decision-making on evacuations. A probabilistic event tree
assigns a probability to each eruptive scenario, and also
describes the possible evolution and concatenation of
scenarios. Assigned probabilities must be updated as a
crisis evolves.
Risk is defined as the likelihood of a hazard occurring
multiplied by the vulnerability of people or infrastructure to
be impacted by the hazard. Because natural phenomena and
their associated hazards cannot generally be modified, risk
mitigation is mainly related to the reduction of the hazard
exposure and of the vulnerability of the exposed systems.
Vulnerability is defined as the inability of physical and
human systems to withstand the impacts of a natural
hazard. Formally, it can be stated in terms of a probability
Chapter | 68 Volcanic Crisis Management 1191
implementation of a policy of land development (discour-
aging new settlements and encouraging emigration into
safer areas).
At this point, continuing education of the population is
required to ensure that awareness of the potential threat is
kept current, especially if a volcano has not erupted for many
years. Dissemination of information about the phenomenon,
its reach and effects, and the mitigation measures should
thus be one of the top priorities in any preparedness actions.
Specifically, the main measures of self-protection, such as
self-evacuation, preparation of emergency kits, or develop-
ment of community or household plans, and response to a
volcanic emergency must be widely explained. There are
many methods of dissemination of messages, and an
assessment of the most effective methods for communica-
tion of hazard information should be undertaken. In devel-
oping countries with basic infrastructure, this may take the
form of community meetings and face-to-face discussions;
in areas with highly developed communication, other mea-
sures such as social media, Internet, and television may be
available.
Some difficulties in this dissemination process may be
anticipated. For instance, in highly developed, industrialized
countries, the possibility of land devaluation may sometimes
pose a problem in the handling of this information.
Within a developing country, there are many re-
quirements for limited resources. For example, if a volcano
has not erupted recently, and communities are subjected to
social deprivation issues such as high levels of crime or lack
of suitable housing, it is likely that the volcanic mitigation
measures are seen as much lower priority than the provision
of basic sanitation or crime prevention. In this case, com-
munities and the authorities together need to decide on the
best use of available resources. Sometimes political prob-
lems, particularly when the volcano is located at or near a
border region, may arise.
In these cases, important aspects of preparedness
include the construction and dissemination of a document
outlining organizational responsibilities, resulting from a
discussion and agreement among the involved parties.
Another important step for risk reduction is the imple-
mentation of infrastructural measures, related mainly to
engineering works aimed at protecting structures and its
inhabitants. Although not much can be done in this respect
for the safekeeping of structures from the most severe
eruptive phenomena, such as pyroclastic and lava flows,
protection at reasonable costs from the fall of tephra can be
achieved with proper roof structure.
Some of the most destructive volcanic phenomena are
mudflows or lahars. The energy and range of lahars may be
reduced through barriers to retain or divert solid materials
in the mudflows. These constructions are rather complex
and expensive. However, they have been extensively
implemented in several countries; for example, in Japan, a