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<DIV class=article-heading><FONT size=4>Tsunami simulations scare
Japan</FONT></DIV></HGROUP>
<DIV>Updated risk assessment warns country to prepare for much larger
earthquakes and waves.</DIV>
<DIV><SPAN class=vcard><A class=fn
href="http://www.nature.com/news/tsunami-simulations-scare-japan-1.10460#auth-1">David
Cyranoski</A></SPAN></DIV><TIME datetime="2012-04-17" pubdate>
<DIV class=pubdate-and-corrections>Nature. Volume: 484<SPAN>,
</SPAN>Pages:296–297 <ABBR
title="Digital Object Identifier">doi</ABBR>:10.1038/484296a</DIV>
<DIV class=pubdate-and-corrections>17 April 2012</TIME></DIV>
<DIV class=dateline><A
href="http://www.nature.com/news/tsunami-simulations-scare-japan-1.10460">http://www.nature.com/news/tsunami-simulations-scare-japan-1.10460</A></DIV>
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<P class=caption><FONT size=1>Japan has been forced to review its tsunami
defences in the wake of last year’s disaster. </FONT><FONT
size=1>STR/epa/Corbis</FONT></P></DIV></DIV>
<DIV class=article-tools>Japan’s government is heeding a key message from last
year’s Tohoku earthquake and tsunami: the underwater faults that encircle the
country can unleash much greater devastation than previously anticipated. Last
week, the cabinet’s disaster-management division briefed local officials on
simulations that raise the spectre of waves even larger and more destructive
than those last March, sending the officials scrambling to rethink their tsunami
defence plans.</DIV>
<P>The estimates come from a government-appointed team of scientists led by
Katsuyuki Abe, a tsunami expert and emeritus professor of the University of
Tokyo. The team updated the nation’s main tsunami model by increasing the
magnitude of the largest expected earthquake to incorporate recent massive
quakes, including the magnitude-9.0 Tohoku and the magnitude 8.8 that hit Chile
in 2010, causing a tsunami that reached as far as Japan. The result: for most
locations, the size of future tsunamis could far outstrip previous estimates,
which were made in 2003. The town of Kuroshio in Kōchi prefecture was steeled
for a maximum 14.1-metre wave; it now faces the threat of a 34.4-metre
inundation. Likewise, the offshore island of Niijima has to prepare for a
29.7-metre wave, rather than 5.4 metres (see <A
href="http://www.nature.com/news/tsunami-simulations-scare-japan-1.10460#waves">‘Making
waves’</A>).</P>
<P>Ninety cities and towns must consider how to withstand a tsunami wave of
higher than 10 metres, whereas just ten locales were expecting this from
the older simulations. Twenty-three have been told to prepare for a tsunami of
20 metres or more, a threat none had previously anticipated. The team also
raised the estimates of the risks posed by the largest earthquakes, with the
number of towns and cities expecting the maximum level of ground shaking — 7 on
Japan’s intensity scale — raised from 35 to 153.<A name=waves></A></P>
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<P>The reappraisal focuses on the Nankai trough, an offshore fault south of
Honshu, Japan’s main island, that regularly produces large quakes, including the
magnitude-8.7 Hoei quake of 1707 that set the ceiling for the 2003 estimates.
The latest simulations use the Tohoku quake, which was roughly three times more
powerful than Hoei, as the maximum. They also take account of recent studies
that used the thickness of sediment layers deposited by past tsunamis to
estimate the size and frequency of major earthquakes and tsunamis that happened
before accurate measurement methods existed. The 2003 models, which predicted
much smaller tsunamis than those that followed the Tohoku earthquake, have been
criticized for not including such sedimentation data (see <A
href="http://www.nature.com/uidfinder/10.1038/483141a"><I>Nature
</I><B>483,</B>141–143; 2012</A>).</P>
<P>The updated model also considers ‘large slip’ areas, in which extensive crust
movement can make parts of tsunamis particularly hefty. Such areas were to blame
for the unexpectedly high waves in parts of Japan last year. The team considered
how coastal regions would be affected in 11 tsunami scenarios, each of which had
large slip areas in different places along the Nankai trough.</P>
<DIV class="related-stories-box box">Magnitude-9 events are expected to occur
very rarely, perhaps once every millennium, says Kenji Satake, a tsunami expert
at the University of Tokyo and a member of the team that updated the models. How
much preparation is needed for such rare, devastating events remains a matter of
debate, but the models have put local governments in a tight spot. Yukihiko
Nakamura, head of earthquake preparedness in the Kōchi prefecture, says that the
briefing was not detailed enough to give administrators a clear path
forward.</DIV>
<P>“We need to know how high the water will come at different points. Will it be
34 metres everywhere?” The government has promised a fine-grained
simulation and inundation maps that detail water levels by the end of May.</P>
<P>Even that information may not help officials to determine how to prepare for
such devastating events. Some towns are installing additional loudspeakers to
warn citizens of an approaching tsunami, but they might give no more than a few
minutes’ warning. Nakamura says it is unreasonable to think that people could
seek shelter — by climbing evacuation towers tens of metres tall, for example —
within that time.</P>
<P>Officials in some regions are also making plans to move city offices to
higher ground, and considering measures such as building large underground
shelters or relocating large numbers of homes — both of which are costly options
that are beyond the capacity of local or prefectural governments. A cabinet
working group is discussing how the central government could help.</P>
<P>Before last year’s devastating earthquake, “we had been preparing for a
magnitude-8 quake they said would have 60% chance of coming in the next 30
years”, says Nakamura. “But after Tohoku, we have to be prepared for the
unexpected, too.”</P></DIV>
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