Understanding the Seismic Signature of Transtensional Opening in the Reykjanes Peninsula Rift Zone, SW Iceland

We analyze seismicity and centroid moment tensors (CMTs) on the Reykjanes Peninsula, Iceland, during the early phase of a widespread unrest period that led to multiple fissure eruptions between 2021 and 2024. We use a dense temporary seismic array, together with fiber‐optic distributed acoustic sensing data, and incorporate first‐motion polarities into the CMT inversion to improve accuracy, generating a total of 300 robust CMT solutions for magnitudes Mw>2.5 $Mw > 2.5$, focusing on 83 reliable Mw>2.7 $Mw > 2.7$ earthquakes for interpretation. The CMTs predominantly exhibit shallow strike‐slip faulting, with a few normal faulting events compatible with tectonic stress. Interestingly, significant positive isotropic components are resolved, contributing up to 15% of the moment release. We also develop a new high‐resolution seismic catalog of 34,407 events and show that larger shallow earthquakes at the plate boundary are preceded by the slow upward migration of microearthquakes from below, suggesting that intruding magmatic fluids interact with the oblique plate boundary to trigger slow slip events. We interpret our results as the seismic response to transtensional motion at the plate boundary in the brittle upper crust under shear, in response to stress changes induced by the intrusion of pressurized fluids in the lower crust. The complex interaction of multiple subparallel dikes with the plate boundary fault contributes to a broader deformation band that accommodates both tectonic and magmatic stresses. While the location and magnitude of the CMTs correlate with reactivated surface fractures and faults, the locations of intense, deep microseismic swarms indicate the sites of future fissure eruptions. Plain Language Summary Magma intrusions can affect the Earth's crust by producing surface deformation, perturbing stress conditions, and triggering seismicity. The combined study of microearthquake swarms and the source mechanism of larger earthquakes provides insights into the interactions between magmatic fluid intrusions and the surrounding rock, which are key to understanding magmatic processes. Strong, widespread, and persistent seismic activity occurs on the Reykjanes Peninsula, Iceland, prior to the Fagradalsfjall and Grindavík eruptions between 2021 and 2024. By taking advantage of the dense deployment of seismic stations and fiber‐optic cables, we are able to accurately resolve the mechanism of larger earthquakes at the plate boundary. The larger, sign