Subsurface Imaging of Geothermal Systems: Insights from a Single Earthquake Record

Distributed Acoustic Sensing (DAS) applied to existing fibre-optic networks enables continuous, high-resolution strain measurements over tens of kilometres, making it possible to image hidden subsurface features. Here we present an initial analysis of passive seismic strain data acquired along a dark fibre in rural northern Iceland, where the cable crosses multiple geothermal systems. The setup allows us to illuminate the geometry of structures that otherwise show little to no surface expression and remain poorly constrained at depth. 

Our study focuses on the strain response to a near-regional M5.0 earthquake, using the dense spatial sampling of DAS to extract structural information along the fibre. Localized strain amplification is observed above the geothermal areas, while autocorrelation analysis reveals reflected arrivals consistent with subsurface layering. Combined with distinct resonance frequencies along the same segments, these observations suggest sediment-filled basin geometries. Records of scattered surface waves and their migration reveal fault and strong scattering zones, indicating highly fractured regions that may provide permeable pathways for fluid flow within the geothermal systems. 

These findings demonstrate that a single earthquake record can be sufficient to extract detailed structural information using DAS, complementing noise-based and other conventional geophysical imaging techniques. The approach builds on existing telecommunication infrastructure and is therefore cost- and labour-effective. Repeated analysis of future seismic events could enable temporal monitoring of reservoir evolution and fluid migration in geothermal and volcanic systems.