Quake may trigger ’gigantic’ tsunami

Local governments in tsunami-prone areas are calling for the government
to give a clear definition as early as possible of the height of "the
largest possible tsunami" predicted by a government panel in preparation
for working out countermeasures.

They have asked the central government for clarification as to what
would be the maximum height of the largest possible tsunami cited by an
expert panel of the Central Disaster Management Council, an advisory
organ of the Cabinet Office.

In a preview of its interim report on tsunami response measures compiled
Sunday, the panel calls for the central and local governments to work
out measures that can deal with "the largest scientifically possible
tsunami."

Feeling the strongest sense of pressure and urgency are those local
governments threatened by Tokai, Tonankai and Nankai earthquakes, which
experts say may occur in succession.

As for those quakes, experts forecast at least a 60 percent probability
of occurrence within 30 years. The researchers who analyzed the Great
East Japan Earthquake and ensuing tsunami warned that there is a
possibility of a “gigantic” tsunami up to two times the size of what was
previously thought to be the largest possible tsunami.

The Nagoya municipal government decided earlier this month to designate
103 buildings it owns in areas close to the sea as tsunami evacuation
centers. The designated buildings, which are at least four stories high,
can accommodate only 10 percent of about 300,000 residents living around
them. The municipal government intends to call for the cooperation from
owners of private buildings.

The height of the tsunami assumed to follow Tokai and Tonankai
earthquakes if they occur in tandem is two to three meters at most. The
city’s disaster response office is concerned about what would happen if
Tokai, Tonankai and Nankai earthquakes occur in conjunction. "We want
the central government to present its estimate of tsunami size soon so
that we can reinforce wave barriers and reexamine hazard maps," a city
official said.

In Osaka Prefecture where barriers five to six meters high have been
built in coastal areas, a possible tsunami is assumed to measure less
than three meters high.

In preparation for a huge tsunami, the prefectural government is
confirming whether evacuation centers exist in low-lying areas. "Drastic
measures will be formulated after the [central] government announces its
estimates of the tsunami’s size," said an official of the prefectural
government’s crisis management section.

The government council’s expert panel pointed out the need to improve
response measures in areas where no tsunami has occurred but the
possibility of one happening cannot be ruled out. One such area is the
coastal area along the Sea of Japan stretching from the southern part of
Akita Prefecture to Yamagata Prefecture.

In Akita Prefecture, tsunami hazard maps have been drawn up in only
three of eight cities and towns along the coast of the Sea of Japan. The
remaining five cities and towns began compiling maps in the aftermath of
the March 11 disaster.

The prefectural government has asked Akita University to study how to
deal with elderly people who cannot escape to higher ground and other
safe places quickly in times of tsunami.

The Yomiuri Shimbun , Jun. 21, 2011
http://www.yomiuri.co.jp/dy/national/T110620005368.htm

 Panel urges tsunami defense / Local govts advised to prepare for possible massive disaster

An expert panel under the government’s Central Disaster Management
Council is urging the central and local governments to prepare for the
largest tsunami deemed possible by science.

The proposal was included in the outline of an interim report on
anti-tsunami measures compiled by the panel in its third meeting Sunday.
Led by Kansai Univ Prof. Yoshiaki Kawata, the panel is expected to
release the interim report later this month.

The panel is calling on the central and local governments to drastically
review their conventional anti-tsunami measures, and the proposal is
expected to have a significant impact on those measures, observers said.

According to Kawata, past measures against tsunami were drawn up based
on the largest tsunami among confirmed cases in the past. Only evidence
that was deemed virtually certain was used to determine wave heights.

However, the panel is calling for more importance to be given to
research and analysis of tsunami traces left in geological layers and
other scientific information.

When compiling antidisaster measures, the panel urged authorities to
draw up plans in anticipation of the largest tsunami that can be
anticipated from such evidence.

Prior to the March 11 disaster, some local governments in the Tohoku
region had drawn up antidisaster plans in anticipation of a magnitude-8
earthquake off Miyagi Prefecture. The area was hit by a magnitude-9
quake in the Great East Japan Earthquake.

Through examination of sediment in old strata, experts concluded that
the 869 Jogan tsunami caused about the same level of inundation as the
Great East Japan Earthquake. However, local governments were unable to
make use of the finding in their anti-tsunami measures in time for the
March 11 disaster.

On the Pacific coast of western Japan, it was previously believed that
the tsunami observed during the 1707 Hoei earthquake, in which Tokai,
Tonankai and Nankai earthquakes occurred in rapid succession, was the
largest ever.

However, a recent survey uncovered materials deposited by a tsunami in a
geological layer about 2000 years ago at a thickness 2.5 times greater
than that left by the Hoei earthquake.

"Previously, we did not take into account something so unlikely [in our
prediction]. From now, we’ll include undocumented cases as well [in our
predictions]."

The panel is expected to compile a final report in autumn.

The Yomiuri Shimbun, June 21, 2011
http://www.yomiuri.co.jp/dy/national/T110620005178.htm