Tectonic stress changes related to plate spreading prior to the 2021 Fagradalsfjall eruption in SW Iceland

The Reykjanes Peninsula in SW Iceland is a part of the Mid-Atlantic plate boundary. It forms its transtensional segment with several volcanic and faulting systems. We focus on the 2017 seismicity that occurred in the central part of Reykjanes at the place of Fagradalsfjall volcano prior to its eruption on March 19, 2021. We invert well-determined focal mechanisms of the 2017 seismicity and provide mapping of tectonic stress in space and time. Our results disclose heterogeneous stress field manifested by mix of shear, tensile and compressive fracturing.  Although the fracturing was diverse, directions of the principal stress axes were stable and consistent with the processes at the transtensional divergent plate boundary. The prominent stress direction was in the azimuth of 120°±8°, which represents the overall extension related to rifting in the Reykjanes Peninsula. The activity initiated on the transform fault segment with predominantly shear strike-slip events. The non-shear fractures occurred later being associated with normal dip-slips and corresponding to the opening of volcanic fissures trending in the azimuth of 30-35°, perpendicular to the extension. The dip-slips were mainly located above an aseismic dike detected in the centre of the 2017 swarm. This dike represents a zone of crustal weakening during a preparatory phase of future 2021 Fagradalsfjall volcanic eruption located at the same place. Moreover, we detected local variation of stress when the stress axes abruptly interchanged their directions in the individual stress domains. These stress changes are interpreted in a consequence of plate spreading and upcoming fluid flow during a preparatory phase of a rifting episode.