Emile A. Okal - emile@earth.nwu.edu
Department of Geological Sciences
Northwestern University
Evanston, IL 60201
Jacques Talandier
LDG-CEA
Boîte Postale 12
91680 Bruyères-le-Châtel, France
Louis Géli
IFREMER
Boîte Postale 70
29280 Plouzané, France
Popular Version of Paper 2aUW7
Presented Tuesday morning, December 3, 1996
3rd Joint ASA/ASJ Meeting, Honolulu, Hawaii
Embarged until December, 3 1996
Acoustic waves are capable of propagating over considerable distances across the basins of the world's oceans, by taking advantage of a waveguide (a channel through which waves are directed) known as the "SOFAR Channel." This waveguide develops typically between depths of 600 and 1800 m below sea level (b.s.l.) as a zone where the speed of sound in the ocean's water (controlled by a combination of pressure, temperature and salinity) exhibits a significant minimum. Combined with the normal properties of sound in water, it allows for extremely efficient sound propagation over large distances, which has found such diverse applications as the distant tracking of submarines and the long-range study of cetaceans such as dolphins and whales.
Upon hitting a coastline or a shoaling, these so-called "T" waves are partially converted into seismic waves, and can therefore be detected by regular seismic stations on land. Indeed, this is how T waves were originally discovered, and in some instances, teleseismic T waves have actually been felt by the island populations. In this presentation, we will focus on seismic detection of underwater volcanic activity by T waves recorded on land-based seismic stations, using the example of the Polynesian Seismic Network. In this context, in the past 25 years, we have detected and monitored a number of active volcanoes on the floor of the Pacific Ocean at distances ranging from 100 to 8500 km.
More recently (1991-1993), remarkable T waves were recorded in Polynesia from a location in the Southern Pacific (54 degrees South; 140 degrees West; they featured both sustained very high amplitudes and exceptionally monochromatic spectra, consisting of a single frequency fluctuating in the vicinity of 8 Hz (i.e., a perfect pitch), in the entire 2-80 Hz frequency range. This is in contrast to other volcanic sources, which generally exhibit a much richer spectrum, and when individual resonance peaks are present, also feature overtones with substantial amplitudes, comparable to the higher modes or "harmonics" giving richness to the sound of a musical instrument.
The source of this T-wave activity is offset some 150 km from the Mid-Oceanic Ridge, in an area featuring a significant anomaly in the Earth's gravity field, suggestive of the presence of upwelling convection currents in the underlying mantle of the Earth. Existing charts showed one seamount rising to 800 m b.s.l. at the exact epicentral location. Following the 1991-1992 swarm, the area was explored by surface ships, including N/O L'Atalante in early 1996. A massive underwater volcanic ridge extending 450 km in length, 20 km in width and rising to 135 m b.s.l. was mapped. No similar structure is known to exist anywhere on the ocean floor, and its origin could be associated with a pinching of the lithosphere, the relatively rigid skin of the Earth.
The nature of the monochromatic source remains a challenge, since most models of resonators would predict the excitation of overtones, as in the case of the resonance of a crack in the rock filled with magma. We speculate that the absence of overtones could be explained by the resonance of a volume of bubbly fluid, in which higher-frequency vibrations (f >13 Hz) are effectively suppressed at the source through the dissipative transfer of heat. The possibility of significant gas-bubble activity is raised by the shallow depths mapped by the ship survey, and is also supported by the preliminary observation of fresh vesicular basalts (i.e. newly created dark rocks, presumably from lava flow or magma activity, containing sac-like features). In another model, the bubbles could be steam-released from seawater entrapped below a long-lived lava lake, and jetting through the lake in a series of geysers.