Fault characterization using seismic and DAS recordings at the Hengill geothermal area, Iceland

The Hengill geothermal area is located in southwest Iceland on the plate boundary between the North American and Eurasian plates and is one of the most active seismic zones on the island with thousands of natural earthquakes per year. In addition, seismicity is induced due to active production and injection operations, including the two largest geothermal power plants in Iceland, Nesjavellir and Hellisheiði. In addition, this area is the next target region for the Iceland Deep Drilling Project (IDDP) in search for supercritical geothermal fluids. Detecting and imaging fault zones at high resolution is therefore an important contribution to evaluate the optimum drilling location. We analyze thousands of microseismic events in the area that occurred between December 2018 and August 2021. These events were recorded on different permanent and temporary seismometer networks in the area. In addition, we recorded distributed acoustic sensing (DAS) data along a 25 km long fiber optic telecommunication cable near the Nesjavellir geothermal power plant. We analyze event clustering with a waveform cross-correlation approach and find a clear spatial separation of event clusters delineating planar structures. Clusters experience different temporal evolutions where some develop steadily and others as sudden bursts. Spatial variations of Gutenberg’s b-value and event stress drops show patterns consistent with tomographic seismic velocity inversions. Furthermore, focal mechanisms indicate very consistent source mechanisms within selected event clusters. Along the fiber path, we study waveform characteristics, which correlate with mapped geological features.  Data segments that are recorded where the fiber crosses fault zones exhibit long-tailed codas that may indicate trapping of seismic energy in low-velocity zones around active faults.