Earthquakes or nuclear explosions?: seismic clues to dirty deeds
Both earthquakes and nuclear explosions generate seismic waves that can be detected thousands of kilometres away. From the 1960s to the 1990s about one underground nuclear explosion was carried out each week. This was how nations tested their nuclear weapons and certified them as ready for deployment. Seismology was often the only way to learn if a foreign power was developing nuclear weapons.
Scientists pick up the seismic waves at monitoring stations. These seismic signals allow scientists to locate the explosion or earthquake and to tell the difference between the two. For example if seismic waves are generated very deep within the Earth then they can only be caused by an earthquake. Also explosions do not generate very strong seismic surface waves, therefore strong surface waves (the ones that cause the most damage to buildings) must come from an earthquake.
In China in April 1999 seismic waves were detected originating from 29 km below the surface. They originated so deep within the Earth that they could, therefore, only be from an earthquake. On the other hand in May 1998, an Indian nuclear test resulted in 72 different seismic signals at stations around the world. On the same day there were 57 other seismic events which scientists were able to distinguish from the nuclear test.
In 1996 the Comprehensive Test Ban Treaty (CTBT) outlawing nuclear weapon tests was signed by 150 nations, although by 1999 it had only been ratified by 19 of the 44 countries which had nuclear power reactors. When the CTBT eventually comes into force an International Data Centre (IDC) in Vienna will receive seismic data from 321 stations around the world. Another data centre in the USA will receive data from a network of hydroacoustic, infrasound and radionuclide sensors around the world.
It is not the responsibility of either the US or the Vienna data centres to determine whether detected events are explosions or earthquakes. They simply provide data to signatory countries for them to make their own analysis. Countries will be very careful to work out whether their neighbours are breaking the ban on nuclear testing. In the meantime these data can also help our understanding of the Earth and our efforts to predict earthquakes.