Relative locations and moment tensors of the Nord Stream pipeline events

Two clear seismic events were observed on 26th September 2022 associated with the reported leaks from the Nord Stream 1 (Event 1, NE of Bornholm) and Nord Stream 2 pipelines (Event 2, SE of Bornholm). Arrivals of both events were detected and associated using data from several arrays in Norway and Finland, including the IMS stations NOA, FINES and ARCES. Additional signal analysis with data from the Swedish National Seismic Network and the Danish station on Bornholm enabled a third event to be identified. Auto-correlation analysis of the Event 2 revealed the third event (Event 2B) about 7 seconds after the main amplitude of the P onset (Event 2A). In contrast, for Event 1 SE of Bornholm no additional events could be identified from auto-correlation analysis, which increases confidence that these additional arrivals are not caused by interaction with geological structures. We also observe an arrival 7 s after the Pn phase before the Pg arrival on the NORES array. However, we cannot exclude that this onset interferes with the arrival of the PnPn phase. We then use the time differences between Event 2A and 2B measured by auto-correlation analysis on the Swedish and Danish network stations to determine relative epicentre locations. The results suggest that the two overlapping events occurred just about 220 m apart from each other. The relative locations fit very well with the distance between both pipelines of Nord Stream 1 at the Westernmost gas plume location (NE of Bornholm). We also estimated preliminary full moment tensors for Event 1 and 2 using seismic waveform data and analysed them on a source-type diagram. The results show positive isotropic parameters consistent with explosion-type mechanisms.