Comprehensive Nuclear-Test-Ban Treaty – a peace-keeping initiative with scientific impact

Any major shaking of the Earth can be recorded on a seismograph regardless of the nature of the source. Earthquakes and large explosions generate waves with similar frequency content. This fact has been used for decades to construct systems to monitor detonations of underground nuclear explosions. The quality of the monitoring system has increased significantly in recent years, and we demonstrate here that the data are useful in Danish earthquake research. One important difference between explosions and earthquakes is the depth of the source, most earthquakes occurring at much larger depths than explosions. Thus the depth determination is important in both fields. However, accurate depth determination of earthquake hypocentres is a challenge even when it comes to large well-recorded earthquakes. The uncertainty of the calculated depth of a Danish earthquake is of the same magnitude as the depth itself when using standard location techniques. A technique utilising crustal phases recorded at large distances has been introduced at the Geological Survey of Denmark and Greenland to improve the determination of the hypocentre depths. Only the largest earthquakes in Denmark and its immediate surroundings produce sufficiently strong signals to be recorded at teleseismic distances, i.e. larger than 3000 km. The signals are discernable at low-noise seismic array stations as far away as North America and Africa. Some of these stations are operated by the United Nations Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO). Data from many of these stations are available for scientific purposes. We demonstrate here how data from Canada and Niger can significantly improve the depth estimates for two earthquakes, one in Skane, Sweden in 2008 and the other in the Danish part of the North Sea in 2010 (Figs 1, 2).