Abstract:
Seismic stations close to island shores can record the seismic waves resulting from the conversion of oceanic T waves at the shoreline. Over the past 25 years, the Polynesian Network has detected and monitored a number of active volcanoes on the floor of the Pacific Ocean. Recently, remarkable T waves were recorded from a location in the Southern Pacific (54(degrees)S; 140(degrees)W); they had both very high amplitudes and exceptionally monochromatic spectra, featuring a single frequency (fluctuating around 8 Hz) throughout the 2--80 Hz range. Other volcanic sources generally exhibit a much richer spectrum, including overtones of any resonance peak. The source of this T-wave activity, 150 km from the Mid-Oceanic Ridge, was explored by surface ships. A massive ridge extending 450 km in length, 20 km in width, and rising to 135 m b.s.l. was mapped. The nature of the monochromatic source remains a challenge. It is speculated that the absence of overtones could be explained by the resonance of a volume of bubbly fluid, significant degassing being allowed by the shallow depths mapped by the ship survey.