ALBERT

Description: The ratio of body-wave to surface-wave magnitude, m (sub b) :M (sub s) , has historically been one of the most effective methods for distinguishing earthquakes from underground explosions. In the context of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), m (sub b) :M (sub s) is currently one of the experimental standard event-screening criteria being provisionally tested at the International Data Centre (IDC). An event in the IDC analyst-reviewed bulletin is screened out if the hypothesis that it is an underground explosion can be rejected with high confidence. Recently, two announced nuclear tests by the Democratic People's Republic of Korea have raised interest because the M (sub s) values for these explosions are high compared to historical explosions with similar m (sub b) . On an m (sub b) :M (sub s) plot, both explosions lie close to the contemporary IDC experimental screening line, M (sub s) =1.25m (sub b) -2.2. Although neither explosion was screened out by the IDC, the two explosions indicate that a revision of the line is advisable to ensure with high confidence that any future underground nuclear explosion is not screened out. Here, m (sub b) and M (sub s) magnitudes for 409 past underground nuclear explosions are collated and presented. The magnitudes include new measurements, an archive of historical measurements made over the years at AWE Blacknest, and a reworking of bulletin data. The revised m (sub b) :M (sub s) screening line based on these magnitudes is M (sub s) =m (sub b) -0.64. The effect of the revised line on event screening at the IDC is assessed. It is found that the new criterion screens out 42% of a set of events from 2008, whereas the old criterion screened out 87%, which is a large reduction. The revised provisional m (sub b) :M (sub s) screening line was agreed upon by the Waveform Expert Group at Working Group B of the CTBT Preparatory Commission in February 2010 and has been tested in operations at the IDC since 3 June 2010.