A Shallow Reflector Beneath Krafla Volcano, NE Iceland, Detected With Seismic Migration of Local Earthquake Phases

This study explores passive reflection seismic imaging at Krafla volcano, NE Iceland. Krafla hosts a large geothermal area and is one of the few places on Earth where magma has been directly encountered through drilling, at depths of 2.1 km at the IDDP-1 borehole and 2.6 km at borehole KJ-39. These in-situ observations offer a unique opportunity to test seismic imaging in complex media, providing rare ground truth for evaluating imaging performance and resolution. We propose a new migration workflow for earthquake reflection imaging and apply it to local seismicity recorded by a dense array of short-period seismometers at Krafla. Additionally, by integrating synthetic data generated with SPECFEM2D, we benchmark our migration workflow and assess its limitations in the context of imaging with passive sources. Our results reveal a prominent reflector at 2.4 km depth in the area of the IDDP-1 borehole. Considering the uncertainties in earthquake locations, this reflector may correspond to the known magma body at IDDP-1 or outline a local brittle-ductile transition zone within the geothermal system. Key imaging challenges are related to the partially unknown source characteristics, the limited aperture of our station network, and the geological complexity of the subsurface. We show that, if not properly accounted for, these factors can contribute to imaging artifacts such as spurious reflectors. Our findings provide new insights into the subsurface structure of the Krafla geothermal system and highlight the potential of migration techniques for passive seismic imaging in complex, highly scattering environments.