Date: Thu, 23 Mar 1995 18:37:17 MST From: firstname.lastname@example.org 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: email@example.com firstname.lastname@example.org email@example.com 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 : firstname.lastname@example.org and email@example.com. 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: firstname.lastname@example.org. Phone: (678) 22 268. Fax: (678) 23 276. E-mail J.-P. Metaxian in France: email@example.com -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: firstname.lastname@example.org. 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.