Recent Emergence of Persistent, Slowly Migrating Seismic Swarms Across the Rhenish Massif

Abstract

Seismicity in the interrift zone between the Upper Rhine graben and the Lower Rhine Embayment has historically been dominated by the Ochtendung seismic zone near the Laacher See Volcano in the East Eifel. However, in recent years, persistent activity has emerged farther south. A seismic swarm initiated in 2018 in the Taunus region near Bad Schwalbach, followed in 2024 by a second swarm 50 km to the west in the Hunsrück region near Mörsdorf. Both sequences are still active. Relocated hypocenters reveal northwest–southeast elongated clusters that migrate systematically from ∼15 km depth toward a structural transition near 10 km, with apparent diffusivities of 0.04  m2/s (Mörsdorf) and 0.01  m2/s (Bad Schwalbach). Moment tensor solutions indicate normal faulting on planes rotated by ∼30° relative to the larger-scale hypocentral trend, suggesting seismic slip on secondary Riedel-type fractures within the damage zones of inherited Variscan faults. The combination of multiyear duration, slow migration, and a structural barrier locally breached by ML 1.5 events distinguishes these sequences from typical tectonic swarms and points to a sustained, deep-seated fluid supply, plausibly connected to the broader mantle-fluid system documented for the Eifel region.