T-wave detection of two underwater explosions off Hawaii on 13 April 2000

We studied two presumed underwater explosions, detonated on 13 April 2000 (approximate times 00:19 and 23:29 coordinated universal time), at a site located approximately 215 km southwest of Oahu, Hawaii, and detected from a combination of T phases recorded at shore-based seismic stations and acoustic waves recorded by hydrophones. The explosions were initially detected by the Polynesian Seismic Network, and a preliminary location obtained in the vicinity of Kauai. With the use of an enlarged dataset, an improved location was obtained, after correcting arrival times for both the influence of the seismic path at the receivers, and the effect of dispersion along the acoustic path. The explosive nature of the source was tested using several criteria: the duration-amplitude discriminant of Talandier and Okal (2001), the variation of spectral amplitude with frequency, the observation of a strong frequency dispersion in the spectrograms, and the identification of a bubble period (0.45 sec) in the cepstra of the signals, which translates into a yield of 275 kg of equivalent TNT for a depth of 50 m. In the context of monitoring the Comprehensive Nuclear Test Ban Treaty, these two explosions provide a perfect opportunity to assess the capabilities of T-phase stations and hydrophones for detection, location, identification, and quantification of these sources. Our study, conducted in the absence of any ground-truth information, stresses the possibility of a powerful synergy between these two types of recording facilities, but also points to several limitations in the performance of certain shore-based seismic stations.