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T-Phase Observations From the May 1999 Ascension Island Experiment

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The Comprehensive Nuclear-Test-Ban Treaty CTBT specifies that an International Monitoring System IMS will be used to detect and locate disturbances that could be related to nuclear testing. In order to monitor disturbances in and near the worlds oceans, the IMS will rely on a network of 11 hydroacoustic stations. This hydroacoustic network will be composed of 6 hydrophone stations and 5 T-phase seismic stations. The hydrophone stations will record pressure variations in the ocean. The T-phase stations will record the seismic waves in the solid earth that are excited when a hydroacoustic wave strikes an island or continental margin. The coupling of hydroacoustic-to-seismic energy is currently an active area of research for CTBT monitoring. We report observations of hydroacoustic waves and their conversion to seismic waves T-waves at the volcanic edifice of Ascension Island. An earthquake to the south of Ascension Island was recorded by International Monitoring System IMS hydrophones and temporary seismic stations deployed on Ascension Island by Lawrence Livermore National Laboratory LLNL. The hydrophone recordings are rich in high-frequency 10-40 Hz energy. However, the converted seismic waves are dominated by much lower frequencies 2-8 Hz. The T-waves demonstrate amplitude and travel-time variations on the Island. In addition to the earthquake records, off-shore airgun shots were recorded as part of a crustal structure study. These shots also produced hydroacoustic-to- seismic conversions. We performed two-dimensional finite difference simulations to investigate the T-phase conversion process at ocean-island margins. The point of these calculations will be to model the transfer function of the conversion process and to determine if T-phase amplitude variability is the result of conversion along the complex bathymetry of the island, propagation on the island or a site effect.

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  • Physical and Dynamic Oceanography
  • Seismology
  • Seismic Detection and Detectors
  • Acoustics

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