Since the first detonation of an atomic bomb in 1945, eight countries have conducted more than 2,000 nuclear weapons tests: the United States, the Soviet Union, the United Kingdom, France, China, India, Pakistan and North Korea.
Groups like the Comprehensive Nuclear Test Ban Treaty Organization are constantly on the lookout for new tests.
However, for security and secrecy reasons, modern nuclear tests are carried out underground, making them difficult to detect. Often the only indication that they have occurred are the seismic waves they generate.
In an article published in International Geophysical MagazineMy colleagues and I have developed a way to distinguish between underground nuclear tests and natural earthquakes with about 99 percent accuracy.
The invention of nuclear weapons sparked an international arms race, as the Soviet Union, the United Kingdom, and France developed and tested increasingly larger and more sophisticated devices in an attempt to keep up with the United States.
Many of the early tests caused serious environmental and social damage. For example, the United States’ Castle Bravo test in 1954, conducted secretly on Bikini Atoll in the Marshall Islands, dumped large volumes of radioactive fallout on several nearby islands and their inhabitants.
Between 1952 and 1957, the United Kingdom carried out several tests in Australia, spreading long-lived radioactive material over large areas of southern Australian bushland, with devastating consequences for local indigenous communities.
In 1963, the United States, the United Kingdom, and the USSR agreed to conduct future underground testing to limit radioactive fallout. However, testing continued unabated and China, India, Pakistan and North Korea also entered the picture over the following decades.
How to detect an atomic bomb
During this period there were significant international efforts to figure out how to monitor nuclear testing. The competitive nature of weapons development means that much research and testing is carried out in secret.
Groups such as the Comprehensive Nuclear Test Ban Treaty Organization today manage global networks of instruments designed specifically to identify potential tests. These include:
- Air testing stations to detect trace amounts of radioactive elements in the atmosphere.
- Aquatic listening posts to listen to underwater evidence.
- Infrasound detectors to capture the low-frequency pops and rumbles of explosions in the atmosphere.
- Seismometers to record Earth tremors caused by underground testing.
A needle in a haystack
Seismometers are designed to measure seismic waves: small vibrations of the ground surface generated when large amounts of energy are suddenly released underground, such as during earthquakes or nuclear explosions.
There are two main types of seismic waves. First are body waves, which travel outward in all directions, even deep into the Earth, before returning to the surface. Second are surface waves, which travel along the Earth’s surface like ripples spreading across a pond.
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The difficulty in using seismic waves to monitor underground nuclear testing is distinguishing between explosions and naturally occurring earthquakes. A key goal of monitoring is to never miss an explosion, but thousands of large natural earthquakes occur around the world every day.
As a result, monitoring underground testing is like searching for a potentially non-existent needle in a planet-sized haystack.
Nuclear weapons versus earthquakes
Many different methods have been developed to aid in this search over the past 60 years.
Some of the simplest include analyzing the location or depth of the source. If an event occurs far from volcanoes and tectonic plate boundaries, it could be considered more suspicious. Alternatively, if it occurs at a depth greater than, say, three kilometers, it is unlikely to have been a nuclear test.
However, these simple methods are not foolproof. Tests could be carried out, for example, in earthquake-prone areas to camouflage, and shallow earthquakes are also possible.
A more sophisticated monitoring method involves calculating the ratio between the amount of energy transmitted in body waves and the amount transported in surface waves. Earthquakes tend to expend more energy in surface waves than explosions.
This method has proven to be very effective in identifying underground nuclear tests, but it is also imperfect. It failed to effectively classify the 2017 North Korean nuclear test, which generated significant surface waves because it was carried out inside a mountain tunnel.
This result underscores the importance of using multiple independent discrimination techniques during follow-up: no single method is likely to be reliable for all events.
An alternative method
In 2023, my colleagues and I from the Australian National University and the Los Alamos National Laboratory in the United States will meet to reexamine the problem of determining the source of seismic waves.
We use a recently developed approach to represent how rocks move at the origin of a seismic event and combine it with a more advanced statistical model to describe different types of events. As a result, we were able to take advantage of fundamental differences between the sources of explosions and earthquakes to develop an improved method for classifying these events.
We tested our approach on catalogs of known explosions and earthquakes from the western United States and found that the method is right about 99% of the time. This makes it a useful new tool in efforts to monitor underground nuclear testing.
Robust techniques for nuclear test identification will continue to be a key component of global surveillance programs. They are critical to ensuring that governments are held accountable for the environmental and social impacts of nuclear weapons testing.
Mark Hoggard, DECRA Research Fellow, Australian National University
This article is republished from The Conversation under a Creative Commons license. Read the original article.