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The Tokai Earthquake
by Andrew Alden
The great Tokai Earthquake of the 21st century has not happened yet, but Japan has been getting ready for it for over 30 years.
All of Japan is earthquake country, but
its most dangerous part is on the Pacific coast of the main island Honshu, just
southwest of Tokyo.
Here the Philippine Sea plate is moving
under the Eurasia plate in an extensive subduction zone.
From studying centuries of earthquake
records, Japanese geologists have mapped out segments of the subduction zone
that seem to rupture regularly and repeatedly.
The part southwest of Tokyo, underlying
the coast around Suruga Bay, is called the Tokai segment.
Tokai Earthquake
History
The Tokai segment last ruptured in 1854,
and before that in 1707. Both events were great earthquakes of magnitude
8.4.
The segment ruptured in comparable events
in 1605 and in 1498.
The pattern is pretty stark: a Tokai
earthquake has happened about every 110 years, plus or minus 33 years.
As of 2012, it has been 158 years and
counting.
These facts were put together in the 1970s
by Katsuhiko Ishibashi.
In 1978, the legislature adopted the
Large-Scale Earthquake Countermeasures Act.
In 1979, the Tokai segment was declared an
"area under intensified measures against earthquake disaster."
Research began into the historic
earthquakes and tectonic structure of the Tokai area.
Widespread, persistent public education
raised awareness about the expected effects of the Tokai Earthquake.
Looking back and visualizing forward, we
are not trying to predict the Tokai Earthquake at a specific date but to
clearly foresee it before it happens.
Worse than Kobe,
Worse than Kanto
Professor Ishibashi is now at the
University of Kobe, and perhaps that name rings a bell: Kobe was the site of a
devastating quake in 1995 that the Japanese know as the Hanshin-Awaji
earthquake.
In Kobe alone, 4571 persons died and more
than 200,000 were housed in shelters; in total, 6430 people were killed.
More than 100,000 houses collapsed.
Millions of homes lost water, power, or both. Some $150 billion in damage was
recorded.
The other benchmark Japanese quake was the
Kanto earthquake of 1923. That event killed more than 120,000 people.
The Hanshin-Awaji earthquake was magnitude
7.3. Kanto was 7.9. But at 8.4, the Tokai Earthquake will be substantially
larger.
Tracking The Tokai
Segment With Science
The seismic community in Japan is
monitoring the Tokai segment at depth as well as watching the level of the land
above it.
Below, researchers map a large patch of
the subduction zone where the two sides are locked; this is what will let loose
to cause the quake.
Above, careful measurements show that the
land surface is being dragged down as the lower plate puts strain energy into
the upper plate.
Historical studies have capitalized on
records of the tsunamis caused by past Tokai earthquakes.
New methods allow us to partially
reconstruct the causative event from the wave records.
Preparation for the
Next Tokai Earthquake
The Tokai Earthquake is visualized in
scenarios used by emergency planners.
They need to create plans for an event
that will likely cause about 5800 deaths, 19,000 serious injuries, and nearly 1
million damaged buildings in Shizuoka Prefecture alone.
Large areas will be shaken at intensity 7,
the highest level in the Japanese intensity scale.
The Japanese Coast Guard recently produced
unsettling tsunami animations for the major harbors in the epicentral region.
The Hamaoka nuclear power plant sits where
the hardest shaking is foreseen.
The operators have begun further strengthening
of the structure; based on the same information, popular opposition to the
plant has increased.
In the aftermath of the 2011 Tohoku
earthquake, the plant's very future existence is clouded.
Weaknesses of the
Tokai Earthquake Warning System
Most of this activity does good, but some
aspects can be criticized.
First is its reliance on the simple
recurrence model of earthquakes, which is based on studies of the historical
record.
More desirable would be a physical
recurrence model based on understanding the physics of the earthquake cycle,
and where the region sits in that cycle, but that is still not well known.
Also, the law set up an alert system that
is less robust than it seems.
A panel of six senior seismologists is
supposed to assess the evidence and tell the authorities to make a public
warning announcement when the Tokai Earthquake is imminent within hours or
days.
All the drills and practices that follow
(for instance, freeway traffic is supposed to slow to 20 kph) assume that this
process is scientifically sound, but in fact, there's no consensus on what
evidence actually foreshadows earthquakes.
In fact, a previous chairman of this
Earthquake Assessment Committee, Kiroo Mogi, resigned his position in 1996 over
this and other flaws in the system.
He reported its "grave issues" in
a 2004 paper in Earth Planets Space.
Maybe a better process will be enacted
someday — hopefully, long
before the next Tokai Earthquake.
Andrew
Alden
Professional
geologist, writer, photographer, and geological tour guide
Thirty-seven
years of experience writing about geological subjects
Six
years as a research guide with U.S. Geological Survey (USGS)
Member
of Geological Society of America (GSA), and American Geophysical Union (AGU)
Experience
Andrew
Alden is a former writer for ThoughtCo who contributed hundreds of articles for
more than 17 years. Andrew works as a geologist, writer, editor, and
photographer. He has written on geological subjects since 1981 and participates
actively in his field. For example, Andrew spent six years as a research guide
with the U.S. Geological Survey, leading excursions on both land land and at
sea. And since 1992, he has hosted the earthquakes conference for the online
discussion platform, The Well, which began as a dialogue between the writers
and readers of the Whole Earth Review.
In
addition, Andrew is a longtime member of the member of the Geological Society
of America — an international society that serves members in academia,
government, and industry; and the American Geophysical Union — a community of
earth and space scientists that advances the power of science to ensure a
sustainable future.
Andrew
lives in Oakland, California; and though he writes about the whole planet and
beyond, Andrew finds his own city full of interest too and blogs about its
geology.
Education
Andrew
Alden holds a bachelor's (B.A.) degree in Earth Science from the University of
New Hampshire, College of Engineering and Physical Sciences, in Durham, N.H.
Awards
and Publications
Andrew
Alden on Earthquakes (The Well Group, Inc.,
2011)
Assessment
of River — Floodplain Aquifer Interactions (Environmental and Engineering
Geoscience, 1997)
Andrew
Alden on Hosting (The Well Group, Inc., 1995)
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