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Earthquake study can inform us about nuclear tests

North Korea: What can earthquake science tell us about the nuclear test?, ABC, 4 Sept 17, The Conversation By Neil Wilkins, University of Bristol “……History of forensic seismology  The use of what’s called “forensic seismology” to detect and identify nuclear tests dates back almost to the birth of nuclear weapons themselves.

In 1946, the US conducted the first underwater test of a nuclear bomb at Bikini Atoll in the Pacific Ocean.

The shock waves created by the huge explosion were picked up at seismometers all over the world, and scientists realised that seismology could be used to monitor these kinds of tests.

In 1963, at the height of the Cold War, nuclear testing moved underground. The seismic waves from underground tests are more difficult to detect, because the shaking felt over such long distances is very small — only around one-millionth of a centimetre.

A seismic array is better able to pick out the small vibrations from a particular source than a single seismometer, and can also be used to work out with greater accuracy where the waves originally come from.

In 1996, the Comprehensive Test Ban Treaty(CTBT) was opened for signatures, aiming to ban all nuclear explosions.

To enforce this treaty, the Vienna-based CTBT Organisation is establishing an International Monitoring System with over 50 seismic monitoring stations to detect nuclear tests anywhere on Earth.

This system doesn’t just use seismometers.

Infrasound instruments listen for very low frequency sound waves, inaudible to the human ear, generated by potential nuclear explosions in the atmosphere; hydroacoustic instruments listen for sound waves travelling long distances through the oceans generated by underwater explosions, and radionuclide detectors “sniff out” radioactive gases released from a nuclear test site.

What do seismic monitors look for?

Any sort of earthquake or explosion, whether natural or man-made, produces different sorts of shock waves which travel through the Earth and can be detected by seismometers, which can measure very small ground movements…..

Distinguishing between earthquakes and explosions

There are a number of ways to do this. One is to measure the depth at which the earthquake occurred.

Even with modern drilling technology, it is only possible to place a nuclear device a few kilometres below the ground; if an earthquake occurs at a depth of more than 10 kilometres, we can be certain it is not a nuclear explosion.

Studies of the numerous nuclear tests that took place during the Cold War show that explosions generate larger P waves than S waves when compared with earthquakes.

Explosions also generate proportionally smaller Surface waves than P waves.

Seismologists can therefore compare the size of the different types of wave to try to determine whether the waves came from an explosion or a natural earthquake.

For cases like North Korea, which has carried out a sequence of nuclear tests since 2006, we can directly compare the shape of the waves recorded from each test.

As the tests were all conducted at sites within a few kilometres of each other, the waves have a similar shape, differing only in magnitude……http://www.abc.net.au/news/2017-09-04/north-korea-nuclear-test-what-earthquake-science-can-tell-us/8869328

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September 6, 2017 - Posted by | 2 WORLD, environment, weapons and war

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