Abstract

Seismic events located in the Gulf of Bothnia, the Baltic Sea, and the North Sea, recorded at the regional seismic arrays NORESS and FINESA, have characteristics of underwater blasts observed hydroacoustically. Spectral analysis of regional phases associated with the events reveals strong time-independent spectral scalloping indicating that the waveforms are made up of correlated “pulse-echo” type signals. Cepstral analysis gives consistent delay times between the pulse-echo pairs with the most common delays between 400 and 600 msec. Some cepstra also have negative peaks at quefrencies between 150 and 350 msec. The cepstral peaks are observed in all phases associated with the event, and events recorded at both arrays have the same delay times. The higher time delays are consistent with bubble-pulse delays commonly observed in underwater blasts recorded hydroacoustically. The lower-quefrency negative peaks are caused by echoes with reversed polarities relative to the primary pulse and are consistent with reflections from the top of the water column. Generally, these events have large Pn/Lg amplitude ratios similar to those observed from quarry blasts. Somewhat smaller high-frequency Pn/Lg ratios are observed for Baltic Sea events compared with the North Sea events, which may be due to differences in the water-bottom geologies of the two regions. Simple source models of underwater blasts explain many of the features of the observed cepstra and indicate that the shot depths had to be shallow (around 40 to 80 m) for shot sizes between 75 and 150 kg. This study shows that the seismic sensor network, called for by the Comprehensive Test Ban Treaty (CTBT), can augment the smaller network of hydroacoustic stations for the detection and identification of clandestine under-water nuclear-explosion tests in the near offshore environment.

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