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Tsunami
Forecasting, Preparedness and Warning
George
Pararas-Carayannis
(Excerpts
from a Paper Presented at the Fifteenth Conference on Broadcast
Meteorology of the American Meteorological Society, April 9-12,
1985, Honolulu, Hawaii)
Copyright © 2005.
All Rights Reserved
Abstract
Tsunami hazard mitigation
precludes accurate and expeditious forecasting, issuance of a
prompt warning, and a program of preparedness that will assure
the effectiveness of the warning.
Forecasting tsunamis
requires adequate understanding of the phenomenon, good and expeditious
collection of earthquake and sea level data, and accurate assessment
and interpretation of this data.
In order for a tsunami
warning to be of value, an expeditious and effective international
communications system is necessary to insure proper dissemination
throughout a large geographical area. Civil Defense Agencies
collaborating into the International Tsunami Warning System,
must have good emergency operations plans for their own countries
or territories, efficient internal communications and a program
of tsunami preparedness which includes organizational infrastructural
coordination and comprehensive public education.
Historical Background
Tsunamis are among the
most terrifying natural hazards known to man and have been responsible
for tremendous loss of life and property throughout history.
Because of their destructiveness, tsunami have important impact
on the human, social and economic sectors of our societies. In
the Pacific Ocean where the majority of these waves have been
generated, the historical record shows tremendous destruction.
In Japan, which has one of the most populated coastal regions
in the world and a long history of earthquake activity, tsunami
have destroyed entire coastal populations. There is also a history
of tsunami destruction in Alaska, in the Hawaiian Islands in
South America and elsewhere in the Pacific, although the historic
records for these areas do not go back sufficiently in time.
Historical records also
document considerable loss of life and destruction of property
on the western shores of the North and South Atlantic, the coastal
regions of north-western Europe, and in the seismically active
regions around the eastern Caribbean. Fortunately tsunami in
the Atlantic and the Caribbean do not occur as frequently as
in the Pacific.
Destructive tsunami
have occurred also in the Indian Ocean and in the Mediterranean
Sea. The most notable tsunami in the region of the Indian Ocean
was that associated with the violent explosion of the volcanic
island of Krakatoa in August 1883. A 30 meter (100 feet) tsunami
resulting from this explosion killed 36,500 people in Java and
Sumatra. The violent eruption and explosion of the volcano of
Santorin, in the fifteenth Century B.C. generated a tremendous
tsunami which destroyed most of the coastal Minoan settlements
on the Aegean sea islands acting as the catalyst for the decline
of the advanced Minoan civilization.
Japan is very vulnerable
to the tsunami hazard. All the major Japanese islands have been
struck by devastating tsunamis. A total of 68 destructive tsunami
have struck Japan between A.D. 684 and 1984 with thousands of
lives lost and with the destruction of-hundreds of villages.
In this century alone, at least 6 major destructive tsunamis
have hit Japan. On 3 March 1933 a tsunami in the Sanriku area
reached a height of about 30 meters and killed over 3,000 people,
injured hundreds more and destroyed approximately 9,000 homes
and 8,000 boats. Other similarly destructive tsunami occurred
in 1944, 1946, 1960, and in 1983. The 1983 event, although not
very destructive in terms of lives lost and property damage,
occurred in the Sea of Japan in an area not known before for
seismic or tsunami activity.
In the Hawaiian islands,
tsunami have struck repeatedly, causing great loss of life and
immense damage to property. Most noteworthy of the recent Hawaiian
tsunami is that of 1 April 1946 which inundated and destroyed
the city of Hilo, killing 159 people. Other recent tsunami that
have hit Hawaii occurred in 1952, 1957, 1960, 1964 and 1975.
One of the most devastating recent tsunami was generated by a
large earthquake in the Moro Gulf in the Philippines on 16 August
1976. The tsunami waves killed over 8,000 people in Mindanao,
leaving 10,000 injured and 90,000 more homeless. In August 1977
a large earthquake in the Lesser Sunda Islands, Indonesia generated
a destructive tsunami which killed hundreds of people on Lombok
and Sumbawa Islands along the eastern side of the Indian Ocean.
Another devastating tsunami occurred on 12 December 1979 in the
south-west corner of Colombia destroying several fishing villages,
taking the lives of hundreds of people and creating economic
chaos in an already economically depressed region of that country.
Many more events have occurred in the last twenty years.
Understanding Tsunami
Source Mechanism and Potential Terminal Runup
Tsunamis are impulsively
generated sea waves by a disturbance at or near the ocean. Earthquakes,
submarine volcanic explosions, landslides and the detonation
of nuclear devices near the sea can give rise to such destructive
sea waves. By far the most destructive tsunamis are generated
from large shallow-focus earthquakes with an epicenter or fault
line near or in the ocean. Vertical displacements of the earth's
crust along the rupture resulting from such earthquakes can generate
destructive tsunami waves which can travel across an ocean spreading
destruction across their path. Similar displacements of the ocean
floor can also be produced by volcanic eruptions and submarine
avalanches or landslides. However, these sources are considered
as point sources and, although the tsunami waves generated can
be very destructive locally, the energy of the waves is rapidly
dissipated as they travel across the ocean.
To forecast tsunamis
and determine terminal runup and destructiveness, one must be
able to evaluate the parameters of the tsunami source mechanism
in real time, often from inadequate data. Tsunami source mechanism
analysis is difficult given the time constraints of a warning
situation. It will suffice to say that forecasting the runup
and potential destructiveness of a tsunami at a distant shore
will depend greatly on determining the seismic parameters of
the source location such as magnitude of the earthquake, its
depth, its orientation, the length of the fault line, the size
of the crustal displacements, and depth of the water. Refraction
and diffraction processes will affect the energy and height of
the tsunami waves as they travel across the ocean. These effects
must also be determined. Finally, terminal height, run-up, and
inundation of the tsunami at a point of impact will depend upon
the energy focusing effect, th travel path of the waves, the
coastal configuration, and the offshore bathymetry, only to name
a few.
Tsunami run-up is the
vertical distance between the maximum height reached by the water
on shore and the mean-sea-level surface. Contrary to meteorological
predictions, tsunami run-up, the final product of earthquake
and tsunami investigations is not possible to forecast with a
great degree of accuracy. The reason for this inadequacy is that
the Tsunami Warning System works in a real time frame of short
duration, often with inadequate data and information. Problems
of communications and lack of sufficient station density, often
complicate the process . Forecasting tsunamis requires adequate
understanding of the phenomenon, good and expeditious collection
of earthquake and sea level data, and accurate and expeditious
assessment and interpretation of this data.
Preparedness and Planning
There is very little
that can be done to prevent the occurrence of natural hazards.
But while these natural disasters cannot be prevented, their
results, such as loss of life and property, can be reduced by
proper planning. Government agencies should formulate land-use
regulations for a given coastal area with the tsunami risk potential
in mind, particularly if such an area is known to have sustained
damage in the past. Tsunami hazard perception by the people of
a coastal area is necessary in mitigating loss of life and damage
to property. Hazard perception by the public is based on a technical
understanding of the phenomenon, at least at the basic level,
and a behavioral response stemming from that understanding and
confidence of the public for the authorities responsible for
warning.
Over warning, based
on inadequate data on which to base the prediction, often leads
to false alarms and lack of compliance with warning and evacuation
attempts. Such false alarms result in a loss of faith in the
capability of a warning system and result in reluctance to take
action in subsequent tsunami events.
Fortunately, forecasting
of tsunami in recent years has been quite good and the image
of the Tsunami Warning System and its credibility have improved
considerably. Forecasting, however, is not an exact science as
the phenomenon itself is very complex and data on which the forecast
is based may often be inadequate for certain areas of the Pacific.
The International Tsunami
Warning System
Following the disaster
caused by the tsunami of 1 April 1946 in the Hawaiian Islands
and elsewhere a rudimentary warning system was established in
1948 to provide watch and warning information to the civil authorities
and various military headquarters in the Hawaiian Islands for
dissemination to military bases throughout the Pacific and to
islands in the Trust Territories. Beginning in October 1953 warning
information was extended to California, Oregon and the State
of Washington.
The great destruction
caused by the May 1960 Chilean tsunami prompted a large number
of countries and territories to Join the TWS. Another catastrophic
tsunami generated by the great Alaskan earthquake of 1964 emphasized
the need for an International Tsunami Warning System.
In 1965, the United
Nations Educational Scientific and Cultural Organization's Intergovernmental
Oceanographic Commission accepted the United States' offer to
expand its existing Tsunami Center in Honolulu to become the
Pacific Tsunami Warning Center (PTWC). Also established was an
International Coordination Group (ICG/ITSU) and the International
Tsunami Information Center (ITIC) to review and coordinate the
activities of the International Tsunami Warning System for the
Pacific (ITWS). The Pacific Tsunami Warning System has become
the nucleus of a truly international system. Twenty-two nations
are now members of ICG/ITSU. Several non-member stations and
territories maintain stations for the ITWS. The present system
makes use of twenty-four seismic stations, fifty-three tide stations
and 101 dissemination points scattered throughout the Pacific
Basin under the varying control of the Member States. PTWC in
Honolulu, operated by the U.S. National Weather Service, Pacific
Region, is the operational Center for the System.
Functioning of the system
begins with the detection by any participating seismic observatory
of an earthquake of sufficient size to trigger the alarms, set
at the threshold of 6.5 on the Richter Scale. PTWC collects the
seismic data, locates the earthquake and computes its magnitude.
When reports from tide stations show that a tsunami has been
generated which poses a threat to the population in part or all
of the Pacific, a warning is transmitted to the dissemination
agencies for relaying to the public. The agencies then implement
predetermined plans to evacuate people from endangered areas.
In addition to the International Tsunami Warning System, a number
of Regional Warning Systems have been established to warn the
population in areas where tsunami frequency is high and where
immediate response is necessary. Such regional tsunami warning
systems have been established in the Soviet Union, Japan, Alaska
and Hawaii.
REFERENCE
Pararas-Carayannis
George. TSUNAMI:
FORECASTING. PREPAREDNESS AND WARNING, Fifteenth Conference on Broadcast Meteorology
of the American Meteorological Society, April 9-12, 1985, Honolulu,
Hawaii
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