Question: How do seismic waves differentiate between an earthquake and a nuclear test?
From the 1960s to the 1990s, about one underground nuclear explosion was carried out each week. These were detected by a network of seismometers.
An earthquake can generate four types of waves – Compressional and Shear waves penetrate the interior of the Earth without reaching the surface, and are called Body waves. Love and Rayleigh waves travel on the surface, and are called Surface waves.
Rayleigh waves are like the ripples when you throw a rock into a pond. Most of the shaking from an earthquake is due to Rayleigh waves, which can be much larger than the other waves. The average speeds of the waves is more or less known. Compressive waves arrive first, then Shear, then Love, and finally Rayleigh waves.
After an earthquake, the difference in arrival times of each wave at seismograph stations are used to find the epicentre.
Unlike an earthquake, an underground nuclear blast generates almost no Surface waves. They mostly generate Body waves that propagate in all directions, and can be traced to a small area, measured in metres.
This difference can be seen in seismograms. Those from an earthquake will have all four types of waves. An underground nuclear blast will show Body waves notably stronger than the Surface waves. For example, when both Body and Surface waves measure similar magnitude of 6.0 on the Richter scale, it probably indicates an earthquake. A Body wave magnitude of 6.0, with a Surface wave of magnitude of 4.0 suggests an underground explosion.
Another clue is that earthquakes originate from a deeper, broader area measured in kilometres, while a nuclear blast occurs closer to the surface.
In 2011, the Tōhoku earthquake had a magnitude of 9.1, releasing energy as much as 45,000 nuclear bombs. Its epicentre was about 70 kilometres off the coast of Japan, and its hypocentre was at a depth of about 29 kilometres.
There is an extensive network of seismometers across the globe and within minutes they can report on an underground nuclear test. This information is usually consistent, and can be corroborated with data from the network of hydroacoustic, infrasound and radionuclide sensors.
Response by: Associate Professor Behzad Fatahi, University of Technology Sydney
Brought to you by the Fuzzy Logic science show, 11am Sundays on 2XX 98.3FM. Send your questions to askfuzzy@zoho.com