Unnamed

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  • Last Known Eruption
  • 53.998°S
  • 139.845°W

  • -1000 m
    -3280 ft

  • 335020
  • Latitude
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The Global Volcanism Program has no activity reports for Unnamed.

The Global Volcanism Program has no Weekly Reports available for Unnamed.

Index of Monthly Reports

Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

03/1991 (BGVN 16:03) Monochromatic acoustic T-wave swarm


Contents of Monthly Reports

All information contained in these reports is preliminary and subject to change.

03/1991 (BGVN 16:03) Monochromatic acoustic T-wave swarm

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 <1,000 m below the ocean surface. Volcanic activity from the seamount has not previously been reported.

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.

Information Contacts: J. Talandier, LDG Tahiti.

Acoustic T-waves from a seismic swarm, possibly associated with magmatic activity, were recorded in 1991 from a location 130 km south of the Eltanin Fracture Zone (Global Volcanism Network Bulletin, 1991). The source was near a seamount shallower than 1000 m in depth near the Pacific-Antarctic Ridge.

Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
[ 1991 Mar 11 ] [ 1991 Mar 19 ] Uncertain 0   Seamount by Pacific-Antarctic Ridge?

The Global Volcanism Program has no synonyms or subfeatures listed for Unnamed.

The Global Volcanism Program has no photographs available for Unnamed.

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Smithsonian Institution-GVN, 1990-. [Monthly event reports]. Bull Global Volc Network, v 15-33.

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Oceanic crust (< 15 km)

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Affiliated Databases

Large Eruptions of Unnamed Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.