RSP stations registered acoustic T-waves from a seismic swarm that may have been centered on a seamount near the Pacific-Antarctic Ridge, ~130 km S of the Eltanin fracture zone (figure 1). The episode lasted from 11 to 19 Mar (peak activity 13-19 Mar), and was followed by weak, sporadic activity until 28 Mar. RSP seismologists believed that the swarm was volcanic, although its characteristics were unusual. The network's aperture for events from this region was ~26°. No location uncertainty was given, but the seamount . . . is the only one on bathymetric maps of the area with a summit

Figure 1. Computer-generated bathymetric map of a portion of the S-central Pacific, showing the Eltanin and Udintsev Fracture Zone systems, and the prominent seamount at about 53.9°S, 140.3°W in the epicentral area of the March 1991 swarm. Contour interval, 200 m. Courtesy of the U.S. Naval Research Laboratory.

The T-waves resolved into distinct signals with durations of several seconds, repeat intervals of 15 minutes, and fluctuating amplitudes (figure 2). Each was perfectly monochromatic, without harmonics detectable above the baseline microseismicity (from 20 to 40 dB below the maximum level). Frequencies were between 3.5 and 10 Hz, principally between 5 and 7 Hz during the peak of the swarm (figure 3). The beginning, and especially the end of the swarm, were characterized by the highest-frequency signals. Wave frequency did not vary within individual signals. The signature of the T-waves was consistent with a source in a vertical plane.

Figure 2. Characteristic seismic signals from the earthquake swarm near the Pacific-Antarctic Ridge, 15-16 March 1991. Spectral analyses of these (and other) events are shown in figure 3. Courtesy of J. Talandier.

Figure 3. Spectral analyses of some seismic events from the March 1991 swarm near the Pacific-Antarctic Ridge, including those shown in figure 2. Courtesy of J. Talandier.

Seismologists noted that the monochromatic character of the seismicity seemed difficult to reconcile with the sounds generated at the interface of lava and sea water during shallow submarine eruptions. Instead, it suggested that these signals could have been emitted by some submarine sources (external or internal), very close to the flanks of the volcano, associated with magmatic activity during or before lava discharge. Explosive volcanism, by contrast, generates a wide spectrum of sound.

The RSP has detected T-waves associated with Macdonald seamount (Austral Islands), Monowai and Raoul (Tonga and Kermadec archipelago), White Island (New Zealand), and a number of volcanoes in Japan, the Marianas, and the Galápagos. None of these seismic events was characterized by monochromatic signals. The volcanic seismicity from episodes at Teahitia in 1982, 1983, 1984, and 1985, which was frequently associated with seismic swarms, was also not comparable. Other T-wave episodes caused by magmatic activity at submarine volcanoes consisted of a large spectrum of submarine noise, as opposed to this swarm's very pure emissions.

Geological Summary. Acoustic T-waves from a seismic swarm, possibly associated with magmatic activity, were recorded in 1991 from a location 130 km S of the Eltanin Fracture Zone (Bulletin of the Global Volcanism Network, 1991). The source was near a seamount shallower than 1,000 m in depth near the Pacific-Antarctic Ridge.

Information Contacts: J. Talandier, LDG Tahiti.