Multiparapmeter Seismic Tomography Across High-Temperature Geothermal Field in Hengill (Iceland) Using a Large-N Nodal Array

The Hengill volcano system and its adjacent geothermal fields are Iceland's most productive harnessed high-temperature geothermal area. The geothermal resources are powered by cooling magmatic intrusions connected to three volcanic systems, with Hengill being the youngest that erupted ~2000 years ago. This area is located at a triple junction and has, therefore, a highly heterogeneous crustal structure in addition to a fissure swarm intersecting the Hengill volcanic center. Although explorations of Hengill began more than half a century ago, outstanding questions remain, such as the geophysical signature of super-critical fluid and a more detailed understanding of the underlying geothermal resources. Our particular focus in this work is to explore how high-resolution 3D isotropic and anisotropic seismic velocity models can help to address relevant questions in Hengill.

We perform ambient noise Rayleigh and Love wave imaging by combining a 498-node dense geophone array and a 44-station seismic backbone network. The backbone netowrk has ~2.5 years of data between late 2018 and 2021, with a >3 km station spacing and up to 40 km aperture. The dense array data, targeting provisioned geothermal subfields in the northern and southern parts of Hengill, was acquired through a 1–2 months campaign in the summer of 2021, with a <500 m station spacing and an aperture of 20 km. We demonstrate that, even with the shorter duration of data, the seismic imaging capacities are greatly enhanced in the top 5 km compared to the images retrieved from the backbone network alone. In addition to the shallow structure (<2 km), the dense sampling with good azimuthal coverage provides essential constraints on the deeper structure (>2 km). We observe a primary slow velocity anomaly at ~4 km depth which we associate with solidified magmatic intrusions. The trend of the anomaly is perpendicular to the NE striking Hengill fissure swarm, coinciding with a previously found deep lying low-resistivity anomaly. We find that most of the earthquake locations are near the margin of velocity contrasts, indicating a structure or permeability change in the subsurface.

From the anisotropy model, we observe a predominant fast direction along the vertical axis in the top 2 km of crust, implying an overall vertical crack formation resulting from extensional stress with ~10 mm/yr westward deformation. From 3 to 5 km depth, the fast direction transitions to the horizontal axis, broadly in agreement with intrusions or lava deposits. Around the same depth, the southwestern Hengill geothermal field remains in the vertically-fast direction. This area resides in the junction of distinct geologic, tectonic, and geodetic manifestations. We hypothesize that the extension of  vertically-oriented formation toward depth relates to crustal thinning and increasing permeability that promote one of the powerful boreholes in Hengill.