One of the developments in tsunami hazard assessment
in the last decade is Probabilistic Tsunami Hazard
Assessment (PTHA) [86,87]. Results of the PTHA are
typically displayed as hazard curves that show the annual
frequency of exceedance of tsunami heights. The hazard
from a large number of possible sources including nonearthquake
source can be aggregated together to develop
a tsunami hazard curve. In addition, multiple sources of
uncertainty related to the source parameters and tsunami
numerical computations can be considered in the PTHA.
Uncertainty can be classified into two types: aleatory
and epistemic. Aleatory uncertainty, or random variability,
relates to the natural or stochastic uncertainty inherent
in a physical system, and cannot be reduced but can be
estimated from repeated observations or experiments.
Epistemic uncertainty is due to incomplete knowledge
and data, and can be reduced by the collection of new
data. Epistemic uncertainty can be treated as logic trees
[88]. A single hazard curve is obtained by integration
over the aleatory uncertainties, and a large number of
hazard curves are obtained for different branches of a
logic-tree representing epistemic uncertainty. The PTHA
for Fukushima Dai-ichi Nuclear Power Station estimated
that the annual exceedance of 10 m high tsunami was
an order of 1 × 10−5, or return period of around 100,000
years.