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Pacific-wide tsunamis are much less frequent, but of far
greater destructive potential in that waves are not only
larger initially, but in transit across the Pacific basin,
many distant coastal areas are subject to destructive
impact. For example, the tsunami of May 22, 1960, spread
death and destruction across the Pacific from Chile to
Hawai`i, Japan, and the Philippines.
A tsunami is a system of gravity waves formed in the sea as
a result of a large-scale disturbance of sea level over a
short duration of time. In the process of sea level
returning to equilibrium through a series of oscillations,
waves are generated which propagate outward from the source
region. A tsunami can be generated by submarine volcanic
eruptions, by displacement of submarine sediments, by
coastal landslides into a bay or harbor, by meteor impact,
or by vertical displacement of the earth's crust along a
zone of fracture which underlies or borders the ocean floor.
The
latter is by far the most frequent cause of tsunamis and for
all practical purposes the primary cause of tsunamis capable
of propagation across an ocean basin. The rupture of the
earth's crust will also generate a major earthquake which
can be detected and measured by seismic instrumentation
throughout the world. However, not all major coastal or
near-coastal earthquakes produce tsunamis. At present, there
is no operational method to determine if a tsunami has been
generated except to note the occurrence and epicenter of the
earthquake and then detect the arrival of the characteristic
waves at a network of tide stations.
When a major earthquake occurs, the resultant energy
released into the earth will propagate over a wide range of
frequencies and velocities. Even though the earth movements
discernible to the viewer may be confined to the general
region of the earthquake origin, the various seismic wave
phases propagating throughout the earth result in small, but
measurable, ground motion which can be detected by a
seismometer. A seismograph then provides a visual record of
the ground motion at that station.
For the purposes of the Tsunami Warning System,
consideration is given to three significant seismic wave
phases. The first, the P-wave, is a compressional wave
traveling through the earth's interior at a velocity varying
from approximately 8.0 km/second near the crust-mantle
interface to about 13.5 km/second at the mantle-core
interface.
As such
it is the first seismic phase to be recorded at any one
seismic station and is the first indication that a distant
earthquake has occurred. The location of the earthquake can
be determined by assuming the "best fit" of the pattern of
P-wave arrivals at several stations compared to a standard
table of P-wave arrival times for various distances and
depths of earthquake focus or, in the case of local
earthquakes in or near the limits of a relatively small area
seismic station network, compared to the calculated arrivals
based on a local crustal seismic velocity model.
The second seismic phase of importance is the S-wave, or
Secondary wave. This phase travels through the earth's
interior as a shear wave, following approximately the same
travel path as the P-wave but at a reduced velocity varying
from approximately 6.7 km/second near the crust-mantle
interface to about 8.0 km/second near the core. These
seismic wave phases are classified as body waves due to
their propagation through the earth's interior. In addition
to providing a location, body waves are useful in
determining the size of an earthquake, especially when the
eathquake's focus is deep within the earth.
The third set of seismic phases to be considered are the
surface waves resulting from ground displacements
propagating outward along the surface of the earth. These
are observed at a seismic station as local or regional
surface waves and are the basis for measuring magnitude on
the Richter scale. This is a logarithmic scale devised by
Charles Richter to use the amplitude of the trace recorded
on a seismograph and the distance from the epicenter to
assign a somewhat consistent indication of size to a
particular earthquake as measured at different stations.
Beno Gutenberg extended the Richter scale to include distant
Love and
Raleigh surface waves.
Though it is a logarithmic scale to the base 10, this is
merely a reference to the Richter scale value being
incremented as a logarithmic function of the trace
deflection as recorded on the seismograph and the distance
of the station from the epicenter.
The
actual energy released for each increment of the Richter
scale is a factor of 32. Thus a magnitude 7.0 earthquake
will release 32 times as much energy as a magnitude 6.0, and
the energy release for a magnitude 8.0 is more than 1000
times greater than a 6.0.
Tsunamis travel outward in all directions from the
generating area, with the direction of the main energy
propagation generally being orthogonal to the direction of
the earthquake fracture zone. Their speed depends on the
depth of water, so that the waves undergo accelerations and
decelerations in passing over an ocean bottom of varying
depth. In the deep and open ocean, they travel at speeds of
500 to 1,000 kilometers per hour (300 to 600 miles per
hour). The distance between successive crests can be as much
as 500 to 650 kilometers (300 to 400 miles); however, in the
open ocean, the height of the waves may be no more than 30
to 60 centimeters (1 or 2 feet), and the waves pass
unnoticed. Variations in tsunami propagation result when the
propagation impulse is stronger in one direction than in
others because of the orientation or dimensions of the
generating area and where regional topographic features
modify both the wave form and rate of advance.
The
tsunamis are waveform extends through the entire water
column from sea surface to the ocean bottom. It is this
characteristic that accounts for the great amount of energy
transmitted by a tsunami.
The successive waves of a tsunami in the deep sea have such
great length and so little height they are not visually
recognizable from a surface vessel or from an airplane. The
passing waves produce only a gentle rise and fall of the sea
surface. During the April 1946 tsunami at Hawai`i, ships
standing off the coasts observed tremendous waves breaking
on shore but did not detect any change in sea level at their
offshore locations.
Upon reaching shallower water, the speed of the advancing
wave diminishes, its wave length decreases, and its height
may increase greatly, owing to the piling up of water.
Configuration of the coastline, shape of the ocean floor,
and character of the advancing waves play an important role
in the destruction wrought by tsunamis along any coast,
whether near the generating area or thousands of kilometers
from it. Consequently, detection of relatively small
tsunamis at any locality warrants immediate reporting --
through the TWS -- to spread the alarm to all coastal
localities of approaching potentially dangerous waves.
At present, detection of tsunamis is possible only near
shore where the shoaling effect can be observed. The first
visible indication of an approaching tsunami is often a
recession of water caused by the trough preceding an
advancing wave. Any withdrawal of the sea, therefore, should
be considered a warning of an approaching wave. A rise in
water level also may be the first event. Tide-gauge records
of the Chilean tsunami of May 22, 1960, generally showed a
rise in water level as the first indication of this tsunami.
This
rise amounted to about one-half the amplitude of the
following decrease in water level. Under certain conditions,
the crest of an advancing wave can overtake the preceding
trough while some distance offshore. This causes the wave to
proceed shoreward as a bore -- a wave with a churning front.
The force and destructive effects of tsunamis should not be
underestimated. At some places, the advancing turbulent
front is the most destructive part of the wave. Where the
rise is quiet, the outflow of water to the sea between
crests may be rapid and destructive, sweeping all before it
and undermining roads, buildings, and other works of man
with its swift currents. Ships, unless moved away from
shore, can be thrown against breakwaters, wharves, and other
craft, or washed ashore and left grounded during withdrawals
of the sea.
In the shallow waters of bays and harbors, a tsunami
frequently will initiate seiching. If the tsunami period is
related closely to that of the bay, the seiche is amplified
by the succeeding waves. Under these circumstances, maximum
wave activity often is observed much later than the arrival
of the first wave.
A tsunami is not one wave, but a series of waves. The time
that elapses between passage of successive wave crests at a
given point usually is from 10 to 45 minutes. Oscillations
of destructive proportions may continue for several hours,
and several days may pass before the sea returns to its
normal state.
During the 101-year period from 1900 to 2001, 796 tsunamis
were observed or recorded in the Pacific Ocean according to
the Tsunami Laboritory in
Novosibirsk. 117 caused casualties and damage most near the source only; at least
nine caused widespread destruction throughout the Pacific.
The greatest number of tsunamis during any 1 year was 19 in
1938, but all were minor and caused no damage. There was no
single year of the period that was free of tsunamis.
17 percent of the total tsunamis were generated in or near
Japan. The distribution of tsunami generation
in other areas is as follows: South America, 15 percent: New
Guinea Solomon Islands, 13 percent; Indonesia, 11 percent:
Kuril Islands and Kamchatka, 10 percent; Mexico and Central
America, 10 percent; Philippines, 9 percent; New Zealand and
Tonga, 7 percent; Alaska and West Coasts of Canada and the
United States, 7 percent; and Hawai`i, 3 percent.
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