Date: Thu, 23 Mar 1995 18:37:17 MST
From: admin@instgeof.ecx.ec
Organization: Instituto Geofisico. Escuela Politecnica Nacional
Subject: AOBA
----------------------------Original message----------------------------
Michel Monzier and Claude Robin, ORSTOM
Instituto Geofisico, Escuela Politecnica Nacional
Quito, Ecuador
phone: (593) 2 225 655
fax: (593) 2 567 847
e-mail: michel@instgeof.ecx.ec
rsantacr@sis.poli.edu.ec
fabian@sis.poli.edu.ec
March, 23, 1995
Since we are in permanent contact with the ORSTOM geophysicist
team working in Vanuatu, we think the following text is the most
representative of the situation in Aoba up to march 18.
Best regards
Michel and Claude
Aoba (Vanuatu) Alert status following strong gas discharge and
large amplitude tremor
Aoba
Vanuatu (Southwest Pacific) (15,38 S, 167.83 E)
The following report is from the ORSTOM volcanological team
working in Vanuatu.
Aoba volcano is an oval shaped, basaltic and dominantly lavic
shield volcano (Eggins, 1993, Gorton, 1977, Robin et al., 1993), by
far the largest of the whole New Hebrides arc (base at -3000 m
below sea level, top at 1500 m a.s.l., approximative volume :
2500 km3). It lies in front of the d'Entrecasteaux collision zone, at
the boundary between the northern and southern Aoba Basins, on
a thinned crust, along a fracture running N100 E, transverse to the
arc (for more information, see Greene et al., 1994). Two summit
concentric calderas (5 km in diameter for the largest) enclose the
main central crater (Lake Voui, 2 km in diameter). Numerous
secondary craters and cones lie along the N100 E fracture, out to
the extremities of the island, where magma-seawater interactions
have produced several maars in the past. The whole island is
covered by a dense rainforest (almost 10 h of hard walk to go
from the shore to the summit).
Due to the presence of : 1/ very young deposits related to
strong explosive eruptions all around the central part of the island
down to the coastlines as well as thick lahar deposits; 2/ the
presence of Lake Voui in the crater; 3/ the long time quiescence of
this volcano ( 300-400 years, Warden, 1970); 4/ strong degassing
which occurred in the lake in 1991, and 5/ a population of 3500
inhabitants living in a radius of 10 km from the crater, Aoba
volcano has been recently considered to be the most potentially
dangerous volcano of the Vanuatu archipelago (Robin and
Monzier, 1993, 1994).
Last activity and deposits : Recent volcanic activity includes
the formation of Lake Voui and Manaro Ngoro summit explosion
craters and cones (some 420 years ago). It also includes N'dui
N'dui lava flows, issued from the N100 E fissure, approximately
300 years ago, which reached the NW coast (Warden, 1970).
Possible lahars (or only mudflows following heavy rains?)
annihilated villages on the SE flanks of the island, about 120 years
ago, producing several casualties. An eruption possibly occurred in
1914 with ashfalls (?) and lahars (12 casualties).
Increased gas discharge in 1991 : Three anomalous "boiling"
areas with large bubbles (10 m in diameter) and burned vegetation
were observed at Lake Voui on July 13, 1991 by a VANAIR pilot. It
was the first time he observed such a phenomenon, and he recalled
that the vegetation was still green in May 1991. On 24 July 1991,
an aerial survey revealed only three areas of discolored water in
Lake Voui. The vegetation was burned up to the crater rim, 120 m
above the water. Thus, an anomalously strong SO2 degassing
probably occurred between May and July (Robin et al., 1991). This
event, unnoticed by island residents, certainly marked the end of a
long time of quiescence.
Evolution of activity since December 1994 : Unusual
seismicity was felt by Aoba inhabitants, from 1st to 7th of
December 1994, with a maximum of 7 small to medium-sized
events on the 5th of December. Records of this seismic crisis by
ORSTOM stations located at Santo (70 km from Aoba) and Efate
(260 km from Aoba) have shown that the peak activity lasted 24 h
with 13 events, the largest having a magnitude of 4.6 (Regnier,
1995). Crustal hypocenters were located under the southern
submarine base of the volcano. On 7 December, a brief
reconnaissance by helicopter was done to assess the possible
resumption of activity at the main crater (Lake Voui) and define
the kind of eruption that could develop. Activity at Lake Voui was
similar to that observed on July 1991 and September 1993, with
small areas of hot, gaseous water rising in Lake Voui. Nevertheless,
the rainforest appeared completely burned up to several hundred
meters all around the crater. Despite the end of the seismic crisis,
we emphasized to the National Disaster Management Office (NDO)
in the need to keep circumspect with Aoba volcano. In mid-
December, according to Robin and Monzier (1994), the following
advice was communicated to NDO : "In the case of a resumption of
volcanic activity in the summit area, it will be wise to evacuate, in
a first phase, the population of coastal villages of the central part
of the island (in a 10 km radius area surrounding Lake Voui)
towards the less hazardous NE and SW extremities of the island. If
the eruption occurs near these extremities, or spreads along
fractures from central vents towards these extremities then it
might be necessary to evacuate part of the population to Santo or
Maewo-Pentecost".
According to a VANAIR pilot, on March 1, 1995, Lake Voui
was calm with gas escapes in numerous places. The following day,
the lake was steaming all over, bubbling up in the centre and its
surface was rough; blowing out black sediments was also reported
by this pilot. Early in the morning of March 3, people of Santo
island, 70 km from the Aoba crater, observed a gas plume rising
up to 2 - 3 km above Lake Voui. Simultaneously, crustal seismicity
with characteristics similar to that observed in December 1994
was recorded.
From 4 to 6 March, geophysicists from ORSTOM, Port-Vila,
moved to Aoba and recorded strong and continuous tremor at
Ndui Ndui site, about 9 km NW from the main crater. This tremor
had the following characteristics : 1/ monochromatic signal with a
1.4 Hz mean frequency; 2/ duration of several hours; 3/ amplitude
of 3-4 times the back-ground noise. Reduced displacement
calculated for this tremor is 58 cm2, which would correspond to
an eruption with a VEI of 3.5 to 4 and with a probability occurence
of 70% (McNutt, 1993). At the same time, local observers were
trained to watch the activity and the collaboration with VANAIR
pilots was reinforced. However, as usual during the tropical
summer, the top of the volcano is covered by thick clouds and
almost always is not visible. 5 march, a gas plume was still visible
above Lake Voui.
On 8 March 1995, after discussions between the ORSTOM
geophysicists based at Port-Vila (Vanuatu) and ORSTOM
volcanologists now based in Quito (Ecuador), the following advice
was communicated to the Vanuatu Government:"...The size of the
gas plume observed above Lake Voui crater on March 3, 1995
probably means that magma is now rising within the volcano....
Thus, Aoba volcano is now dangerous and it seems necessary to
envisage the evacuation of the population of coastal villages
located in a 10 km radius area surrounding Lake Voui towards the
less hazardous NE and SW extremities of the island..."
Variations in the level of Lake Voui have been also noted by
an Aoba inhabitant who stayed several days in the summit area :
on the 4 th and 6 th of March, the l
feet below the usual level and the entire hot lake was steaming.
Soft mud was also blown all over the shores.
Recorded tremor activity remained constant on Aoba
between the 9th and 13th of March but with significantly less
intensity than during the 4 to 6 March period. In addition, a
shallow, local micro-seismicity has been noted since the 11 th of
March. During an aerial survey made on the 13 th of March, the
entire lake was steaming and a strong sulfur smell had been
reported all above the summit area of Aoba.
In case of increasing activity in the central crater, magma-
water interactions would produce falls of ash, dense lapilli and
accretionary lapilli, as well as pyroclastic flows, base surges and
lahars. Lava flows may also erupt from N100 E or flank fissures.
The ORSTOM seismological team in Vanuatu will be reinforced on
the 17 th of March by the arrival of a new seismologist. 5 to 7
portable seismic stations will be deployed as soon as possible
around Aoba Island to improve the focal locations and delineate
possible areas of attenuation. Also, a new permanent seismological
station will be installed on Aoba, operated by ORSTOM in Vanuatu.
Daily contacts are maintained between geophysicists based in
Vanuatu and geologists now based in Ecuador; these last ones are
ready to move to Vanuatu.
References
-Eggins, S., 1993. Origin and differenciation of picritic arc magmas,
Aoba (Aoba), Vanuatu, Contrib. Mineral. Petrol., 114: 79-100.
-Gorton, M. P., 1977. The geochemistry and origin of quaternary
volcanism in the New Hebrides, Geochimica et Cosmochimica Acta,
41, 1257-1270.
-Greene, H.G., Collot, J.-Y., Stokking, L.B., et al., 1994. Proc. ODP,
Sci. Results, 134: College Station, TX (Ocean Drilling Program).
-McNutt, S.R., 1993. Volcanic tremor amplitude correlated with
eruption explosivity and its potential use in determining ash
hazards to aviation. Taller internacional sobre el complejo
volcanico Galeras, 11-16 Enero 1993, Pasto, Colombia, 11 p.
-Regnier, M., 1995. Rapport pr'eliminaire sur la crise sismique
d'Aoba de d'ecembre 1994. Rapport ORSTOM, Port-Vila, 4 p.
-Robin, C. and Monzier, M., 1993. Volcanic hazards in Vanuatu.
Disaster Management Workshop by National Disaster Management
Office, Republic of Vanuatu, 24-28 May 1993, Port-Vila, 8 p.
-Robin, C. and Monzier, M., 1994. Volcanic hazards in Vanuatu,
ORSTOM and Dept. of Geology, Mines and Water Resources of the
Vanuatu Government report, 15 p.
-Robin C., Monzier M., Crawford A.J. et Eggins S.M., 1993. The
geology, volcanology, petrology-geochemistry, and tectonic
evolution of the New H'ebrides island arc, Vanuatu. IAVCEI
Canberra 1993, Excursion guide, Record 1993 / 59, Australian
Geological Survey Organisation, 86 p.
-Robin C., Monzier, M., Lardy M., Charley D., Mortimer C. and
Eissen J.-P., 1991. 1990-91 volcanic activity in Vanuatu. Global
Volcanism Network Bulletin, vol. 16, n 7, p. 16-17.
-Warden, A. J., 1970. Evolution of Aoba caldera volcano, New
Hebrides, Bull. Volcanol., XXXIV, 1, 107-140.
Information Contacts:
-C. Robin and M. Monzier (geologists) ORSTOM and Department of
Geology, Mines and Water Resources of the Vanuatu Government
(present adress: ORSTOM, A. P. 17-11-6596, Quito, Ecuador). E-
mails : robin@orstom.ec and monzier@orstom.ec. Phone: (593) 2
543 211 (ORSTOM main office, Quito) and (593) 2 225 655
(Instituto Geofisico, Escuela Politecnica Nacional, Quito). Fax:
(593) 2 567 847 (Instituto Geofisico, E. P. N., Quito).
-M. Lardy (geophysicist), M. Regnier, J.-P. Metaxian, R. Decourt
(seismologists), D. Charley (technical assistant), ORSTOM, B.P. 76,
Port-Vila, Vanuatu. E-mail: lardy@vanuatu.orstom.fr. Phone: (678)
22 268. Fax: (678) 23 276. E-mail J.-P. Metaxian in France:
metaxian@univ-savoie.fr
-Mario Ruiz (seismologist) Instituto Geofisico, Escuela Politecnica
Nacional, Quito, Ecuador. Phone: (593) 2 225 655. Fax: (593) 2
567 847.
-J.-P. Eissen (geologist), ORSTOM, Centre de Brest, B.P. 70, 29280
Plouzane, France. E-mail: eissen@orstom.orstom.fr. Phone: (33) 98
22 46 65. Fax: (33) 98 22 45 14.
Figure Caption
-Aoba Island in Central Vanuatu (ADOBE ILLUSTATOR).
-Simplified topographic map of Aoba Island (PAPER).
-B&W photography (SLIDE):
Summit of Aoba Island, view toward NW (photographied during
World War II by an US pilot). Two concentric calderas (5 km in
diameter for the largest) enclose the main central crater (Lake
Voui, white, 2 km in diameter). In the foreground, the black
Manaro Lake corresponds to a pond trapped in the caldera.