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.

A comprehensive catalogue of historical earthquakes, with accurate epicentres and harmonised magnitudes is a crucial resource for seismic hazard mapping. Here we update and combine catalogues from several sources to compile a catalogue of earthquakes in and near Iceland, in the years 1900–2019. In particular the epicentres are based on local information, whereas the magnitudes are based on teleseismic observations, primarily from international on-line catalogues. The most reliable epicentre information comes from the catalogue of the Icelandic Meteorological Office, but this is complemented with information from several technical reports, scientific publications, and newspaper articles. The catalogue contains 1281 moment magnitude (MW) ≥ 4 events and the estimated completeness magnitude is MW 5.5 in the first years, going down to MW 4.5 for recent years. The largest magnitude is MW 7.0. Such merging of local data and teleseismic catalogues has not been done before for Icelandic earthquakes, and the result is an earthquake map with much more accurate locations than earlier maps. The catalogue also lists additional 5640 earthquakes on the Mid-Atlantic Ridge, north of 43°, with both epicentres and magnitudes determined teleseismically. When moment magnitudes are not available, proxy MW values are computed using chi-squared-regression, normally on the surface-wave magnitude, but exceptionally on the body-wave magnitude. Magnitudes MW ≥ 4.5 have associated uncertainty estimates. The actual combined seismic moment released in the Icelandic earthquakes is found to be consistent with the moment estimated using a simple plate motion model, indicating that the seismic activity of the catalogue period might be typical for any 120 year timespan. The catalogue is named ICEL-NMAR and it is available online on http://data.mendeley.com.