On the challenge of constraining non-double couple moment tensor components, a case study of volcano-tectonic earthquakes in Iceland

This study addresses the challenge of reliably constraining non-double couple components (NDCCs) in moment tensor solutions for volcano-tectonic earthquakes in Iceland. While double-couple models adequately describe most global seismicity, Iceland's complex tectonic setting, featuring rifting and a hotspot, produces diverse seismic sources, some exhibiting significant NDCCs. These components, often dismissed as artifacts, may reflect actual source complexity. We analyze two recent volcanic events: seismicity related to the 2014-2015 Bárðarbunga caldera collapse and subsequent uplift, and the ongoing Reykjanes Peninsula unrest that started in 2021. Both events featured intense seismic swarms, with numerous moment tensors exhibiting pronounced NDCCs. However, the origin and interpretation of these components differ. At Bárðarbunga, CLVD components likely arise from inverting ring-fault geometries as point sources, while in Reykjanes, isotropic components may indicate magma intrusions. To assess the reliability of NDCCs, we conduct a rigorous uncertainty analysis of moment tensor solutions for both regions. This approach examines the stability of NDCCs under different inversion parameters and explores the pitfalls of constraining these components based on their potential causes. Our findings provide criteria for identifying reliable NDCCs and contribute to a better understanding of the limitations of tectonic interpretations based on moment tensor solutions in volcanic environments.