A capability approach for seismic risk analysis and management - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 259

10.3.2 Risk acceptability and tolerability
In a capability approach, judgment about the acceptability of a risk due to
a natural hazard such as an earthquake is based on a comparison of the
likely impact on capabilities with two thresholds (Murphy and Gardoni
2008). The fi rst is a threshold for acceptable risk. This threshold identifi es
the level of capabilities it is acceptable in principle for individuals to have
in the aftermath of a hazard. Put another way, the threshold for acceptable
risk identifi es a level below which the capabilities of individuals should not
fall. Because there is uncertainty surrounding what impact a given hazard
will have upon capabilities, it is necessary to specify the probability that
capabilities will be above the acceptable threshold.
The second is a threshold for tolerable risk. This threshold is lower than
the acceptable threshold. Specifying a tolerable threshold is a response to
the recognition that it may not be reasonable to demand that communities
guarantee the acceptable level of capabilities even in the immediate after-
math of an earthquake. Communities might permissibly fall below the
acceptable threshold in the emergency phase, provided this is a temporary
and reversible situation. At the same time, there should be a limit to the
impact of capabilities that communities permit even at that time. The toler-
able threshold responds to both of these concerns. It identifi es a level of
capabilities below which it is never permissible to go, even if this level is
temporary and reversible.
To illustrate the basic idea behind these two thresholds, consider the
capability for shelter. Having a genuine opportunity to be sheltered in a
secure manner depends upon the presence of a materially adequate, given
the environmental conditions of a particular locale, and stable dwelling. It
may be permissible for a community to allow a certain probability of a
specifi ed level of damage to dwellings that, if experienced, would make it
necessary for some individuals to fi nd alternate dwelling. The potential loss
of the use of a stable dwelling would mean that the capability for shelter is
to some extent insecure. Available alternate dwelling, if for example tem-
porarily provided by the government, would not satisfy the acceptable level
of shelter. However, such temporary housing may be tolerable, provided it
is temporary and reversible.
The process of developing and applying these thresholds can respond to
the concerns about involving the public in the process of evaluating risks,
while acknowledging the concern about the limits to the public's knowledge
and understanding of the technical dimensions of risks (Murphy and
Gardoni 2008). Acceptable and tolerable thresholds articulate certain value
judgments about the kind of opportunity a community is committed to
ensuring its members enjoy. Because of this, it is appropriate for the public
to play a role in determining what these thresholds are. Acceptable and