Seismic activity adds to evidence that explosion caused Ukraine dam collapse

Evidence was mounting Friday to support the theory that the destruction of the Kakhovka dam — the cause of one of the biggest humanitarian and ecological crises since the start of Russia's 15-month invasion of Ukraine — was caused by explosive devices.

Seismic signals captured by geologists and testimony on social media suggest a huge ground-shaking noise was heard in the early hours of Tuesday, while structural engineering experts said it was highly unlikely that such an enormous structure could have spontaneously collapsed through negligence.

The dam has been under Russian control for months, but Moscow has rejected Kyiv’s accusation that it intentionally destroyed the dam and hydroelectric power plant and has instead accused Ukrainian artillery of causing the damage.

Researchers at the Norwegian seismological monitoring service NORSAR said in a statement Thursday that they detected multiple seismic events at the dam in the early hours of Tuesday, first at 2:35 a.m. local time (7:35 p.m. ET Monday) and then a much larger one at 2:54 a.m.

NORSAR said the data indicates the second event was an explosion equivalent to between 1 and 2 on the seismic chart for measuring earthquakes.

Social media groups apparently used by residents in Ukraine's southern Kherson region, where the dam is based, began to mention loud bangs slightly earlier than this.

“Damn... such [loud] explosions...” one person said in a Telegram group used by locals at 2:20 a.m. local time Tuesday (7:20 p.m. ET Monday). “And I hear it continuing,” someone replied 11 minutes later.

The dam had sustained damage at several points in the war — the Kherson region has been a focal point of a bitter struggle for territory in Ukraine's south. Ukraine retook the city of Kherson in November in jubilant scenes, but the left of the eastern bank of the Dnieper river, including the dam, has remained in Russian hands.

Andrew Barr, an expert in effects of blast damage on structures at the University of Sheffield in northern England, said that while the true cause of the dam's destruction was uncertain, an explosion was the likeliest cause.

“It’s conceivable that prior damage to the dam combined with record high water levels could precipitate a failure, but my assessment continues to be that a failure initiated by a large explosion is much more likely,” he said via email.

Col. Hamish de Bretton-Gordon, a British munitions expert and former army officer, also told British broadcaster Sky News that it was implausible for Ukraine to have destroyed the dam using artillery. (Sky News is owned by Comcast, the parent company of NBC News.)

“If it is true that the Ukrainians fired a HIMAR [rocket artillery] in November and took out one gate, they would have needed to fire 20-30 HIMARs to cause the damage we see here. But nobody is reporting that, there is no evidence to suggest that happened,” de Bretton-Gordon said.

“In order to create this much devastation, it must have been a very sophisticated bunch of explosives put in the right places. It was enough, and calculated enough, to destroy the whole dam,” he added.

Meanwhile, Ukraine’s domestic security service said Friday it had intercepted a telephone call proving a Russian “sabotage group” blew up the dam and hydroelectric station.

The Security Service of Ukraine posted 1 1/2-minute audio clip of the alleged conversation on its Telegram channel. It featured two men who appeared to be discussing the fallout from the disaster in Russian.

One says of the dam: “That was our sabotage group. They wanted, kind of to scare [blackmail] by the dam, but it went unplanned, more than they planned.”

NBC News could not independently verify the recording, and Russia did not comment on its content.

So far, 2,500 people, including 140 children, have been evacuated from flooded areas in Kherson, local Ukrainian officials said Friday afternoon. The threat of landmines, washed away from their marked positions, could pose a threat to communities for years to come, experts have warned.

Ukraine has begun criminal proceedings under the charge of ecocide against the perpetrators of the dam collapse.

This article was originally published on NBCNews.com