Seismic analysis of the Nord Stream underwater explosions

On 26 September 2022 the seismic networks in northern Europe detected two seismic events near the Danish island of Bornholm in the Baltic Sea. Soon after, pressure drops were reported on three out of four strings of the Nord Stream 1 and 2 pipelines, and the Swedish and Danish Maritime Authorities reported four sea surface gas leaks. The automatic seismic locations were within 6 - 9 km of the surface leaks, indicating a causal relationship. Subsequent analysis of the seismic data showed that the first event occurred at 00:03:25 UTC southeast of Bornholm (event SE). Auto-correlation and three-component analysis of the second event, together with careful manual inspection, revealed that the event consists of three separate blasts (NE-A, NE-B, NE-C), located northeast of Bornholm and occurring in approximately seven second intervals, with the first at 17:03:50 UTC. Absolute location places the seismic events within a few kilometers of the observed surface leaks, which are within the uncertainty ellipses. Relative location of the three NE events indicate that NE-A and NE-B occurred approximately 220 m apart in a NW-SE direction whereas NE-C most probably was located about 6 km to the southwest of NE-A. Anchoring NE-A to the large surface leak on the Nord Stream 1 pipeline in the Swedish economic zone (SEZ) implies that NE-A and NE-B occurred on the two Nord Stream 1 strings and that NE-C occurred on one of the Nord Stream 1 strings in the Danish EZ. The relative location analysis suggests that none of the three observed NE seismic events are associated with the observed surface leak on Nord Stream 2 in the SEZ. The data indicate a local magnitude (ML) of 1.9 - 2.2 for the SE event and 2.3 - 3.0 for the combined NE events. Using existing relations between ML and yield for underwater explosions, and amplitude data from other blasts in the south Baltic with known yield, would indicate yields of 100 - 200 kg TNT equivalents for SE and 200 - 400 kg TNT for the combined NE events. Although the waveforms exhibit properties of underwater blasts, the early P-phases do not show spectral peaks from bubble oscillations at frequencies predicted by the inferred yields. Newly developed models of high pressure gas emission from a damaged pipeline combined with an explosive source indicate that the pipelines were targeted with explosives of less than 50 kg TNT equivalent.