Comprehensive Risk Assessment for Natural Hazards - Planat

Chapter 4
VOLCANIC HAZARDS
4.1 INTRODUCTION TO VOLCANIC RISKS
Every year several of the 550 historically active volcanoes on
earth are restless and could pose a threat to mankind (see
Table 4.1); two recent examples are particularly relevant. On
19 September 1994, the Vulcan and Tavurvur volcanoes in
the Rabaul Caldera, Papua New Guinea, began to erupt.
Monitoring of precursors and awareness of the population
of the eruptions allowed the safe evacuation of 68 000 people.
The economic damage due to ash fall was significant.
On 18 July 1995, a steam blast explosion occurred on the
dormant Soufrière Hills volcano, Montserrat, West Indies.
This event was followed by an ongoing activity that included
a larger event on 21 August 1995, which generated an
ash-cloud that menaced the capital, Plymouth. About 5 000
out of 12 500 inhabitants of the island were temporarily
evacuated from the southern high hazard area towards the
centre and the north of the island. Since then, the volcanic
activity progressively developed to the point where it affected
Plymouth on 6 August 1997. Eighty per cent of the buildings
were either badly damaged or destroyed, but the previously
evacuated population were safe, although for greater security,
they were moved further north. These two cases
demonstrate that with a good understanding of the hazardous
phenomenon, appropriate information, and
awareness of the population and the authorities, it is possible
in most cases to manage a difficult situation. This, of
course, does not alleviate all personal suffering, but contributes
to its reduction.
Before entering into a description of volcanic hazards
and the different ways in which they can be surveyed, it is
important to present the way in which they are integrated
into risk analyses (Tiedemann, 1992). This approach provides
the basis for developing sound mitigation measures.
Figure 4.1 gives a global view of the problem, whilst its different
aspects will be presented later in this chapter.
Volcanic risk may be defined as: The possibility of loss
of life and damage to properties and cultural heritage in an
area exposed to the threat of a volcanic eruption.
This definition can be summarized by the following
formula (UNDRO, 1980):
Risk* = f(hazard,vulnerability,value)
* See the glossary for the different definitions
The volcanic hazard, denoted H v , can also be written in
the following form:
H v = f (E,P) (4.1)
with E being an event in terms of intensity or magnitude,
duration and P being the probability of occurrence of that
type of event. The product of the vulnerability, denoted V u ,
times the value of the property, denoted V a ,is a measure of
the economic damages that can occur and is given by the
relation:
D = V u • V a (4.2)
Table 4.1 — Examples of major volcanic eruptions during the 20th Century
Year Volcano Country Type of eruption Consequences
1980 Mount St Helens USA Collapse with explosion, 57 deaths, major environmental
pyroclastic flow, debris flow destruction
1982 El Chichon Mexico Explosive, pyroclastic flow 3 500 deaths, high atmosphere
effects
1985 Nevado del Ruiz Columbia Explosive, ice melting, lahars 22 000 deaths, related mainly to
the lahar passing through Armero
1986 Oku volcanic field, Cameroon Carbon dioxide gas released 1 700 persons perished due to the
lake Nyos by the lake lethal gas; 845 were hospitalized
1991 Pinatubo Luzon, Explosive, pyroclastic flow, 900 deaths, 1 000 000 people
Phillipines ash fall and lahars affected by the devastation
1991 Unzen Kyushu, Japan Preatic eruption, extrusion 43 deaths, 6 000 people were
and growth of lava domes, evacuated; 338 houses were
pyroclastic flow
destroyed or damaged
1994 Rabaul Caldera Papua New Ash eruption from two Large portion of the town Rabaul
(Tavurvur and Guinea volcanic cones Vulcan was destroyed by ash fall;
Vulcan) and Tavurvur 50 000 people evacuated safely
from damaged areas
1995– Soufrière Hills Montserrat, Phreatic eruption, dome 19 deaths; of the 11 000 people of
1998 Caribbean, (UK) growth and collapses, the island 7 000 were evacuated
explosion, pyroclastic flow,
ash fall