US Geological Survey says equipment in Beijing often picks up mining blasts but not explosions, and estimates the Tianjin blasts as magnitude 2 or 3

An explosion in China that killed at least seven people and injured hundreds more was registered by the US Geological Survey as two small seismological actions followed by one big one at a seismometer station 160km (100 miles) away in Beijing.

The USGS equipment is meant to detect seismological activity like earthquakes, but also frequently picks up activity from mining blasts.

USGS geophysicist John Bellini said it is rare to detect seismological activity from other events, like the explosion in Tianjin. “Blasts that are not mine-related are rare to record, just because they don’t get transferred into the ground very well,” he said.

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Bellini said that it looks like multiple blasts were recorded at the Beijing monitoring facility, but the agency is not completely certain that the activity came from the explosion in Tianjin. The nature of the blast means that the seismological monitor does not give a completely accurate assessment of the explosion’s magnitude, he said.

“I can’t really give you an accurate magnitude picture – because it is one station and I don’t know how much air blast and how much of it is ground vibration, so I can’t give any precise magnitude measure for it,” Bellini said.

He did say that a safe estimate for the explosion is that it registered between a magnitude 2 and 3 on the Richter scale.

“That doesn’t accurately portray the amount of energy in the explosion, just because it isn’t transferred to the ground very well,” said Bellini.

Citing the verified Weibo account of the China Earthquake Networks Centre, AFP reported the magnitude of the first explosion was the equivalent of detonating three tons of TNT, while the second was the equivalent of detonating 21 tons of the explosive.