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.