September 4, 2017
Ferenc Dalnoki-Veress
The following is an excerpt from a September 4, 2017 article for the BBC.
The nuclear test that North Korea conducted on Sunday is thought to be the biggest ever conducted by Pyongyang. But what does this really mean and how will we find out more about the bomb? Physicist Ferenc Dalnoki-Veress explains.
A nuclear explosion is an extremely large explosion, so large that it shakes the ground just as an earthquake does and is detected by seismic sensors thousands of kilometres away.
The magnitude of the shaking is a measure of the immense energy released by the event. A parameter known as the body-wave magnitude (Mb) is used.
This is not a linear scale. A magnitude-6 event, for example, releases 30 times more energy than one of magnitude 5.
In all, 34 stations that are part of the Comprehensive Test Ban Treaty Organisation’s (CTBTO) vast seismic monitoring network detected North Korea’s explosion and it was so intense that it actually “saturated” the detectors. In other words for this monitoring network, which is sensitive to extremely small nuclear test explosions, this test was so high it essentially went off scale.
There have been widely differing calculations of the power of this blast, ranging from 50-150 kilotonnes. The force is measured in kilotonnes to indicate what would happen if one kilotonne of TNT was exploded.
The yields predicted so far vary because it depends on the precise formula used: which scaling relation of the yield as a function of body wave magnitude is used – and that depends on a variety of factors such as the depth and type of rock where the test was conducted, for example.
A recent scaling equation takes into account the depth at which an explosion took place. This was developed by Miao Zhang and Lianxing Wen from the University of Science and Technology of China and Stony Brook and is appropriate for North Korea.
Read the full article at the BBC
