ALBERT

Description: The ratio of body-wave to surface-wave magnitude, mb:Ms, 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), mb:Ms 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 Ms values for these explosions are high compared to historical explosions with similar mb. On an mb:Ms plot, both explosions lie close to the contemporary IDC experimental screening line, Ms=1.25mb-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, mb and Ms 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 mb:Ms screening line based on these magnitudes is Ms=mb-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 mb:Ms 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.

Description: 〈span〉〈div〉Abstract〈/div〉In this study, seismic and hydroacoustic signals from underwater explosions in 2001, 2008, and 2016 near Florida are analyzed. These 10,000 lb chemical explosions were detonated by the United States Navy to validate the ability of new classes of ships to withstand explosions. For many of the explosions, the ground‐truth (GT) epicenters are known. These epicenters are used to improve the accuracy of the locations of explosions with no GT data by performing a relative relocation using a Bayesian hierarchical seismic‐event locator. Seismic and hydroacoustic signals are also used to characterize the underwater explosion sources. Bubble pulse modulations, characteristic of underwater explosions, are identified at seismic stations in the United States, and the observed bubble pulse frequency is consistent with published GT information. The absence of clear modulations in the spectra caused by reverberations in the water column means that the depth of the explosions in the water and hence the trinitrotoluene (TNT) equivalent charge weight of the explosion cannot be resolved from the frequency of the bubble pulse modulations. Published estimates of the local magnitudes ML and the known charge weights of these explosions are compared with data from previous underwater explosions. A relationship between charge weight and ML from previous well‐calibrated explosions detonated in the Dead Sea is shown to provide reasonable estimates of the charge weight once corrected for the salinity of the seawater near Florida. Hydroacoustic signals from the Florida underwater explosions are also observed as 〈span〉H〈/span〉 phases on hydrophone sensors near Ascension Island. The bubble pulse is not observed as clearly at the hydrophone sensors at Ascension Island possibly as a result of signal distortion in the shallow water close to Florida. This has implications for event identification using hydrophone stations and demonstrates the importance of combining seismic and hydroacoustic observations.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉In this study, seismic and hydroacoustic signals from underwater explosions in 2001, 2008, and 2016 near Florida are analyzed. These 10,000 lb chemical explosions were detonated by the United States Navy to validate the ability of new classes of ships to withstand explosions. For many of the explosions, the ground‐truth (GT) epicenters are known. These epicenters are used to improve the accuracy of the locations of explosions with no GT data by performing a relative relocation using a Bayesian hierarchical seismic‐event locator. Seismic and hydroacoustic signals are also used to characterize the underwater explosion sources. Bubble pulse modulations, characteristic of underwater explosions, are identified at seismic stations in the United States, and the observed bubble pulse frequency is consistent with published GT information. The absence of clear modulations in the spectra caused by reverberations in the water column means that the depth of the explosions in the water and hence the trinitrotoluene (TNT) equivalent charge weight of the explosion cannot be resolved from the frequency of the bubble pulse modulations. Published estimates of the local magnitudes ML and the known charge weights of these explosions are compared with data from previous underwater explosions. A relationship between charge weight and ML from previous well‐calibrated explosions detonated in the Dead Sea is shown to provide reasonable estimates of the charge weight once corrected for the salinity of the seawater near Florida. Hydroacoustic signals from the Florida underwater explosions are also observed as 〈span〉H〈/span〉 phases on hydrophone sensors near Ascension Island. The bubble pulse is not observed as clearly at the hydrophone sensors at Ascension Island possibly as a result of signal distortion in the shallow water close to Florida. This has implications for event identification using hydrophone stations and demonstrates the importance of combining seismic and hydroacoustic observations.〈/span〉