138
7.2.2 New Tsunami Classification System
The 2011 Tohoku Earthquake Tsunami led to major discussions within the Japanese
coastal engineering and management community about whether hard measures
(such as breakwaters or dikes) are preferable over soft measures (such as tsunami
warning systems and evacuation plans) to protect the coastline and the communities
situated next to it (Shibayama et al. 2013). Eventually, the concept of Level 1 and
Level 2 tsunamis emerged based on ideas concerning time scales and the likelihood
of an area being affected by such events. These concepts are widely used today, and
formed the cornerstone of reconstruction philosophy in the aftermath of the event
(as will be expounded upon later in this chapter). It is important to note that this
classification is based on the frequency of these events, and that the exact period of
return of each of the events has yet to be fixed, though there is a clear feeling that
one of them relates to human life-spans, and the second to civilization time scales.
The two levels would be:
Level 1 Tsunami Events with a return period of several decades to 100+ years
(essentially, the Japanese expression which has been used in coastal engineering
discussions would translate as a return period from 50–60 to 150–160 years).
Although the height of the wave would depend on the event and location, Level 1
Tsunamis refer to waves which are comparatively low in height, typically less than
7–10 m.
Level 2 Tsunami These events have return periods of between one hundred to a few
thousand years. The tsunami heights are much higher, and encompass waves over
10 m in height, and sometimes even up to 20–30 m. Clearly, both the 2004 Indian
Ocean and 2011 Tohoku Earthquake Tsunamis fall under this category.
It is important to note that given the nature of the propagation process, a given
event might represent a Level 2 tsunami for a certain area or country, yet only a
Level 1 event for other places. For instance, the 2011 Tohoku Earthquake Tsunami
was clearly a Level 2 event in northern Japan, though by the time it reached Chile
the tsunami was only a Level 1 event.
Events such as meteorite impacts or underwater landslides, which can cause devastating waves over 50 m in height, are outside the scope of this classification. These
types of hazards would have return periods of tens or hundreds of thousands of
years, and would completely devastate a coastline, reaching dozens of kilometres
inland and probably rendering useless any evacuation strategy in place. One could
actually talk about “Level 3” events, and it is unclear whether present day technology is advanced enough to protect human society from them. To the authors’ knowledge, no strategies are currently in place anywhere in Japan (arguably the country in
the world which has invested the most (Mori et al. 2012) in improving resilience
against natural disasters in general, and tsunamis in particular) or any other country
to protect against such events. This highlights how the time scales involved in an
event dictate the type of actions (or complete lack of actions, for events with very
high return periods) employed to improve resilience and mitigate the consequences
of a given hazard.
M. Esteban et al.
7.2.2 New Tsunami Classification System
The 2011 Tohoku Earthquake Tsunami led to major discussions within the Japanese
coastal engineering and management community about whether hard measures
(such as breakwaters or dikes) are preferable over soft measures (such as tsunami
warning systems and evacuation plans) to protect the coastline and the communities
situated next to it (Shibayama et al. 2013). Eventually, the concept of Level 1 and
Level 2 tsunamis emerged based on ideas concerning time scales and the likelihood
of an area being affected by such events. These concepts are widely used today, and
formed the cornerstone of reconstruction philosophy in the aftermath of the event
(as will be expounded upon later in this chapter). It is important to note that this
classification is based on the frequency of these events, and that the exact period of
return of each of the events has yet to be fixed, though there is a clear feeling that
one of them relates to human life-spans, and the second to civilization time scales.
The two levels would be:
Level 1 Tsunami Events with a return period of several decades to 100+ years
(essentially, the Japanese expression which has been used in coastal engineering
discussions would translate as a return period from 50–60 to 150–160 years).
Although the height of the wave would depend on the event and location, Level 1
Tsunamis refer to waves which are comparatively low in height, typically less than
7–10 m.
Level 2 Tsunami These events have return periods of between one hundred to a few
thousand years. The tsunami heights are much higher, and encompass waves over
10 m in height, and sometimes even up to 20–30 m. Clearly, both the 2004 Indian
Ocean and 2011 Tohoku Earthquake Tsunamis fall under this category.
It is important to note that given the nature of the propagation process, a given
event might represent a Level 2 tsunami for a certain area or country, yet only a
Level 1 event for other places. For instance, the 2011 Tohoku Earthquake Tsunami
was clearly a Level 2 event in northern Japan, though by the time it reached Chile
the tsunami was only a Level 1 event.
Events such as meteorite impacts or underwater landslides, which can cause devastating waves over 50 m in height, are outside the scope of this classification. These
types of hazards would have return periods of tens or hundreds of thousands of
years, and would completely devastate a coastline, reaching dozens of kilometres
inland and probably rendering useless any evacuation strategy in place. One could
actually talk about “Level 3” events, and it is unclear whether present day technology is advanced enough to protect human society from them. To the authors’ knowledge, no strategies are currently in place anywhere in Japan (arguably the country in
the world which has invested the most (Mori et al. 2012) in improving resilience
against natural disasters in general, and tsunamis in particular) or any other country
to protect against such events. This highlights how the time scales involved in an
event dictate the type of actions (or complete lack of actions, for events with very
high return periods) employed to improve resilience and mitigate the consequences
of a given hazard.
M. Esteban et al.