Volcanic Hazards

The multiplicity of events that can occur from a single volcano adds to the complexity of hazard prediction and civil defence response compared to straightforward earth-shaking events or a chemical spill. The following section delves into the many types of hazards associated with volcanic eruptions, following on from 1.5 in the previous section. In addition to the following section, you should read pages 218-228 of Chapter 12 “Volcanoes as a hazard” of your textbook by Bryant (1991).

Hazard versus risk

Volcanic hazard is defined as the probability of an area being affected by potentially destructive volcanic processes or products within a given period of time. Volcanic risk is the possibility of a loss (e.g. of life, property or production potential) within the area subject to the hazards. Assessment of risk involves calculation of the following relationship:

Risk = Value x Vulnerability x Hazard

where “value” can include number of lives, property, civil works and production capacity threatened, and “vulnerability” is a measure of the proportion of the value (0-100%) likely to be lost in a given hazardous event.

Volcanic disasters versus other types of disasters

On a global basis, volcanic and related hazards occur less frequently, affect fewer people, cause fewer casualties and smaller economic losses than many other natural or man-made disasters (Fig. 5A, B).

Figure 5.

The deadliest eruption in history was in 1815, Tambora, Indonesia which killed 92 000 people. However, 500 000 people were killed in the worst hurricane in the Ganges Delta, Bangladesh in 1970, and the worst natural disaster in history was the Huahsien Earthquake in China in 1556 which resulted in more than 820 000 fatalities.

In the U.S.A. the annual economic loss due to volcanic activity is an order of magnitude smaller than that from earthquakes, and almost two orders of magnitude less than the loss from floods and ground failures. It has been suggested that in the U.S. the average person is more likely to die from a heart attack shovelling snow or from a lightning strike than from either an earthquake or volcanic eruption. However, for more densely populated countries such as Japan, Indonesia and the Philippines, volcanic hazards have a far greater potential for causing enormous economic and social losses.

Historical volcanic disasters

Table 5. Selected volcanic disasters and their respective human fatalities in the period from 1000-1985 A.D. Adapted from Tilling (1989).

The loss of human lives is probably the best basis by which to compare historic eruptions, other effects such as economic losses are not completely known or difficult to compare for different times and places. The historical record, presented in Table 5, is dominated by a few particularly destructive events. Between 1000 A.D. and 1985, more than 300 000 people have been killed directly or indirectly by volcanic activity. Most of these lives have been lost in the circum-Pacific area, with over 65% of the losses in only three countries - Colombia, Indonesia and Japan.

Table 6. Proportions of historical human fatalities from differing volcanic hazards, adapted from Tilling (1989).

The average number of fatalities per year in the 20th Century is higher than that of the preceding three centuries (Table 6), despite developments made in the scientific study of volcanic activity over this time. However, in the 20th Century the incidence of fatalities caused by indirect hazards (including starvation and tsunamis) has decreased (Table 6), probably due to improved communications and relief systems. These conclusions are however biased by the short interval of time (compared to geologic time) considered, and that there were no large tsunami-producing eruptions in the 20th Century. The large increase in the number of deaths associated with pyroclastic flows, debris avalanches and debris flows in the 20th century is biased by the two disastrous events at Mont Pelée in 1902, and Nevado del Ruíz in 1985.

Short and long term volcanic hazards

There are two broad levels of volcanic hazards:

(1) those that occur frequently (i.e. more than once per century) and which most people living near a volcano are likely to experience at least once during their lifetimes - here termed short or immediate term hazards;

(2) those that occur less frequently (much less than once per century) and which people are unlikely to experience in their lifetimes - termed long term or potential hazards. An example of short term hazards are lahars on the slopes of Ruapehu, affecting ski slopes in 1969 and 1995. An eruption of Taupo volcano can be considered a long term hazard, its average eruption recurrence interval is of the order of 2000 years and no activity is known for the last c. 2000 years.

These two hazard levels need to be planned for in different ways. Where short term hazards are clearly present, the volcano should be kept under intensive surveillance. Areas at risk need to be zoned and the activities within them restricted. At volcanoes where only long term hazards are likely, some instrumental monitoring is desirable and although it is difficult to restrict land uses, in some areas developmental planning will need to take the potential hazards into account.