Risk and Avoidance - Tsunami - page 200

tsunami warnings. Following the Chilean Tsunami of 1960,
communities threatened by tsunami in Japan were identified
and protective seawalls built (Fig. 10.9 ). Now they line
40 % of its 34,751 km of coastline with some up to 12 m
high. As will be described subsequently, they have failed
badly. Large tsunami still require evacuation. Near-field
tsunami are a threat in Japan, especially along the Sanriku
coastline of northeastern Honshu, where only 25-30 min of
lapse time exists between the beginning of an earthquake
and the arrival at shore of the resulting tsunami. If it is
assumed that most people can be evacuated within 15 min
of a warning, then tsunamigenic earthquakes here must be
detected within the first 10 min. The P wave for any local
earthquake can be detected within seconds using an exten-
sive network of 180 high frequency and low magnification
seismometers. The Japanese Warning System also utilizes
The Geostationary Meteorological Satellite to disseminate
information in case the ground base network is destroyed in
a tsunamigenic earthquake. The Earthquake Early Warnings
system established in August 1, 2006 now utilizes the early
characteristics of seismic waves to predict seismic intensity
before the S, Rayleigh, and Love waves arrive. Warnings
can be issued within 3 min of an earthquake happening. In
the Noto Peninsula Earthquake having a magnitude of 6.9
on March 25, 2007, a tsunami warning was issued within
1 min and 40 s of the earthquake.
Based upon this information, a tsunami bulletin is issued
as a warning, watch, or no danger advisory. Warnings are
passed through central government offices, which include
the Maritime Safety Agency, which transmits warnings to
harbor authorities, fishing fleets, and fishermen, and the
Nippon Broadcasting Corporation, which broadcasts warn-
ings nationally on radio and television. At the same time,
warnings are transmitted to prefectures and to local
authorities via LADESS, the Local Automatic Data Editing
and Switching System. At the local level, warnings are then
issued via the Simultaneous Announcement Wireless Sys-
tem (SAWS). This system can switch on sirens and bells,
and even radios in individual homes. Mobile loudspeakers
mounted on fire trucks will also cruise the area broadcasting
the warning. In extreme cases, a network of individual
contacts has been established and the tsunami warning can
be transmitted by word of mouth or over the telephone.
Warnings issued within 15 min may not be good enough to
save lives. Local authorities may hesitate to initiate SAWS
and wait for confirmation of a tsunami warning for their
particular coast to appear in map form on television. These
maps take time to be drawn and do not appear as part of the
initial warning. Even where a direct warning is heeded, it
may be insufficient. In the Sea of Japan, the lapse time
between the beginning of an earthquake and the arrival at
shore of the resulting tsunami can be as little as 5 min. For
example, a tsunami warning was broadcasted directly to the

public, via television and radio, within 5 min of the
Okushiri, Sea of Japan earthquake of July 12, 1993.
However, by then, the tsunami had already reached shore
and was taking lives.
Most of the above measures failed during the T ¯hoku
Tsunami of March 11, 2011 despite a warning being issued
within 8 s of the first P wave being detected. Scientists and
government agencies had underestimated the potential for
faults offshore of the Sanriku coast to generate massive
earthquakes. Initial warnings issued by the Japanese Mete-
orological Agency also underestimated the size of the
earthquake and tsunami. The size of the predicted tsunami
was continually increased hours after it had devastatingly
impacted the coast. Following the event, the Agency
changed its procedures. Height estimates of run-up for any
earthquake of magnitude 8 or larger will no longer be
issued. Instead, the warning will be simply the possibility of
a huge tsunami. The T ¯ hoku Tsunami also exceeded the
limits of inundation maps prepared using historic events.
For example, in Unosumai, 86 % of the 485 deaths occurred
outside the mapped hazard area (Earthquake Engineering
Research Institute 2011 ). Over half of survivors complained
that they thought they were safe. Local knowledge appeared
just as unreliable. Following the 1896 Meiji Tsunami and
again after the 1933 Showa Tsunami, many towns erected
tsunami stones marking the limit of run-up, warning about
building seaward and showing safe havens (Murata et al.
2010 ). Of the 317 such stones identified, 40 % were washed
away by the 2011 Tsunami (Nishio 2013 ). Modern warning
signs assumed a 45-year recurrence for tsunami and were
set far too close to the shoreline. In many places, the 2011
tsunami exceeded the height of these markers by 10 m
(Earthquake Engineering Research Institute 2011 ). Walls
built to protect towns were just as unreliable. At Ryoishi
and similar towns along the Sanriku coast, these walls had
been extended after 1960 to protect them against a tsunami
equivalent to the Meiji Tsunami of 1896 (Fig. 10.9 ). And in
1985, it was decided to extend the wall at Ryoishi to a
height of 12 m to match the most probable tsunami that
included reflection from the gigantic break-wall built in
nearby Kamaishi Bay. However budget constraints restric-
ted the wall to an elevation of 9.5 m. The T ¯ hoku Tsunami
had a flow depth of over 15 m along this coast and either
overtopped or breached most all of these walls (Fig. 10.10 ).
At Ryoishi, 40 residents lost their lives.
People's plans for avoiding the tsunami were also mis-
guided. People had also been told to evacuate vertically, to
the third story or above in concrete buildings. Hundreds of
these buildings were identified in urban areas. Over 100
failed to provide safety because they did not stand up to the
force of the flow or were lower than the depth of water, which
in many locations exceeded three floors (Fig. 10.11 ). Many
buildings were too close to the coast. Once people had fled to