Geoscience Reference
In-Depth Information
10.3
Warning Systems
Seventh, if headlands concentrate tsunami energy because
of refraction, then gullies do the same because of funneling.
The highest run-up measured during the Hokkaido Nansei-Oki
Tsunami of July 12, 1993 was 31.7 m in a narrow gully (Shuto
and Matsutomi 1995 ). On the adjacent coastline the wave did
not reach more than 10 m above sea level. It is safer to climb as
far as you can up a steep slope rather than flee from a tsunami
by running up a gully—even one that appears sheltered
because it is hidden from the ocean.
Eighth, tsunami are not blocked by cliffs. Compared to
the long wavelengths of a tsunami, which can still have a
wavelength of 12 km at the base of a cliff dropping 20 m
into deep water, the height of a cliff is minuscule. Steep
slopes are similar to cliffs. Tsunami waves 1-2 m in height
have historically surged up cliffs or steep slopes to heights
of 30 m or more above sea level. If one has any doubt of
this then turn to Fig. 3.6 and look at the limit of run-up in
the background of the photograph. This photograph was
taken at Riang-Kroko following the December 12, 1992
Tsunami. While the wave had a height of only a couple of
meters approaching the coast and was stopped on gentle
slopes by forest, it ran up to a height of 26.2 m above sea
level on steeper slopes and bulldozed slopes clear of veg-
etation (Yeh et al. 1993 ). The view from cliffs is great, but
anyone standing there during a tsunami may have a unique
life experience. Never do what 10,000 people did at San
Francisco following the Great Alaskan Earthquake of 1964.
When they heard that a tsunami was coming, they raced
down to vantage points on cliffs to watch it come in. For-
tunately, the tsunami was a fizzler along this part of the
Californian coast. However, it killed 11 people at Crescent
City to the north.
Finally, tsunami are enhanced in the lee of circular-
shaped islands. Not only do they travel faster here, the
height of their run-up can also be greater, especially if the
initial wave is large. Two examples of this effect were
presented in this text in Chap. 2 . The December 12, 1992
Tsunami along the north coast of Flores Island, Indonesia,
devastated two villages in the lee of Babi, a small coastal
island lying 5 km offshore of the main island (Yeh et al.
1994 ). Wave heights actually increased from 2 to 7 m
around the island. Similarly, the July 12, 1993 Tsunami in
the Sea of Japan destroyed the town of Hamatsumae lying
on a sheltered part of Okusihir Island (Shuto and Matsutomi
1995 ). The tsunami ran up 30 m above sea level—more
than three times the elevation recorded at some communi-
ties fronting the wave on the more exposed coast. Over 800
people were killed in the first instance and 300 people in the
latter. Lee sides of islands are particularly vulnerable to
tsunami
10.3.1
The Pacific Tsunami Warning Center
As shown in Chap. 6 , the most devastating ocean-wide
tsunami occur in the Pacific Ocean. For that reason, tsunami
warning is best developed in this region. The lead-time for
warnings in the Pacific is the best of any ocean, anywhere
up to 24 h depending upon the location of sites relative to
an earthquake epicenter. Following the Alaskan Tsunami of
1946, the U.S. government established tsunami warning in
the Pacific Ocean under the auspices of the Seismic Sea
Wave Warning System (Bryant 2005 ; Murata et al. 2010 ;
International Tsunami Information Center 2013 ). In 1948,
this system evolved into the Pacific Tsunami Warning
Center (PTWC). Warnings were initially issued for the
United States and Hawaiian areas, but following the 1960
Chilean earthquake, the scheme was extended to all coun-
tries bordering the Pacific Ocean. Japan up until 1960 had
its own warning network, believing at the time that signif-
icant tsunami affecting that country originated locally. The
1960 Chilean Tsunami proved that any submarine earth-
quake in the Pacific Ocean region could spread ocean-wide.
The Pacific Warning System was significantly proven fol-
lowing the Alaskan earthquake of 1964. Within 46 min of
that earthquake, a Pacific-wide tsunami warning was issued.
This earthquake also precipitated the need for an Interna-
tional Tsunami Warning System (ITWS) for the Pacific that
was established by the Intergovernmental Oceanographic
Commission (IOC) of UNESCO at Ewa Beach, Oahu,
Hawaii, in 1968. At the same time, other UNESCO/IOC
member countries integrated their existing facilities and
communications into the system. Presently there are 31
participants in the Pacific Tsunami Warning System. Many
of these countries also operate national tsunami warning
centers, providing warning services for their local area.
After 2004, the Pacific Tsunami Warning Center took on
additional
responsibilities
for
the
Indian
Ocean,
South
China Sea, and Caribbean.
The objective of the International Tsunami Warning
System is to detect, locate, and determine the magnitude of
potentially tsunamigenic earthquakes occurring anywhere in
the world. The warning system operates 24 h per day, each
day of the year. It relies on the detection of any earthquake
with a surface wave magnitude of 6.5 or greater registering
on one of 31 seismographs outside the shadow zones of any
P or S waves originating in the Pacific region (Fig. 10.6 ).
These seismographs automatically relay information to the
United States National Earthquake Information Center in
Denver where computers analysis the short period waves for
potentially tsunamigenic earthquakes. This detection pro-
cess occurs within a few minutes. Once a suspect earthquake
because
long
waves
wrap
around
these
small
obstructions
as
solitary
waves,
becoming
trapped
and
increasing in amplitude.
 
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