Swarm of T-wave events during October 2005

After more than 17-years in quiescence, Macdonald seamount entered into a short phase of hydroacoustic activity starting at 1400 UTC on 13 October 2005. The seamount is ~ 6,000 km S of Honolulu, Hawaii (figure 2). When hydroacoustic activity has been of sufficient amplitude and duration, it has often been inferred to suggest submarine eruptions. In this case the signals were relatively weak and of modest duration, indicating seismic swarms without a clear association with volcanism.

Over five days the Polynesian Seismic Network (RSP for Réseau Sismique Polynésien) recorded 423 small- and medium-amplitude T-wave events from the seamount (figure 3). The best inland seismic station recording the event was at East Tuamotu, ~ 750 km from Macdonald. The ground response there was in the range of 50-300 nanometers.

According to Robert Dziak, hundreds of medium to large amplitude T-wave events were also recorded on the NOAA/PMEL hydrophone array in the eastern Equatorial Pacific located some 2,200 km away from Macdonald. There was an absence of Pn and Sn seismic phases (the P- and S-wave phases that propagate at the base of ocean crust along the Moho discontinuity) suggesting the swarm was composed of comparatively low-magnitude events. Accordingly, geophysicists from both the RSP and NOAA/PMEL all interpreted the activity as a modest seismic swarm.

Many of the October 2005 T-wave signals were well located, with an error ellipse of less than 1 km in diameter. The strongest signals were seen on Tahiti, Rangiroa, and parts of the East Tuamotu networks; and consequently, these yielded the smaller error ellipses. The weakest T waves were seen on parts of the East Tuamotu network, resulting in the large NE-trending error ellipses.

The signals stopped at 0700 UTC on 17 October 2005. Dominique Reymond and Olivier Hyvernaud of the RSP were not aware of any witnessed eruptive signs or any detected hydroacoustic activity at or near Macdonald following the 2005 episode through 2006.

Additional research. Our previous reports on Macdonald seamount discussed activity during the 1980s, most recently during January 1989 (SEAN 14:01). Since then, several papers have shed light on the region (including Stoffers and others, 1989; McNutt and others, 1997; Sleep, 1997). McNutt and others (1997) presented high-resolution swath bathymetry in the Southern Austral islands, and noted more than one lone seamount in the area of Macdonald.The seamounts at Macdonald were morphologically distinct from their neighbors to the W along the Austral island chain in both their tall form and their slopes, which curved concave-upward.

References. Bonneville, A., (undated-post 2002), The Cook-Austral volcanic chain, URL: http://www.mantleplumes.org/Cook-Austral.html.

McNutt, M.K., Caress, D.W. Reynolds, J., Jordahl1, K.A., and Duncan, R.A., 1997, Failure of plume theory to explain midplate volcanism in the southern Austral islands: Nature, v. 389, p. 479-482.

Sleep, N., 1997 (2 October), Earth Science: The puzzle of the South Pacific: Nature, v. 389, p. 439-440.

Stoffers, P., Botz1, R., Cheminée, J-L., Devey, C.W., Froger, V., Glasby, G.P., Hartmann, M., Hékinian, R., Kögler, F., Laschek, D., Larqué, P., Michaelis, W., M?he1, R.K., Puteanus, D., and Richnow, H.H., 1989, Geology of Macdonald Seamount region, Austral Islands: Recent hotspot volcanism in the south Pacific: Journal Marine Geophysical Researches, v. 11, no. 2/June, p. 101-112.

Information Contacts: Dominique Reymond and Olivier Hyvernaud, Laboratoire de Géophysique, Commissariat a l'Energie Atomique, CEA/DASE/LDG, PO Box 640, Papeete, Tahiti, French Polynesia; Robert P. Dziak, Oregon State University, 2115 SE OSU Drive Newport OR 97365, USA.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

November 1981 (SEAN 06:11) Cite this Report

Six submarine eruptions since December 1977

Hydrophones recorded a submarine eruption from Macdonald Seamount 29 May 1967 (Johnson, 1970). After more than 10 years of apparent inactivity, acoustic waves (T-phase) from six eruptions were detected by French Polynesian seismic stations (at Tubuai, 23.3°S, 149.5°W; Tahiti, 17.6°S, 149.5°W; Moorea, 17.5°S, 149.8°W; Rangiroa 15.0°S, 147.7°W; Hao, 18.1°S, 141.0°W; and Rikitea, 23.1°S, 135.0°W) between December 1977 and February 1981. The acoustic record of each eruption began with intense explosive signals, followed by a few hours to a few days of amplitude-modulated noise that was frequently punctuated by brief periods of additional explosive activity, less commonly by strong (but apparently non-explosive) increases in noise amplitude.

The first eruption, which was the longest and most vigorous, was first detected at 1630 on 10 December 1977 and returned continuous noise for about 92 hours, during which nearly 50 periods of explosive activity could be recognized. Occasional bursts of noise continued for another 30 hours before activity ended at about 1830 on 15 December. A strong explosion recorded at 0246 on 30 September 1979 was followed by fewer than 5 hours of diminishing activity. About 20 minutes of frequent explosions first recorded at 1330 on 12 February 1980 were followed by roughly 12 hours of eruption noise. Explosions that began to appear on the records at 0109 on 10 November 1980 remained strong and frequent for about 7 hours and eruption noise continued for about 15 additional hours. On 24 December 1980, seismic instruments detected intermittent explosions from 0610 until roughly 0900, continuous eruption noise for about 4 more hours, then intermittent noise that lasted until early the next morning. Explosions at 0611 and 0620 on 15 February 1981 were followed by about 12 hours of eruption noise. No additional activity had been recorded as of late October.

Macdonald was discovered after the 1967 eruption and bathymetric work in December 1973 defined a submarine edifice reaching to within 49 m of the ocean surface (Johnson, 1980). More recent bathymetry by the French National Marine vessel La Paimpolaise indicates that further growth of Macdonald has occurred, bringing its summit to 23 m below sea level.

Reference. Johnson, R.H., 1970, Active submarine volcanism in the Austral Islands: Science, v. 167, p. 977-979.

Further Reference. Talandier, J. and Okal, E.A., 1982, Crises sismiques au Volcan Macdonald (Ocean Pacifique Sud): C.R. Acad. Sci. Paris, ser. II, v. 295, p. 195-200.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.

February 1982 (SEAN 07:02) Cite this Report

Renewed activity; 7th known eruption

Renewed submarine activity at Macdonald Seamount was detected 1 March by Polynesian seismic stations. Acoustic waves (T-phase) were recorded at Moorea, Tubuai, Vaihoa at Rangiroa, and Rikitea in the Gambier Islands. Explosive signals were first recorded at 1337 and were followed by continuous noise of varying intensity. The initial phase, a few hours long, was succeeded by sporadic activity that lasted until about 0400 on 2 March. The latest activity is comparable in length, intensity and development to that of February 1980.

Bathymetric work that ended in February 1982 and included dredging rocks from the summit peak and adjacent plateau precisely located the submarine volcanic edifice. Its top was 27 m below sea level.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.

April 1983 (SEAN 08:04) Cite this Report

Eighth known eruptive episode detected

On 14 March, four RSP stations recorded acoustic waves (T-phase) from activity interpreted as a shallow submarine eruption at Macdonald Seamount. The signals were received at Moorea, Vaihoa on Rangiroa, Tubuai, and Rikitea in the Gambier Islands. Strong explosive signals began at 0914, and were followed by continuous noise of varying intensity. The activity began in the same manner as the seven previous eruptions, and was probably comparable in strength and development to that of February 1981. However, high background noise levels caused by the passage of Cyclone Reva over Polynesia made interpretation difficult.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.

August 1983 (SEAN 08:08) Cite this Report

Renewed submarine activity

In May, the RSP recorded seismicity from renewed eruptive activity at Macdonald. Its eight previous eruptions had begun with explosive events, but the May activity did not, and probably was a continuation of the March eruption. Reconaissance by a Marine National Française vessel did not show a perceptible increase in the volcano's summit altitude since the bathymetric survey of February 1982.

Further Reference. Talandier, J. and Okal, E.A., 1984, New Surveys of Macdonald Seamount Following Volcanoseismic Activity, 1977-1983; Geophysical Research Letters, v. 1, no. 9, p. 813-816.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.

April 1984 (SEAN 09:04) Cite this Report

Renewed submarine activity in 1983

Between May 1983 and January 1984, the RSP recorded acoustic waves (T-phase) from three shallow submarine eruptions at Macdonald Seamount. On 17 May 1983, eruptive activity began gradually with a few explosive sequences and lasted 4.5 days until 21 May. Activity resumed explosively on 27-28 October and continued for 15 hours with numerous explosive sequences. On 24 December, activity began gradually with no explosive sequences and continued for almost 10 days until 3 January 1984. This last event was the 12th and longest eruption recorded since the volcano was discovered in 1967.

Information Contacts: J. Talandier, Lab. de Géophysique, Tahiti.

May 1986 (SEAN 11:05) Cite this Report

First seismic crisis in 4 years

The RSP detected acoustic (T-phase) waves from . . . a seismic crisis at Macdonald. The seismicity began on 16 May at 1550 with explosive activity, followed by continuous noise of variable amplitudes with episodes of higher intensity. The crisis ended 18 May at about 2100, but sporadic activity was recorded until 20 May around [0400]. This crisis is the third recorded since 1977 and the first since December 1983. The May activity is comparable in duration and intensity to that of June 1982 (Talandier and Okal, 1984).

Reference. Talandier, J., and Okal, E.A., 1984, New surveys of Macdonald Seamount, south central Pacific, following volcanoseismic activity, 1977-1983: Geophysical Research Letters, v. 1, no. 9, p. 813-816.

Information Contacts: J. Talandier, LDG Tahiti.

May 1987 (SEAN 12:05) Cite this Report

Acoustic waves from moderate eruption

An eruption was detected by acoustic waves (T-phase) received by RSP stations on 4 June. Explosions began around [0400] and were followed by a continuous noise of variable level with some episodes of stronger intensity. The eruption ended ~4 hours later [but see 12:09]. This eruption was the 15th recorded since 1977 and one of the shortest of the moderate-intensity eruptions.

Information Contacts: J. Talandier, LDG Tahiti.

June 1987 (SEAN 12:06) Cite this Report

Acoustic waves recorded from eruption

An eruption was detected by acoustic waves (T-phase) received by RSP stations 6-7 July. Explosions began around 0100 on 6 July and were followed by continuous noise of variable level, with episodes of stronger intensity. The 27.5 hour crisis [but see 12:09] was . . . comparable to the November 1980 eruption in duration and intensity.

Information Contacts: J. Talandier, LDG Tahiti.

September 1987 (SEAN 12:09) Cite this Report

Eruption jolts research ship; products sampled

The New York Times reported that the seamount erupted on 11 October as the RV Melville was over the vent. When the ship first reached the area, a large crescent-shaped zone of sea water was discolored green. While researchers dredged rock specimens from the summit and took water samples, large bubbles suddenly broke the sea surface and turned the water dark brown. Gas bubbles shook the ship, causing loud "clangs and clamors" and a sulfur smell. Harmon Craig (Univ of California, San Diego) described one large bubble that rose 2 m above the ocean surface and burst, forcefully ejecting gas and exposing 20-30 volcanic clasts in its core. The rocks floated briefly, and one piece of dark volcanic glass ~20 cm long was recovered while still too hot to touch. When sampled after the eruption, the seamount was covered with fresh volcanic glass. Coral was also found on the volcano, suggesting significant repose periods between some eruptive episodes. Eruptive activity was declining as the ship departed 12 October [see 12:10 for more details].

RSP stations have been detecting acoustic waves (T-phase) since June. Intense seismic crises occurred 4-7 June and 6-15 July; both began explosively. Activity began gradually 7 August and was continuous but weak until 10 August. Sporadic moderate-level activity was recorded 10-12 August and 19 August-4 October. The 10-13 October activity was continuous and of high intensity. On 3 July 1987, divers from the Cousteau Society's RV Calypso photographed and filmed intense hydrothermal activity.

The August-October seismicity did not include crises that began explosively, indicating to French seismologists that semi-continuous activity could have been occurring during the entire period. This episode brings to 20 the number of distinct crises detected since 1977.

Information Contacts: J. Talandier, LDG Tahiti; The New York Times.

October 1987 (SEAN 12:10) Cite this Report

Explosion seismicity continues; more observations from research vessel

Harmon Craig provided additional observations of the 11-12 October eruption. When the RV Melville first arrived . . . on 11 October, bright green discolored water had spread in a crescent-shaped zone ~2.5 km wide that extended 1.5 km from a point source. The water remained discolored during the 2 days that the ship was in the area. Episodes of erupting gas bubbles were most intense during the afternoon, with bubbles rising sporadically at 10-minute to 1-hour intervals. Following eruptions of bubbles, an area of chocolate-brown water, perhaps 100 m across, appeared in the center of the green discolored water. Craig saw a total of ~30-40 basaltic rocks brought to the surface by gas bubbles. The depth to the summit was 30-100 m; the summit had a flat area at 100 m depth and pinnacles rose as much as 70 m above this surface.

The RSP reported that a seismic crisis began again on 14 October at [1200]. Continuous intense noise was recorded until the onset of explosive phenomena at about [1730], persisting until [0320 the next morning] when it was followed by weak but continuous activity that was ongoing as of 16 October.

Information Contacts: H. Craig, Scripps Institute of Oceanography; J. Talandier, LDG Tahiti.

April 1988 (SEAN 13:04) Cite this Report

Eight months of frequent eruptive seismicity

During the past 8 months, the RSP has detected frequent seismic swarms from Macdonald Seamount. The recorded acoustic waves (T-phases) are generated during eruptions at the interface between lava and sea water. Seismicity was recorded 19 August-27 October, 1987; 28 December 1987-3 March 1988; and 28 March-26 April, 1988. With the exception of a strong signal in October, activity was weak and varied from continuous to sporadic. Seismologists suggested that the absence of explosive onsets for the swarms detected since August could indicate that activity has been quasi-continuous through the end of April and beyond.

Information Contacts: J. Talandier, LDG Tahiti.

May 1988 (SEAN 13:05) Cite this Report

Activity since 1977 summarized

Over the last 11 years, the Polynesian Seismic Network has recorded two dozen eruptive episodes (see figure 1, below). Activity has been particularly frequent since June 1987.

Information Contacts: J. Talandier, LDG Tahiti.

September 1988 (SEAN 13:09) Cite this Report

Two weeks of submarine eruptive activity

Numerous shallow submarine explosions of moderate intensity occurred . . . from 2155 on 2 September until 5 September, recorded as acoustic waves (T-phase) by the RSP. Sporadic weak activity persisted from 6 September until the eruption's end at about 0700 on 18 September. Periods of activity were also detected 6-18 May and 11 June 1988. No explosions accompanied the onsets of active periods during August 1987-June 1988, suggesting to seismologists that the eruption may have been continuous during that time.

Information Contacts: J. Talandier, LDG Tahiti.

December 1988 (SEAN 13:12) Cite this Report

Frequent eruptive seismicity in 1988

Seismicity . . . was substantially more vigorous in 1987 and particularly in 1988 than in previous years since 1977 (figure 1). Acoustic T-waves generated from a submarine eruption 11-17 November 1988 were the most intense since the Polynesian Seismic Research station began to collect data more than 25 years ago. Since the end of the 3-18 September eruption nearly continuous weak seismicity from Macdonald was detected 26 September-5 October, 12-24 October, and 12-14 December, and weaker more sporadic explosions 25-29 December.

Information Contacts: J. Talandier, LDG Tahiti.

January 1989 (SEAN 14:01) Cite this Report

Eruptions seen from ship and submarine

The volcano entered a new eruptive phase 19-20 January after 2 weeks of quiet. T-waves from a series of explosions were registered by the RSP. After 3 days without activity, eruptions began just as the support ship for a joint French-German oceanographic study of hot spots in the Society and Austral Islands (with the submersible Cyana) arrived . . . . On 24 January at [0345], flashes of light, probably resulting from gas combustion within surface bubbles, accompanied a strong H₂S odor. Bubbling at the sea surface emitted water vapor and H₂S gas [at 0400-0430 and 0530-0600]. A green slick formed on the ocean surface, rapidly growing to nearly 2 km, while the seismic station at Papeete detected explosion signals from Macdonald. The green slick (apparently 5-25 m thick) drifted away . . . at roughly 0.5 km/hour and disappeared during the following night.

No surface activity was observed early 25 January, but the seismic network registered small signals. At [1215], the appearance of a green surface slick coincided with an explosive event recognized on the seismographs. The slick was composed of floating patches of foam, volcanic ash, and orange-yellow bits of sulfur with dark grey interiors. Numerous dead fish floated on the ocean surface in and around the slick, their gills choked with volcanic ash.

No further activity was observed at Macdonald until 27 January. At [1400], sprays of water and vapor were observed on the sea surface as Cyana was submerging to study the volcano. These were accompanied by large black bubbles of foam, composed of an emulsion of volcanic ash and gas, iron sulfide, and steam. The bubbles exploded at the surface, emitting a large volume of gas. Degassing from fissures at the base of one of the craters was observed from Cyana at the sea bottom. Similar surface activity occurred again [at 1730-1900], accompanied by short, red flashes.

At [0435] the next day, three green surface slicks formed, accompanied by large explosions that were felt on the support ship. Many explosive events that were registered on board the ship by a 3.5 kHz echosounder were also recorded at the RSP in Tahiti. By the time the eruption ended at [1501], a number of events had been detected by the echosounder during surface activity. Two additional dives by Cyana allowed reconnaissance studies of Macdonald's volcanic structures between 1,500 m depth and the summit, 50 m below sea level. During the eruption, 16 sets of water samples (each consisting of 12 samples from different depths) were taken. While the support vessel was in the area, a hydrophone was deployed, recording all underwater acoustic events. (see figure 1)

Information Contacts: J-L. Cheminée, IPGP; P. Stoffers, Christian-Albrechts Univ zu Kiel, Germany; J. Talandier, LDG Tahiti; G. MacMurtry, Univ of Hawaii; H. Richnow, Univ Hamburg, Germany; N. Binard, Lab de Géologie Marin, France; R. Huber, Univ Regensburg, Germany; E. Okal, Northwestern Univ, IL.

January 2007 (BGVN 32:01) Cite this Report

Swarm of T-wave events during October 2005

After more than 17-years in quiescence, Macdonald seamount entered into a short phase of hydroacoustic activity starting at 1400 UTC on 13 October 2005. The seamount is ~ 6,000 km S of Honolulu, Hawaii (figure 2). When hydroacoustic activity has been of sufficient amplitude and duration, it has often been inferred to suggest submarine eruptions. In this case the signals were relatively weak and of modest duration, indicating seismic swarms without a clear association with volcanism.

Over five days the Polynesian Seismic Network (RSP for Réseau Sismique Polynésien) recorded 423 small- and medium-amplitude T-wave events from the seamount (figure 3). The best inland seismic station recording the event was at East Tuamotu, ~ 750 km from Macdonald. The ground response there was in the range of 50-300 nanometers.

According to Robert Dziak, hundreds of medium to large amplitude T-wave events were also recorded on the NOAA/PMEL hydrophone array in the eastern Equatorial Pacific located some 2,200 km away from Macdonald. There was an absence of Pn and Sn seismic phases (the P- and S-wave phases that propagate at the base of ocean crust along the Moho discontinuity) suggesting the swarm was composed of comparatively low-magnitude events. Accordingly, geophysicists from both the RSP and NOAA/PMEL all interpreted the activity as a modest seismic swarm.

Many of the October 2005 T-wave signals were well located, with an error ellipse of less than 1 km in diameter. The strongest signals were seen on Tahiti, Rangiroa, and parts of the East Tuamotu networks; and consequently, these yielded the smaller error ellipses. The weakest T waves were seen on parts of the East Tuamotu network, resulting in the large NE-trending error ellipses.

The signals stopped at 0700 UTC on 17 October 2005. Dominique Reymond and Olivier Hyvernaud of the RSP were not aware of any witnessed eruptive signs or any detected hydroacoustic activity at or near Macdonald following the 2005 episode through 2006.

Additional research. Our previous reports on Macdonald seamount discussed activity during the 1980s, most recently during January 1989 (SEAN 14:01). Since then, several papers have shed light on the region (including Stoffers and others, 1989; McNutt and others, 1997; Sleep, 1997). McNutt and others (1997) presented high-resolution swath bathymetry in the Southern Austral islands, and noted more than one lone seamount in the area of Macdonald.The seamounts at Macdonald were morphologically distinct from their neighbors to the W along the Austral island chain in both their tall form and their slopes, which curved concave-upward.

References. Bonneville, A., (undated-post 2002), The Cook-Austral volcanic chain, URL: http://www.mantleplumes.org/Cook-Austral.html.

McNutt, M.K., Caress, D.W. Reynolds, J., Jordahl1, K.A., and Duncan, R.A., 1997, Failure of plume theory to explain midplate volcanism in the southern Austral islands: Nature, v. 389, p. 479-482.

Sleep, N., 1997 (2 October), Earth Science: The puzzle of the South Pacific: Nature, v. 389, p. 439-440.

Stoffers, P., Botz1, R., Cheminée, J-L., Devey, C.W., Froger, V., Glasby, G.P., Hartmann, M., Hékinian, R., Kögler, F., Laschek, D., Larqué, P., Michaelis, W., M?he1, R.K., Puteanus, D., and Richnow, H.H., 1989, Geology of Macdonald Seamount region, Austral Islands: Recent hotspot volcanism in the south Pacific: Journal Marine Geophysical Researches, v. 11, no. 2/June, p. 101-112.

Information Contacts: Dominique Reymond and Olivier Hyvernaud, Laboratoire de Géophysique, Commissariat a l'Energie Atomique, CEA/DASE/LDG, PO Box 640, Papeete, Tahiti, French Polynesia; Robert P. Dziak, Oregon State University, 2115 SE OSU Drive Newport OR 97365, USA.