Preventive measures in response to an eruption
Throughout history mankind has tried to propitiate the gods or demons believed to inhabit active volcanoes. The Native Americans in Central America, for example, used to throw virgins into the crater of Masaya to calm its outbursts. The practice, however, can have neither encouraged virginity, nor appeased the volcano, for its eruptions have continued unabated. Even when an eruption can be predicted, it cannot be prevented. It is possible, though, to limit the damaging effects of many types of activity and evacuate the threatened population. However, it is easier for some populations to move than others. The rich can pick up their cheque books and drive off, but poor peasants in poor countries have to stay with their fields because they have nothing else.
Much also depends on what the volcano is going to emit. Unless they are encased in a valley, lava-flows are the easiest formations to influence, partly because they move so slowly and partly because of the way they solidify. However, even lava- flows cannot be mastered: they can only be tamed like lions confined to their cages. The lower slopes of Etna provide the most propitious terrain in Europe for experiments in flow control, as Diego Pappalardo demonstrated in 1669 when he dug holes into the solidified sides of the flow advancing on Catania to divert it into a new direction. Unfortunately, however, it turned towards Paterno, whose inhabitants forced Pappalardo to abandon the scheme. As a result, until recently, lava-flow diversion has been forbidden and left in the hands of the Almighty in Sicily, although sometimes the veil of St Agatha was paraded before a flow to induce it to halt. Efforts at diversion, however, were made in Spring 1983, when explosives were used to break up one side of a flow that was threatening seven villages, and four ash and cinder dams were constructed to contain the lavas nearer their source. But it took much effort to move 750 000 m3 of material in 50 days at a cost of US$3 million. It may, in fact, have been cheaper to let the lava-flow proceed unhindered, but the experience gained was invaluable when Zafferana was threatened.
The eruption that began on 14 December 1991 presented a major challenge because it was the largest lava emission on Etna since 1669, and, given its initial discharge rates and the local relief, it was thought that the flow would overwhelm Zafferana, a town with 7000 inhabitants. Considerable resources were marshalled to prevent this disaster. A barrier of ash and cinders, 234m long and 21 m high - the highest yet built to control lavas - was thrown across the narrow exit of the Valle del Bove to impede the current and buy time to defend Zafferana. When the lavas spilled over this barrier on 7 April 1992, three further dams were built down valley, but, to avoid possible legal consequences, care was taken not to divert the flow onto property that would not otherwise have been affected. Continued copious effusions, however, had taken the snout to within 700m of the town by May 1992. Attention had, therefore, to be focused on the point of emission, 2000m high an d 8km from Zafferana, where the current might be more effectively stemmed. On four occasions, concrete and steel blocks were dropped from helicopters onto the flow to impede its progress. Each intervention gave about two weeks of respite before the snout resumed its advance towards Zafferana. The fifth and most successful intervention took place on 29 May 1992. A deep channel was dug branching at a small angle from the flow. At its head, the solidified walls of the flow were first thinned and then blasted open. Two-thirds of the molten current opted for the new channel, enabling its former course to be blocked with boulders. Deprived of its supply, the natural snout immediately halted, while the snout of the diverted lavas began to advance over the initial solidified flow, 6 km from Zafferana. The struggle to protect the town had thus gained time and distance. All this effort had its reward on 1 June 1992, when the rate of lava discharge from the vent fell by half and further reduced the threat to Zafferana, before the eruption soon waned considerably and ceased altogether in March 1993. These were the most comprehensive lava control measures yet undertaken. The diversion near the vent took seven weeks of co-operative effort and could only have been undertaken in a sparsely populated area - and even then some complained that ecologically valuable habitats had been destroyed ...
In spite of Etna's marked increase in activity since 1971, the people living on it usually have a respectful and fatalistic attitude to the volcano. "Etna is like a mother to us, she feeds and protects us, even though she is now acting like a cruel step- mother", said an inhabitant of Zafferana as the lavas advanced. Indeed, evacuation of homes has never seemed a serious option until lava has entered the kitchen; and the owner of one of the few houses to be overwhelmed by the flow in 1992 left an offering of bread and red wine on the table - and an imprecation against the government written on the wall.
The measures that can be taken against docile lava-flows would have little effect on lahars,jokulhlaups, blasts, poisonous gas, or nuees ardentes. Settlements in valleys radiating from dangerous strato-volcanoes would have to be surrounded by huge fortifications like the grandest of medieval citadels - and they would probably be thrown down or overridden anyway. After all, the nuee ardente from Montagne Pelee on 8 May 1902 had no difficulty in demolishing every building to its very foundations in the northern part of St Pierre. The only palliative to these lethal and fast-moving features is instant departure from the danger zone.
Falling ash causes damage to property but relatively few deaths. When ash rains down on a settlement, the inhabitants often panic and flee, especially since it can bring suffocating darkness too. Fear may also save lives, because the weight of ash accumulated often causes rooftops to collapse and bury those who stay behind. Fine ash spreads as all-penetratng clouds, choking humans, their animals, their cars, and even, on occasions, the engines of the their jet aircraft. Few effective measures, except perhaps shovelling, can be taken against ash, as both the Pompeiians and the people of east Washington State have discovered in their time. The main threat from ash comes when it destroys crops and animals, such as when ash from Irazu destroyed the Costa Rican coffee crop in 1965. In the past, ashfalls have provoked widespread famines, most notoriously when the eruption of Tambora in 1815 caused 92000 deaths in Indonesia. Replacement of destroyed crops and animals, resettlement and international aid are usually the sole solutions. As a consolation for the hungry, however, ash increases future yields by adding valuable nutrients to the soil. Indeed, it is reported that some Papuan tribes pray for ashfalls to make their crops flourish.
The most sophisticated methods of disaster alleviation or population protection have been developed in Japan, where Sakurajima, in particular, has virtually become a surveillance and disaster-plan laboratory that could serve as a model for the rest of the world. Sakurajima has been continually active since 1955 and its slightest whim can be forecast and analyzed a few hours in advance in the world's most modem volcano observatory. The government has spent a fortune on counteracting the effects of these eruptions, but does not apparently offer financial help to those who wish to leave the area and cannot sell their homes. Closely spaced concrete volcanic bomb shelters have been built along the roads; children wear helmets to go to school; many expensive dams and canals to slow down and divert lahars have been constructed; all information is computerized, televised and automatically transmitted to the competent authorities. Every year a full-scale evacuation is rehearsed so that everyone knows where to go and what to do when the alert is given. Temporary lodgings are always ready. Few populations are so well protected. . .
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