Crustal Characterization of the Hengill Geothermal Fields: Insights From Isotropic and Anisotropic Seismic Noise Imaging Using a 500‐Node Array.

The Hengill volcano and its associated geothermal fields represent Iceland's most productive harnessed high‐temperature geothermal fields, where resources are fueled by cooling magmatic intrusions connected to three volcanic systems. The crustal structure in this area is highly heterogeneous and shaped by the intricate interplay between tectonic forces and magmatic/hydrothermal activities. This complexity makes detailed subsurface characterization challenging. In this study, we aim to push the current resolution limits using a 500‐node temporary seismic array and perform an isotropic and, for the first time, radially‐anisotropic velocity model of the area. The high‐resolution isotropic velocity model reveals the characteristic N30ºE fissure swarm that crosses the area within the top 500 m and outlines a deep‐seated low‐velocity body composed of cooling magmatic intrusions at 5 km depth. This deeper body is located near the eastern part of the three volcanic centers and connected to a shallower body at 2–3 km depth that strikes westward toward Hengill volcano. Additionally, our study discovered that non‐induced earthquakes deeper than 2 km align with velocity contrasts that reflect structural variability, indicating the potential to identify deep permeable pathways using dense array imaging. The anisotropic model indicates that the shallow crust of Hengill within the top 2 km is dominated by vertical fractures or cracks, likely attributed to overall divergent deformation from rifting in the study area. This characteristic is diminished at depths greater than 2–3 km, replaced by a layering pattern where the lava flows and/or subhorizontal intrusions become the primary factors influencing the observed anisotropy. Plain Language Summary: In this study, we explore the subsurface beneath Iceland's most productive geothermal fields, situated on the flanks of the Hengill volcano, around 30 km to the east of Reykjavik. Using 500 temporary seismic sensors, we use seismic waves to image the subsurface and unveil structures with unprecedented detail. We observe both shallow and deep structures within the top 5 km. Specifically, we identify a shallow crust that coincides with a fractured region that is observed on the surface, along with regions where the seismic waves travel slower and we interpret to be hot magmatic intrusions at 4–5 km depth that power the geothermal fields above. Additionally, we observe that natural seismicity tends to occur near velocity contrasts that likely indicate structural boundaries. We interpret this observation as potential permeable pathways where hot fluids ascend to the shallow crust. Furthermore, our study sheds light on the structural fabrics influenced by the drifting of the tectonic plates and the magmatic deposits as distinct layering at the base of volcanoes. Key Points: Dense array seismic noise imaging can resolve iso‐ and anisotropic structures beneath the Hengill geothermal fields in the top 5 km depth with unprecedented detailThe VVOIGT isotropic image delineates the shallow fractured crust, deep‐seated solidified magmas, and potential deep permeable pathwaysThe anisotropic image illuminates structural fabrics related to rifting and lithology in Hengill [ABSTRACT FROM AUTHOR]