Seismological evidence for fluid accumulation beneath the dormant East Eifel volcanic region, Germany

Abstract The East Eifel region is a Cenozoic-Quaternary intraplate volcanic field, well known as the type locality for maar volcanism. Its most recent major eruption occurred at the Laacher See volcano about 13,000 years ago and is regarded as the largest late Quaternary volcanic eruption in central Europe. Despite its dormant state, ongoing regional mantle-derived CO 2 degassing and persistent microseismicity, particularly within the Ochtendung Seismic Zone, indicate active subsurface processes, likely related to fluid migration. Using the Eifel large-N seismic network, we recorded over 500 local microearthquakes from September 2022 to August 2023. Earthquakes within the Ochtendung Seismic Zone exhibit distinct secondary S-wave phases, indicating strong impedance contrasts associated with fluid accumulations within the crust. By extracting travel-time differences between direct and reflected S waves and applying an eikonal-based reverse-time-migration-like approach, we image localized fluid-rich zones at depths of 12–14 km beneath the microearthquakes, suggesting the accumulation of magmatic fluids aligned near the seismic zone. Microearthquakes within the Ochtendung Seismic Zone occur as burst-like sequences reflecting successive ruptures. We propose that the sustained microseismicity results from slip reactivation induced by transient fluid flux, representing an example of natural hydraulic fracturing.