Integrating IMS data in the analysis of the Nord Stream underwater blasts in the Baltic Sea

On September 26, 2022 the International Monitoring System (IMS) of the CTBTO detected two seismic events near the Danish island of Bornholm as reported in the Reviewed Event Bulletin (REB). In the REB phases include both seismic and infrasound data up to 1600 km away and the events are located near the positions where large gas leaks from the Nord Stream pipelines were also observed. The events were recorded clearly by the Danish seismograph network with the closest two seismographs located on Bornholm at distances of 40 – and 70 km. Furthermore, seismographs in the surrounding countries picked up the signals and data were analysed at the national data centers. The waveforms exhibited clear properties of underwater blasts with significant P-energy and much smaller S-energy, as well as other characteristics not associated with natural earthquakes. Further analysis revealed that the second event may consist of multiple blasts close in time and space. Analysis of the events included integration between IMS data and regional data from Denmark, Sweden, Germany and Norway, communication with CTBTO experts, and served as an exercise in collaboration during an international crisis.