Combined (DAS/Seismometers) Seismic Analysis of an Underwater Explosion in the Baltic Sea

In November 2024, an Ml ~1.5-2.0 underwater explosion occurred in Danish waters north of Bornholm Island. It was recorded by the Danish and Greenlandic national seismic networks as well as the Swedish national network. In addition, the event was captured by a nearby Distributed Acoustic Sensing (DAS) system deployed along a 120 km-long underwater fiber-optic cable. We investigated the event location by integrating data from these complementary recording systems, systematically assessing the trade-off between the number of DAS channels with respect to the number of permanent seismometers and the quality of the picks. Our results indicate horizontal uncertainties on the order of 2–3 km for the final event location. To further constrain the depth of the explosion and its yield, we conducted a spectral analysis of the seismograms, joined with a non-linear inversion of the P-waveforms at the closest station. The inferred source parameters are consistent with the known water depth and velocity at the explosion site, revealing that the event probably involved two distinct detonations located less than 10 m above the seafloor, each with an approximate 30 kg TNT equivalent yield. These findings highlight the advantages of combining conventional permanent seismic instrumentation with underwater DAS, thereby improving the detection and characterization of anthropogenic seismic sources and offering enhanced protection for critical submarine infrastructures.