such conditions, the optimal choice of data handling strategy and plume model is of particularly relevance.The geo-referencing and photo analysis was conducted under the EU Framework 7 FutureVolc project (2012–2016). This work contributes to project MAXI-Plume, supported by the Icelandic Research Fund (Rannís), grant Nr. 206527-051. TD was supported by the IRF (Rannís) Postdoctoral project grant 206527–051.Pre-print (óritrýnt handrit

Deep crustal melt plumbing of Bárðarbunga volcano, Iceland

Understanding magmatic plumbing within the Earth's crust is important for understanding volcanic systems and improving eruption forecasting. We discuss magma plumbing under Bárðarbunga volcano, Iceland, over a 4 year period encompassing the largest Icelandic eruption in 230 years. Microseismicity extends through the usually ductile region of the Earth's crust, from 7 to 22 km depth in a subvertical column. Moment tensor solutions for an example earthquake exhibits opening tensile crack behavior. This is consistent with the deep (>7 km) seismicity being caused by the movement of melt in the normally aseismic crust. The seismically inferred melt path from the mantle source is offset laterally from the center of the Bárðarbunga caldera by ~12 km, rather than lying directly beneath it. It is likely that an aseismic melt feed also exists directly beneath the caldera and is aseismic due to elevated temperatures and pervasive partial melt under the caldera

Seismic amplitude ratio analysis of the Bárðarbunga-Holuhraun dike propagation and eruption

Magma is transported in brittle rock through dikes and sills. This movement may be accompanied by the release of seismic energy that can be tracked from the Earth's surface. Locating dikes and deciphering their dynamics is therefore of prime importance in understanding and potentially forecasting volcanic eruptions. The Seismic Amplitude Ratio Analysis (SARA) method aims to track melt propagation using the amplitudes recorded across a seismic network without picking the arrival times of individual earthquake phases. This study validates this methodology by comparing SARA locations (filtered between 2 and 16 Hz) with the earthquake locations (same frequency band) recorded during the 2014–2015 Bár urn:x-wiley:jgrb:media:jgrb52508:jgrb52508-math-0003arbunga‐Holuhraun dike intrusion and eruption in Iceland. Integrating both approaches also provides the opportunity to investigate the spatiotemporal characteristics of magma migration during the dike intrusion and ensuing eruption. During the intrusion SARA locations correspond remarkably well to the locations of earthquakes. Several exceptions are, however, observed. (1) A low‐frequency signal was possibly associated with a subglacial eruption on 23 August. (2) A systematic retreat of the seismicity was also observed to the back of each active segment during stalled phases and was associated with a larger spatial extent of the seismic energy source. This behavior may be controlled by the dike's shape and/or by dike inflation. (3) During the eruption SARA locations consistently focused at the eruptive site. (4) Tremor‐rich signal close to ice cauldrons occurred on 3 September. This study demonstrates the power of the SARA methodology, provided robust site amplification; Quality Factors and seismic velocities are available

Deep crustal melt plumbing of Bárðarbunga volcano, Iceland

Understanding magmatic plumbing within the Earth’s crust is important for understanding volcanic systems and improving eruption forecasting. We discuss magma plumbing under Bárðarbunga volcano, Iceland, over a four-year period encompassing the largest Icelandic eruption in 230 years. Microseismicity extends through the usually ductile region of the Earth’s crust, from 7-22 km depth in a sub-vertical column. Moment tensor solutions for an example earthquake exhibits opening tensile crack behavior. This is consistent with the deep (> 7 km) seismicity being caused by the movement of melt in the normally aseismic crust. The seismically inferred melt path from the mantle source is offset laterally from the center of the Bárðarbunga caldera by ~12 km, rather than lying directly beneath it. It is likely that an aseismic melt feed also exists directly beneath the caldera and is aseismic due to elevated temperatures and pervasive partial melt under the caldera.Funding was by research grants from the NERC and the European Community’s Seventh Framework Program grant 308377 (Project FUTUREVOLC), and a number of graduate studentships from the NERC

Seismic imaging of the shallow crust beneath the Krafla central volcano, NE Iceland

We studied the seismic velocity structure beneath the Krafla central volcano, NE Iceland, by performing 3-D tomographic inversions of 1453 earthquakes recorded by a temporary local seismic network between 2009 and 2012. The seismicity is concentrated primarily around the Leirhnjúkur geothermal field near the center of the Krafla caldera. To obtain robust velocity models, we incorporated active seismic data from previous surveys. The Krafla central volcano has a relatively complex velocity structure with higher P wave velocities (V_p) underneath regions of higher topographic relief and two distinct low-V_p anomalies beneath the Leirhnjúkur geothermal field. The latter match well with two attenuating bodies inferred from S wave shadows during the Krafla rifting episode of 1974–1985. Within the Leirhnjúkur geothermalreservoir, we resolved a shallow (−0.5 to 0.5 km below sea level; bsl) region with low-V_p/V_s values and a deeper (0.5–1.5 km bsl) high-V_p/V_s zone. We interpret the difference in the velocity ratios of the two zones to be caused by higher rock porosities and crack densities in the shallow region and lower porosities and crack densities in the deeper region. A strong low-V_p/V_s anomaly underlies these zones, where a superheated steam zone within felsic rock overlies rhyolitic melt

Deep long period seismicity preceding and during the 2021 Fagradalsfjall eruption, Iceland

AbstractWe use a dense seismic network on the Reykjanes Peninsula, Iceland, to image a group of earthquakes at 10–12 km depth, 2 km north-east of 2021 Fagradalsfjall eruption site. These deep earthquakes have a lower frequency content compared to earthquakes located in the upper, brittle crust and are similar to deep long period (DLP) seismicity observed at other volcanoes in Iceland and around the world. We observed several swarms of DLP earthquakes between the start of the study period (June 2020) and the initiation of the 3-week-long dyke intrusion that preceded the eruption in March 2021. During the eruption, DLP earthquake swarms returned 1 km SW of their original location during periods when the discharge rate or fountaining style of the eruption changed. The DLP seismicity is therefore likely to be linked to the magma plumbing system beneath Fagradalsfjall. However, the DLP seismicity occurred ~ 5 km shallower than where petrological modelling places the near-Moho magma storage region in which the Fagradalsfjall lava was stored. We suggest that the DLP seismicity was triggered by the exsolution of CO2-rich fluids or the movement of magma at a barrier to the transport of melt in the lower crust. Increased flux through the magma plumbing system during the eruption likely adds to the complexity of the melt migration process, thus causing further DLP seismicity, despite a contemporaneous magma channel to the surface.</jats:p

Deep long period seismicity preceding and during the 2021 Fagradalsfjall eruption, Iceland

AbstractWe use a dense seismic network on the Reykjanes Peninsula, Iceland, to image a group of earthquakes at 10–12 km depth, 2 km north-east of 2021 Fagradalsfjall eruption site. These deep earthquakes have a lower frequency content compared to earthquakes located in the upper, brittle crust and are similar to deep long period (DLP) seismicity observed at other volcanoes in Iceland and around the world. We observed several swarms of DLP earthquakes between the start of the study period (June 2020) and the initiation of the 3-week-long dyke intrusion that preceded the eruption in March 2021. During the eruption, DLP earthquake swarms returned 1 km SW of their original location during periods when the discharge rate or fountaining style of the eruption changed. The DLP seismicity is therefore likely to be linked to the magma plumbing system beneath Fagradalsfjall. However, the DLP seismicity occurred ~ 5 km shallower than where petrological modelling places the near-Moho magma storage region in which the Fagradalsfjall lava was stored. We suggest that the DLP seismicity was triggered by the exsolution of CO2-rich fluids or the movement of magma at a barrier to the transport of melt in the lower crust. Increased flux through the magma plumbing system during the eruption likely adds to the complexity of the melt migration process, thus causing further DLP seismicity, despite a contemporaneous magma channel to the surface.</jats:p