Microseismicity Reveals Fault Activation and Fluid Processes Beneath the Neuwied Basin and Laacher See Volcano, East Eifel, Germany

The Neuwied Basin within the East Eifel Volcanic Field (EEVF) is characterized by increased microseismicity, long hypothesized to be linked to the subsurface Ochtendung Fault Zone (OFZ). However, the source of this unrest remained elusive due to limited hypocenter resolution. Here, we present an extended local earthquake catalogue, compiled from a year-long Large-N deployment and a machine learning-based detection and location approach, including over 1,000 microearthquakes recorded between September 2022 and August 2023. This high-resolution dataset reveals new seismicity clusters, repeated waveforms, and distinct temporal bursts of activity, suggesting fluid-induced earthquake triggering. Probabilistic moment tensor inversion for 192 high-quality events (Mw 0.6-2.7) resolves predominantly strike-slip faulting along the OFZ, with localised clusters of normal faulting nearby, potentially associated with a previously unknown border fault of the NWB. Notably, we observe systematic rotations in P-axis orientations along the OFZ, which we interpret as localized stress perturbations induced by an overpressured reservoir beneath the Laacher See volcano - the youngest explosive eruption centre in the EEVF. These patterns, coupled with elevated magmatic CO2 emissions in the region and high waveform similarity, suggest that active magmatic and transcrustal fluid processes are influencing the stress regimes and driving the seismicity in the NWB. Our high-resolution seismicity and moment tensor catalogue offers new insights into the interplay between tectonics and fluid-driven processes beneath the youngest volcanoes in the EEVF.