Modelling of earthquake swarms suggests magmatic fluids in the upper crust beneath the Eger Rift

Abstract Earthquake swarms are enigmatic seismic phenomena that occur across diverse tectonic settings, from volcanic to intraplate regions, and are often associated with fluid migration, magmatic activity, or stress redistribution. The Northwest Bohemia/Vogtland region is a globally recognized hotspot and natural laboratory for such activity. This study examines the recent activation of an earthquake swarm in the region after a century of dormancy. By integrating high-resolution seismicity patterns and earthquake source mechanisms with models of fluid migration, we provide a detailed reconstruction of the swarm’s temporal and spatial evolution. Our seismicity modeling suggests the activation of a pre-existing fault by natural hydro-fracturing and hydro-shearing under the influence of ascending magmatic fluids, beginning with a high-pressure CO 2 -rich fluid intrusion, followed by transitions to low-pressure hydro-shearing on the fault, likely associated with CO 2 -magma mixtures. Our results emphasize the pivotal role of fluid overpressure and fault zone weakening in controlling swarm dynamics.