On September 26 last year seismologists detected two underwater explosions in the Baltic Sea. According to Norsar, a Norwegian monitoring organisation, the first occurred at 02.03 local time, about 25km southeast of Denmark’s Bornholm island. The second happened 17 hours later, some 50km northeast of Bornholm.
It soon became clear that the blasts had ruptured pipelines of the Nord Stream system laid beneath the sea to provide Germany with natural gas from Russia. But there was a puzzle. When investigators arrived at the location where seismologists had detected the second blast they found the pipes had been hit by three separate explosions, all in a relatively small area. So how was it that two of the explosions had not been reported by monitoring stations?
This is one question seismologists will be discussing in Vienna next month at the annual meeting of the European Geophysical Union (EGU).
Another question scheduled for discussion is whether initial estimates about the quantity of explosives used were accurate. The answer to that could be important. One theory is that the attacks were carried out by six people aboard a 15-metre yacht, the Andromeda. But given the small size of the boat, use of a very large quantity of explosives would tend to cast doubt on that scenario. Meanwhile, a very large quantity could point to involvement by a state actor
A statement from Norsar on the day of the attack said the first blast had a magnitude of 1.8 (categorised as a microearthquake) and equivalent to 190–320kg of TNT.
The second explosion detected was larger, with a magnitude of 2.2 or 2.3 which the seismologists described as corresponding to “a noticeable earthquake” and estimated as equivalent to 650–900 kg of TNT.
A few days later, German security officials told Der Spiegel they believed “massive explosive devices with the force of 500kg of TNT” had been used. They had reached this conclusion based on seismic date, Der Spiegel said, adding that “The strength of the explosive used also suggests that the attack was carried out by a state rather than a terrorist organisation.”
Denmark and Sweden were more vague about the quantity, saying in a joint report to the UN that the blasts were equivalent to the power of “several hundred kilograms of explosive”.
(“TNT equivalent” is a standard way of describing the energy released in an explosion, even if TNT is not used. Different types of explosive vary in their destructive power, so an explosion equivalent to 100kg of TNT would not necessarily mean 100kg of explosive was used: it could be more, or less.)
Norsar’s statement explained how it had calculated the TNT equivalence: “In order to get an estimate of the explosive power of the explosions, we have used a ratio between magnitude and charge weight derived from analyzes of previous underwater explosions with known charge weights. In addition, the relationship has been validated using data from previous military explosions in the North Sea.”
Highly pressurised gas
However, the Nord Stream explosions were not typical. They targeted pipes containing vast quantitities of highly pressurised gas and nobody is quite sure how a sudden release of gas caused by the explosion would affect the seismological data, including estimates of the TNT equivalent.
Nord Stream’s pipes were designed to withstand internal pressures of more than 170 bar (roughly 170 times normal atmospheric pressure). The pipelines were not operational at the time of the explosions, so the pressure is likely to have been lower — possibly in the region of 100 bar — but still very significant.
In response to questions sent by email, Norsar seismologist Ben Dando replied: “We are still actively investigating the Nord Stream explosions and we are working closely with European seismologists on this topic.”
He continued: “The influence of the pressurised gas is an important consideration. The explosive force (or TNT equivalent) that we state should not be interpreted as a definitive detonation charge size … the total energy released will include the degassing and depressurisation from the pipeline.”
One of the forthcoming papers for the Vienna conference decribes the problem of estimating the TNT equivalent of the explosions as “non-trivial”. Its abstract says: “The contribution to the seismic energy from suddenly outflowing methane gas is under investigation and results will be included in the presentation.”
Another paper, co-authored by Dando, may shed some light on the situation northeast of Bornholm where three explosions occurred but the initial seismological reports mentioned only one.
An obvious explanation might be that all three bombs were set to be detonated simultaneously by a single trigger signal, but that doesn’t quite fit the seismological evidence. Further analysis of the data appears to confirm that more than one explosion occurred, though they were not completely synchronised: there was a seven-second gap between two of them.
Seismological papers for next month’s Vienna conference (abstracts):
● Integrating IMS data in the analysis of the Nord Stream underwater blasts in the Baltic Sea
● The Nord Stream underwater explosions: location, classification and yield estimation
● Relative locations and moment tensors of the Nord Stream pipeline events
APRIL UPDATE: The conference papers mentioned above have not been published and the seismologists are now working on a joint paper which may take some time.