Non-double-couple earthquakes in 2017 swarm in Reykjanes Peninsula, SW Iceland: Sensitive indicator of volcano-tectonic movements at slow-spreading rift

•Analysis of seismicity and moment tensors along transtensional rift in SW Iceland.•Details in non-double-couple ISO components indicate activation of fault segments.•They suggest interactions between tectonic and magmatic processes.•Non-DC components map tectonic processes in complex volcano-tectonic setting. The analysis of the 2017 earthquake swarm along the obliquely divergent Reykjanes Peninsula plate boundary revealed the most frequent focal mechanisms corresponding to main activated fault, which relates to transform faulting of the North Atlantic Rift in Iceland. Detailed double-difference locations, focal mechanisms and non-double-couple (non-DC) volumetric components of seismic moment tensors indicate an activation of three fault segments suggesting continuous interactions between tectonic and magmatic processes. They are related to inflation/deflation of a vertical magmatic dike and comprise: (1) shearing at strike-slip transform fault with left lateral motion; (2) collapses at normal faulting with negative volumetric components due to magma/fluid escape, and (3) shear-tensile opening at oblique strike-slip faulting with positive volumetric components connected to flow of trapped over-pressurized fluids. The identification of three regimes of complex volcano-tectonic evolution in divergent plate movement proves an enormous capability of the non-DC volumetric components to map tectonic processes in such settings.