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Disaster risk and two
recent episodes in Japan
Kaoru Takara, Disaster Prevention Research Institute, Kyoto University, Japan
T
he following equation is often used to conceptually
explain disaster risk: DR = H x E x V. In this equation
DR stands for disaster risk, H is hazard, E is expo-
sure and V is vulnerability. Hazard is a natural phenomenon
such as earthquake, volcano eruption, landslide, storm,
flood and heatwave. Exposure is defined as something to be
affected by natural disasters, such as people and property.
Vulnerability is defined as a condition resulting from physi-
cal, environmental, social or economic factors or processes,
which increases the susceptibility of people or a community
to the impact of a hazard.
Imagine a rock island in an ocean with a tsunami approaching
it. If the rock island has no population and property, there will
be no disaster risk, because exposure (people and property) is
zero. In general, disaster risk is defined as an expectation value
of losses (such as deaths, injuries and property) that would
be caused by a hazard. Growing exposure and vulnerability
increases the number of natural disasters and the levels of loss.
Living land is a place where people and properties are
exposed to natural hazards. It should be protected from
hazards by countermeasures such as infrastructures, specifi-
cally flood control facilities and anti-earthquake buildings.
The countermeasures can reduce the vulnerability of the
society or living land to natural hazards, or reduce exposure
by adopting a policy of land use restriction.
Figure 1 explains disaster risk in living land, in which the
safer place is coloured green and the vulnerable place is indi-
cated by a yellow portion. The blue portion indicates exposure;
people and properties are located in both the safer and vulner-
able places. When a hazard attacks this living land, there will
be different disaster risk situations as indicated in the diagram:
(a) serious disaster damage is possible because people and
properties are exposed by the hazard in the vulnerable place
(b) people and properties are affected by the hazard but there
is no damage because they are located in the safer place
(c) the hazard hits the vulnerable place but there is no damage
because there are no people or properties there
(d) no damage because there is no exposures in the safer place
(e) risky but no damage; exposures in the vulnerable place are
about to be hit by the hazard, but it does not reach them.
Readers may easily recognize that if vulnerability becomes
bigger then disaster risk becomes bigger. Likewise, the more
exposures, the more disaster risk. Countermeasures should
make the area (a) smaller, namely reducing vulnerability by
protecting the vulnerable place and decreasing exposures by
relocating people and properties to the safer place.
Earthquake and tsunami in Sendai
Sendai is the largest city in the Tohoku region of Japan and
the capital city of Miyagi Prefecture. A tsunami hazard map
in the Sendai city area (786 km
2
with 1.04 million popula-
tion), as seen in Figure 2, shows the areas where tsunami
waves had been assumed to come up (the purple coloured
portion). Caused by the Great East Japan Earthquake on 11
March 2011, the actual tsunami waves came up to the orange
coloured portion and killed many people there. This suggests
a danger of hazard maps. People who lived in the orange
coloured areas may have misunderstood that there was no risk
of tsunami. Usually hazard maps for floods and tsunamis are
based on simulation results obtained from assumed 100-year
events. In March 2011, however, the tsunami event was much
larger than the envisioned tsunami. The tsunami hazard map
was not useful for such a tremendous tsunami event.
Sendai city revised the tsunami hazard map (Figure 3), which
shows the Level 1 tsunami area (orange portion) and Level 2
tsunami area (yellow portion). Two major roads have been
reconstructed in order to protect against tsunamis: one is called
the Shiogama-Watari Line (the 42.1 km long Prefectural Road
Figure 1: A conceptual explanation of disaster risk,
hazard, exposure and vulnerability
Source: Kaoru Takara
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