The 2021 Fagradalsfjall dike intrusion marked the initiation of a new era of volcanism on Iceland’s Reykjanes Peninsula. In this study, we present a large automatic catalog consisting of more than 80,000 earthquake hypocenters spanning the full period of the dike intrusion, which were derived from seismic data recorded by a dense network of seismic stations. The 9 – 10 km long dike exhibits a two-segment geometry of similar lengths. Linear regression on a relatively relocated subset of over 12,000 earthquakes revealed a strike of 029° with a standard deviation of 2° in the southern segment, and 046° with a standard deviation of 1° in the northern segment of the dike. A total of 97 detailed fault plane solutions from relative relocations of selected subsets of events provide new insight into the controls on faulting, showing almost exclusively right-lateral strike-slip/oblique-slip faulting associated with the dike intrusion, and a lack of left-lateral strike-slip fault motion. The alignment of fault planes is consistent with the orientation of pre-existing fractures, within uncertainty estimates. In light of these observations, we conclude that the likelihood of faulting being related to classical dike tip fracture of new rock ahead of the dike tip is low. Instead, our preferred explanation for the dominant controlling factor on the orientation of dike-related faulting is the extensive network of pre-existing fractures formed by the active transtensional plate boundary along the Reykjanes Peninsula.

Microearthquakes reveal the kinematics of the Bárðarbunga caldera ring fault; both during the 2014-2015 volcanic rifting event and gradual caldera collapse, and its subsequent, ongoing re-inflation. Manual analysis of earthquake phase arrivals has been used to produce reliable hypocenter locations with tightly constrained focal mechanisms for events both during and after the eruption. Phase arrival polarities are reversed between events that occurred during the caldera collapse and those that have occurred since. Both precise relative relocations of the seismicity and focal mechanism solutions confirm that this is due to slip reversal on the same ring fault structure. The fault planes are steeply dipping (averaging 78 ± 6°). Furthermore, the spatial distribution of aftershocks following large-magnitude post-eruptive events provides constraints on the shape and size of the fault plane and the amount of slip that typically occurs in caldera events.

The Icelandic crust is a product of its unique tectonic setting, where the interaction of an ascending mantle plume and the mid-Atlantic Ridge has caused elevated mantle melting, which has accreted and cooled in the crust to form an oceanic plateau. Here, we investigate the strength, orientation and distribution of seismic anisotropy in the upper crust of the Northern Volcanic Zone using local earthquake shear wave splitting, with a view to understanding how the contemporary stress field may influence sub-wavelength structure and processes. This is achieved using a dataset comprising >50,000 earthquakes located in the top 10 km of the crust, recorded by up to 70 stations over a 9 year period. We find that anisotropy is largely confined to the top 3–4 km of the crust, with an average delay time of 0.10 ± 0.08 s and an average orientation of the fast axis of anisotropy of N15° ± 33°E, which closely matches the spreading direction of the Eurasian and North American plates (~N16°E). These results are consistent with the presence of rift-parallel cracks that gradually close with depth, the preferential opening of which is controlled by the regional stress field. Lateral variations in the strength of shear wave anisotropy reveal that regions with the highest concentrations of earthquakes have the highest SWA values (~10%), which reflects the presence of significant brittle deformation. Disruption of the orientation of the fast axis of anisotropy around Askja volcano can be related to local stress changes caused by underlying magmatic processes.

Hydraulically-forced crevassing is thought to reduce the stability of ice shelves and ice sheets, affecting structural integrity and providing pathways for surface meltwater to the bed. It can cause ice shelves to collapse and ice sheets to accelerate into the ocean. However, direct observations of the hydraulically-forced crevassing process remain elusive. Here we report a new, novel method and observations that use icequakes to directly observe crevassing and determine the role of hydrofracture. Crevasse icequake depths from seismic observations are compared to a theoretically derived maximum-dry-crevasse-depth. We observe icequakes below this depth, suggesting hydrofracture. Furthermore, icequake source mechanisms provide insight into the fracture process, with predominantly opening cracks observed, which have opening volumes of tens to hundreds of cubic meters. Our method and findings provide a framework for studying a critical process, key for the stability of ice shelves and ice sheets, and hence rates of future sea-level rise.

Detecting and locating microearthquakes from continuous waveform records is the fundamental step in microseismic processing. Dense local networks and arrays have introduced the possibility to detect large numbers of far weaker events, but when viewed on seismic records from individual stations their waveforms are often obscured by noise. Furthermore, areas of interest for microseismic monitoring often feature extremely high event rates, highlighting the limitations of traditional techniques based on phase picking and association. In order to maximise the new insights gained, we require fully automated techniques which can exploit modern recordings to produce highly complete earthquake catalogues containing few artefacts. QuakeMigrate is a new modular, open-source python package providing a framework to efficiently, automatically and robustly detect and locate microseismicity. The user inputs continuous seismic data, a velocity model or pre-calculated look-up table and list of station locations. Instead of reducing the raw waveforms to discrete time picks, they are transformed (by amplitude, frequency and/or polarisation analysis) to continuous functions representing the probability of a particular phase arrival through time. These ‘onset functions’ from stations across the network are then migrated according to a travel-time look-up table and stacked to perform a grid-search for coherent sources of energy in the subsurface. This enables detection of earthquakes at close to or below the signal-to-noise ratio at individual stations, and implicitly associates phase arrivals even at very small inter-event times. We demonstrate the flexibility and power of this approach with examples of basal icequakes detected at the Rutford Ice Stream, Antarctica, dike- and caldera-collapse induced seismicity at Bárðarbunga central volcano, Iceland, and the aftershock sequence from a M5 earthquake at Mt. Kinabalu, North Borneo. The modular nature of the workflow and wide range of automatic plotting options makes parameter choice straightforward, and robust event location uncertainty statistics facilitate filtering to produce a robust catalogue. QuakeMigrate also outputs phase picks and local magnitude estimates, with an architecture designed to promote further community-driven extension in future.

Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect, reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strength.