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VOLCANIC RISK ASSESSMENT APPROACH AND EXAMPLE By HENRY GAUDRU, UNISDR scientific adviser for volcanic risk mitigation |
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AREQUIPA AND MISTI VOLCANO (PERU) Southern Peru is characterized by a high density of volcanoes. Owing to the high population around some of them, two stratovolcanoes are considered as significants threats : Ubinas, 6 km away and 2.3 km above several villages located in the Ubinas valley (5000 people) and El Misti, about 17 km away above the centre of Arequipa, second largest city of Peru. The Arequipa town. >1 M. inhabitants developed at the Western bottom of the Misti and Chachani active stratovolcanoes. Since the Pliocene, the Arequipa area is center of explosive activity. Thick ignimbrite beds of dacitic composition are related to the Chachani resurgent caldera formation. During the Pleistocene to Holocene. effusive and explosive activities occurred inside the caldera. and at their southern wall. on the Misti volcano. In addition, catastrophic flank failure affected the Western and Southern flanks of the extinct Pichu-pichu and Misti volcanoes. and produced debris avalanche that covered the S of Arequipa basin with mobilitv values of 0.11 to 0.12. Since the Holocene, a new cone grew-up on the Southern flank of the ancient Misti volcano. The voluminous modern strato-cone (ca 70 km3) anf high (5825 m) consists of a series of stubby lava flows and thick pyroclastic flows pilling up to 2.8 km in thickness. The present summit crater, 500 m across and 200 m deep including a youthful plug, is located in an older one.The latest plinian events produced about 15-m thick andesitic scoria deposits on the volcano summit. The deposit blanket NE of the volcano. indicate a NE downwind direction. From satetellite data LANDSAT-TM Based on reflection intensity ( Garcia Zuniga F.F., 1998), two partially superimposed fan-shaped deposits were identified where lower intensity can be followed up to 35 km awav from the vent and higher intensity can be followed up to 25 km. Comparing existing models of tephra falls dispersion with that of Misti, it could have been produced from eruptive columns about 20 km high and 10 to 20 m/s wind velocities. These data define the range of plinian events that have occurred at Misti and could help mitigate volcanic hazards in Arequipa region in the case of volcanic reactivation and change of wind directions. The district has grown at an accelerated rate over the last 20 years, with little planning and without consideration of risk-management criteria, evidenced by the fact that settlements now occupy high-risk areas located in the streams along the sides of the volcano. VOLCANIC HAZARDS Based on the tephrostratigraphic study, the most important volcanic hazards for Arequipa population are the following (Thouret et al., 1994). 1) Plinian fallout. They are the most common products around El Misti. Low subplinian columns (4 to 5 km) might cause ash fallout in the city; for greater plinian eruptions (>0.1 km general statistical data from Newhall (in Scott, 1989) as well as observed deposits at El Misti suggest that the thickness of the relevant fallout layer could be about 50 cm in the city; 2) Pyroclastic flows. Tens of recent pyroclastic flow deposits are found in the SW to SE valleys. This corresponds to the outer crater structure opened to the south. The small block-and-ash flows usually travel 10- 12 km but some more voluminous and mobile pumice pyroclastic flows are found in the suburbs and up to the centre of Arequipa. Due to the great elevation (3500 m) of El Misti above the city, a H/L ratio smaller than 0,22 allows a flow to reach the first suburbs, 14 km from the crater. Many small-volume pyroclastic flows reported in the literature are more mobile, having a H/L ratio smaller than this value (Hayashi and Self, 1992). 3) Lahars. In Arequipa's dry climate, lahars are not likely to be very common. However, lahar deposits can be seen in many valleys and are probably related either to the melting of the temporary snow-cap by pyroclastic flowage or to the sudden release of water by the collapse of a natural dam in the very deep and narrow Rio Chili canyon. Heavy during the wet season (December to March) are commonly responsible for floods in the lowest part of the city. 4) Debris avalanche. The steep slopes Misti and its location on an active fault raises the Altiplano some 2000 m above Arequipa basin make it possible for the flank of the volcano to collapse toward city. At least one large debris avalanche deposit can be observed in this area. A H/L ratio as high as 0.15 would be sufficient debris avalanche to devastate the whole city Considering the H/L data available volcanic debris avalanches (Hayashi and Self 1992; Siebert et al., 1984) which contained between 0.05 and 0.13, Arequipa would have very little chance to be spared should such an event occur. **************************************************************************************************************************************************************************** Concern over the potential threat of Misti volcano (2001) In the recent past years (December 2001), geologists expressed concern over the potential threat to nearly one million people living on the flanks of El Misti Volcano in Peru. The volcano currently shows no sign of activity, but a detailed study on El Misti. published in the Geological Society of America Bulletin. stressed future risk and offered new support for safer development. “Future eruptions of El Misti, even if moderate in magnitude, will entail considerable hazards to the densely populated area of Arequipa,” warned Jean-Claude Thouret of the Université Blaise Pascal in France and his team of French, Peruvian and British scientists. Spain founded the town of Arequipa at an oasis on the bank of the Rio Chili in 1540, but the town also happened to lie in a depression between two ancient volcanic flows. With the global population increase after World War II the town surged from 86,000 in 1940 to 677,000 in 1995, 750,000 in 2001 and >1 million today in 2009 people live in Arequipa, while the city center lies 17 kilometers from the summit vent and 3.5 kilometers below it in elevation. “Anyone who has flown into the airport at Arequipa is very aware of the risk,” says Simon Young, a volcanic hazards consultant in Florida. Thouret and colleagues studied El Misti's eruptive history and provided geologic maps depicting the main drainage channels and areas of debris avalanche deposits; rockslides; and pyroclastic, pumice and lava fiows. The last eruption occurred between 1440 and 1470 with phreatic events recorded in 1677, 1784 and 1787. Since them, the volcano has been moderately active; fumarolic activity at the summit plug resumed in 1949, 1984, and 1895. “We think that the recent studies of Jean-Claude Thouret are in agreement with a large consensus of volcanologists,” says Henry Gaudru, president of the European Volcanological Society and International Commission on Mitigation of Volcanic Disasters in Geneva, Switzerland. “This recent work is very interesting and should be integrated in the future risk assessment plan to mitigate volcanic impact for the population living in Arequipa.” While earthquakes regularly shake this region and landslides may pose more of an immediate threat, “the possible impact of Misti on Arequipa is as worrisome as that of Vesuvius near Napoli,” Thouret says. He and his colleagues urge the implementation of emergency response policy and land-use planning to help regulate city growth. “Future growth ofthe city should be preferentially oriented southeast and west ofthe depression but beyond 25 kilometers from the vent.” Extract from Geotimes, February 2002 ******************************************************************************************************************************************************************************** Volcanic hazard map - Years 2006 In recognition of El Misti’s enormous potential volcanic threat,
the national geological agency of Peru –Instituto Geológico Minero y Metalúrgico del Jersy Mariño , Instituto Geológico Minero y Metalúrgico (Peru), Marco Rivera, Instituto Geológico Minero y Metalúrgico (Peru), Lourdes Cacya, Instituto Geológico Minero y Metalúrgico (Peru) , Jean Claude Thouret, Université Blais Pascal (France), Luisa Macedo, Instituto Geológico Minero y Metalúrgico (Peru), Claus Siebe, Universidad Nacional Autónoma de México (Mexico), Robert Tilling, PMA GCA (United States) ******************************************************************************************************************************************************************************* ISDR INFORMS - PARTNER IN ACTION - Disaster Reduction in the Americas (2008) Mitigation of volcanic risks in the district of Alto Selva Alegre In recent years, and in response to a number of natural events that have caused disasters in different parts of the world, the study of geological hazards has become a priority for geological services. These studies constitute a basic foundation for developing emergency plans and land-use planning, and for implementing prevention and mitigation measures. Thanks to the support and philosophy of the Multinational Andean Program–Geosciences for the Andean Communities (MAP-GAC), the Geological, Mineral and Metallurgical Institute of Peru (INGEMMET) began developing the Misti Volcano hazard map in 2005. Later, the Project for Education, Dissemination, and Awareness-Raising about Volcanic Hazards in Alto Selva Alegre, Arequipa began in 2006. A number of institutions are participating in this program, including INGEMMET, the Multinational Andean Project, the local government of Alto Selva Alegre, the local Civil Defense Committee, leaders and residents of the Bella Esperanza and Javier Heraud settlements, the Diego Thomson Educational Institute, the National Civil Defense Institute (INDECI), the Center for Disaster Studies and Prevention (PREDES), the provincial municipality of Arequipa, the regional government of Arequipa, the national police, and representatives of the healthcare sector, among others. Project Goals The primary goal of the project is to disseminate and use geo-scientific knowledge, particularly as it relates to volcanic hazards, in development planning, land-use planning, and disaster prevention in the district of Alto Selva Alegre. It aims to promote greater awareness of prevention regarding natural phenomena that generate hazards, and contribute to the implementation of risk mitigation measures. Methodologie The Misti Volcano hazard map identified various high-risk places in the urban areas of Arequipa. These areas are located along the Chili River basin and the streams of San Lazaro, Huarangal, and Pastores. The affected districts are mainly Alto Selva Alegre, Miraflores, Mariano Melgar, Paucarpata, and El Cercado. An effort supported by the passage of a new municipal ordinance in Alto Selva Alegre (201/2007– MDASA) which was actually called upon by the local population. The authorities have defined the limits of the district's expansion using UTM coordinates, and these limits have been signposted. A local Civil Defense Committee has been set up in the district and is working on emergency and contingency plans in case the Misti Volcano becomes active. In addition, educational materials have been developed with the local population. Recently, illegal squatters have been evicted, in accordance with the municipal ordinance. Within the education community, in particular at the Diego Thomson School, teachers and students have learned more about the environment in which they live, and now understand that Misti is not a “dead” volcano, a mere local symbol or a tourist attraction as some thought, but that it is an active volcano that could erupt again at any moment. Thus, they have learned about volcanic hazards, prevention-related issues, the civil defense system, rescue techniques, first aid, etc. A number of institutions have participated in these trainings, including the national police and doctors from the district's health care sector, among others. After these trainings, the students organized and formed the Prevention Club, which supports the project by disseminating what they have learned throughout the district. Two drawing contests demonstrated the students' level of understanding about volcanic hazards, and shows that these issues are being incorporated in cross-cutting ways into their studies. Next steps - Distribute the hazard map to all public agencies in the city. Luisa Macedo 1, Jersy Mariño1, Roxana Amache 2, Fernando Muñoz 3, and Ana Arguedas 4 1 - Geological, Mineral and Metallurgical Institute of Peru (INGEMMET). Av. Canadá 1470, San Borja, Lima, Peru: lmacedo@ingemmet.gob.pe . 2 - Center for Disaster Studies and Prevention (PREDES). 3 - Multinational Andean Program – Geosciences for the Andean Communities (MAP-GAC) 4 - National Civil Defense Institute (INDECI), Arequipa. For further information, please contact: From ISDR (EIRD) revista, 15, 2008 ********************************************************************************************************************************************************************************* UNIDSR contact for volcanoes: HENRY GAUDRU |
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