A multi-reference event approach to earthquake relocation: insights from the West Bohemia Swarm Zone (Czech Republic, 2000–2025)

Abstract

The intraplate West Bohemia region represents one of the most prominent areas of recurrent earthquake swarm activity in Central Europe, with ongoing seismicity observed through 2025. We relocated this activity using the new multi-reference event (MRE) method and compared the results with GrowClust (GC) relative locations. The MRE approach provides precise relative locations even for small events and can be applied efficiently to long-term, large catalogs. Its key advantage lies in the ability to incrementally incorporate new events without reprocessing the entire dataset, making it suitable for continuous and real-time monitoring. The comparison demonstrates that the newly developed MRE approach produces results that are consistent with those of the established GC method. Both methods produce a consistent image of the southern and northern parts of the swarm, while the central segment of the active Nový Kostel seismic zone shows the strongest differences. The MRE results are sensitive to the quality of reference event locations, occasionally leading to scattered clusters where input locations are less reliable. In contrast, GC systematically sharpens the seismicity distribution and produces more compact structures. Overall, these results provide the first comprehensive view of the relocated seismicity in the West Bohemia region, extended through 2025, and reveal characteristic migration trends in the activity.