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Figure 5: GPS data at Ojika
Early Warning
Pre-signal
15 days before
Early warning is the most diffi cult task in
disaster management. Early warning of the
Tsunami had intended to be based on GPS
wave height recorder as shown in Figure
6. However, although there were about 15
GPS wave height recorders installed 15
to 20 km offshore along the Pacifi c coast,
there was only one Tsunami GPS wave
height recorder located 15 km offshore of
Kamaishi City near the epicenter, which
could record valuable wave form of the
Tsunami. It revealed the fi rst occurrence
of a 6.6 m high Tsunami 30 minutes
after the earthquake and subsequently a
total of seven occurrences up to 6 hours
afterwards. This would be the fi rst record
of a giant Tsunami in the world (see Figure
7). The Tsunami wave offshore will be
increased up to about three times (about
20m) as it crosses the coast depending on
the sea bottom topography and will fl ow
inland up to almost 30 to 40 m above the
Figure 7: Tsunami Height Record
36 | Coordinates January 2013
Pre-signal
3 days before
Preparedness
sea level. Though the
GPS wave height recorder
is useful to record the
Tsunami wave forms, it
would not work as early
warning system because
the Tsunami wave will
arrive at the coast within
about 5 minutes with the
velocity of about 100
km per hour. But the
Japanese government is
attempting to increase
the number of GPS
wave height recorders
offshore in the Pacifi c
Ocean for early warning.
Preparedness against future earthquakes
and Tsunamis are most important in order
to avoid a similar catastrophe. Many
people tried to escape responsibilities
for not preparing for such a disaster by
saying that they did not expect such a large
Tsunami. Now they cannot avoid being
responsible for one of the worst examples
of making an incorrect assumption leading
to a lack of preparedness. Archaeological
excavations have investigated ground
layers along the coast lines in Japan to
reveal the traces of past giant Tsunamis
as shown in Figure 8-1. The Tsunami
layers are easily detected from such
ground excavations as shown in Figure
8-2. The profi le shows that giant Tsunamis
occurred about every one thousand years.
According to such records, simulations
of Tsunamis have been implemented
under an assumption of the three worst
Figure 6: GPS Wave Height Recorder
cases of giant earthquakes of Nanaki,
Tonakai and Tokai, which occurred at
the Nankai Trough in 1707 (called Hoei
Great Earthquake with Tsunami and the
eruption of Mt. Fuji 49 days afterwards).
The result of the simulations in this worst
case showed that a Tsunami higher than
10 to 30 m would occur along wide areas
of the Pacifi c coast as shown in Figure
9. The detail topographic survey of the
land portion, as well as the sea bottom
are very necessary for the simulations.
Relocation of residential
zones to higher land
Relocation of residential zones to higher
ground will be a key point to avoid future
devastation and loss of life. However
Figure 8: 1 Archaeological excavation
Figure 8: 2 Geological layers
showing the past Tsunami