Seismicity Patterns and Multi-Scale Imaging of Krafla, N–E Iceland, With Local Earthquake Tomography

Krafla, one of five central volcanoes of the Northern Volcanic Zone in Iceland, is utilized for geothermal energy production. Due to scientific and industrial interests, the volcano and its geothermal system have been imaged and monitored with various geophysical methods over the last decades leading to a better knowledge of its complex geological setting. Nonetheless, the unexpected encounter of magma at relatively shallow depths during drilling of the IDDP-1 well in 2009 proved that imaging small-scale structures remains challenging in such heterogeneous geological settings. With data from a local permanent 12 station seismic network owned by Landsvirkjun and operated by Iceland GeoSurvey since 2013, and a dense temporary network of 98 seismic nodes deployed for one month in 2022 in the center of Krafla caldera, we conducted a multi-scale analysis based on local earthquake tomography. This analysis enables us to identify small-scale velocity structures and improve earthquake locations. The newly obtained high-resolution 3D models for P- and S-wave velocities offer a glimpse into the subsurface structure of the volcanic system with both wave types being responsive to distinct rock/fluid properties. The relocated seismic activity highlights active structures pinpointed through the tomography, in particular the seismogenic zone at the boundary of high to low $V_p/V_s$ ratios, close to where magma was repeatedly encountered. By comparing the newly obtained high-resolution velocity models with available well log data, such as formation temperature, we aim to enhance the understanding of the interconnected volcanic and geothermal systems in areas lacking in situ measurements.