A tsunami may be started by a sea bottom slide, an earthquake or a volcanic eruption.
The most infamous of all was launched by the explosion of the island of Krakatoa in 1883;
it raced across the Pacific at 300 miles an hour, devastated the coasts of Java and Sumatra with waves 100 to 130 feet high, and pounded the shore as far away as San Francisco.

The ancient Greeks recorded several catastrophic inundations by huge waves.
Whether or not Plato's tale of the lost continent of Atlantis is true, skeptics concede that the myth may have some foundation in a great tsunami of ancient times.
Indeed, a tremendously destructive tsunami that arose in the Arabian Sea in 1945 has even revived the interest of geologists and archaeologists in the Biblical story of the Flood.

One of the most damaging tsunami on record followed the famous Lisbon earthquake of November 1, 1755;
its waves persisted for a week and were felt as far away as the English coast.
Tsunami are rare, however, in the Atlantic Ocean;
they are far more common in the Pacific.
Japan has had 15 destructive ones (eight of them disastrous) since 1596.
The Hawaiian Islands are struck severely an average of once every 25 years.

In 1707 an earthquake in Japan generated waves so huge that they piled into the Inland Sea;
one wave swamped more than 1,000 ships and boats in Osaka Bay.
A tsunami in the Hawaiian Islands in 1869 washed away an entire town (Ponoluu), leaving only two forlorn trees standing where the community had been.
In 1896 a Japanese tsunami killed 27,000 people and swept away 10,000 homes.

The dimensions of these waves dwarf all our usual standards of measurement.
An ordinary sea wave is rarely more than a few hundred feet long from crest to crest -- no longer than 320 feet in the Atlantic or 1,000 feet in the Pacific.
But a tsunami often extends more than 100 miles and sometimes as much as 600 miles from crest to crest.
While a wind wave never travels at more than 60 miles per hour, the velocity of a tsunami in the open sea must be reckoned in hundreds of miles per hour.
The greater the depth of the water, the greater is the speed of the wave;
Lagrange's law says that its velocity is equal to the square root of the product of the depth times the acceleration due to gravity.
In the deep waters of the Pacific these waves reach a speed of 500 miles per hour.

Tsunami are so shallow in comparison with their length that in the open ocean they are hardly detectable.
Their amplitude sometimes is as little as two feet from trough to crest.
Usually it is only when they approach shallow water on the shore that they build up to their terrifying heights.
On the fateful day in 1896 when the great waves approached Japan, fishermen at sea noticed no unusual swells.
Not until they sailed home at the end of the day, through a sea strewn with bodies and the wreckage of houses, were they aware of what had happened.
The seemingly quiet ocean had crashed a wall of water from 10 to 100 feet high upon beaches crowded with bathers, drowning thousands of them and flattening villages along the shore.

The giant waves are more dangerous on flat shores than on steep ones.
They usually range from 20 to 60 feet in height, but when they pour into a V-shaped inlet or harbor they may rise to mountainous proportions.

Generally the first salvo of a tsunami is a rather sharp swell, not different enough from an ordinary wave to alarm casual observers.
This is followed by a tremendous suck of water away from the shore as the first great trough arrives.
Reefs are left high and dry, and the beaches are covered with stranded fish.
At Hilo large numbers of people ran out to inspect the amazing spectacle of the denuded beach.
Many of them paid for their curiosity with their lives, for some minutes later the first giant wave roared over the shore.
After an earthquake in Japan in 1793 people on the coast at Tugaru were so terrified by the extraordinary ebbing of the sea that they scurried to higher ground.
When a second quake came, they dashed back to the beach, fearing that they might be buried under landslides.
Just as they reached the shore, the first huge wave crashed upon them.

A tsunami is not a single wave but a series.
The waves are separated by intervals of 15 minutes to an hour or more (because of their great length), and this has often lulled people into thinking after the first great wave has crashed that it is all over.
The waves may keep coming for many hours.
Usually the third to the eighth waves in the series are the biggest.

Among the observers of the 1946 tsunami at Hilo was Francis P. Shepard of the Scripps Institution of Oceanography, one of the world's foremost marine geologists.
He was able to make a detailed inspection of the waves.
Their onrush and retreat, he reported, was accompanied by a great hissing, roaring and rattling.
The third and fourth waves seemed to be the highest.
On some of the islands' beaches the waves came in gently;
they were steepest on the shores facing the direction of the seaquake from which the waves had come.
In Hilo Bay they were from 21 to 26 feet high.
The highest waves, 55 feet, occurred at Pololu Valley.

Scientists and fishermen have occasionally seen strange by-products of the phenomenon.
During a 1933 tsunami in Japan the sea glowed brilliantly at night.
The luminosity of the water is now believed to have been caused by the stimulation of vast numbers of the luminescent organism Noctiluca miliaris by the turbulence of the sea.
Japanese fishermen have sometimes observed that sardines hauled up in their nets during a tsunami have enormously swollen stomachs;
the fish have swallowed vast numbers of bottom-living diatoms, raised to the surface by the disturbance.
The waves of a 1923 tsunami in Sagami Bay brought to the surface and battered to death huge numbers of fishes that normally live at a depth of 3,000 feet.
Gratified fishermen hauled them in by the thousands.

The tsunami-warning system developed since the 1946 disaster in Hawaii relies mainly on a simple and ingenious instrument devised by Commander C. K. Green of the Coast and Geodetic Survey staff.
It consists of a series of pipes and a pressure-measuring chamber which record the rise and fall of the water surface.
Ordinary water tides are disregarded.
But when waves with a period of between 10 and 40 minutes begin to roll over the ocean, they set in motion a corresponding oscillation in a column of mercury which closes an electric circuit.
This in turn sets off an alarm, notifying the observers at the station that a tsunami is in progress.
Such equipment has been installed at Hilo, Midway, Attu and Dutch Harbor.
The moment the alarm goes off, information is immediately forwarded to Honolulu, which is the center of the warning system.

This center also receives prompt reports on earthquakes from four Coast Survey stations in the Pacific which are equipped with seismographs.
Its staff makes a preliminary determination of the epicenter of the quake and alerts tide stations near the epicenter for a tsunami.
By means of charts showing wave-travel times and depths in the ocean at various locations, it is possible to estimate the rate of approach and probable time of arrival at Hawaii of a tsunami getting under way at any spot in the Pacific.
The civil and military authorities are then advised of the danger, and they issue warnings and take all necessary protective steps.
All of these activities are geared to a top-priority communication system, and practice tests have been held to assure that everything will work smoothly.

Since the 1946 disaster there have been 15 tsunami in the Pacific, but only one was of any consequence.
On November 4, 1952, an earthquake occurred under the sea off the Kamchatka Peninsula.
At 17:07 that afternoon (Greenwich time) the shock was recorded by the seismograph alarm in Honolulu.
The warning system immediately went into action.
Within about an hour with the help of reports from seismic stations in Alaska, Arizona and California, the quake's epicenter was placed at 51 degrees North latitude and 158 degrees East longitude.
While accounts of the progress of the tsunami came in from various points in the Pacific (Midway reported it was covered with nine feet of water), the Hawaiian station made its calculations and notified the military services and the police that the first big wave would arrive at Honolulu at 23:30 Greenwich time.

It turned out that the waves were not so high as in 1946.
They hurled a cement barge against a freighter in Honolulu Harbor, knocked down telephone lines, marooned automobiles, flooded lawns, killed six cows.
But not a single human life was lost, and property damage in the Hawaiian Islands did not exceed $800,000.
There is little doubt that the warning system saved lives and reduced the damage.

But it is plain that a warning system, however efficient, is not enough.
In the vulnerable areas of the Pacific there should be restrictions against building homes on exposed coasts, or at least a requirement that they be either raised off the ground or anchored strongly against waves.

The key to the world of geology is change;
nothing remains the same.
Life has evolved from simple combinations of molecules in the sea to complex combinations in man.
The land, too, is changing, and earthquakes are daily reminders of this.
Earthquakes result when movements in the earth twist rocks until they break.
Sometimes this is accompanied by visible shifts of the ground surface;
often the shifts cannot be seen, but they are there;
and everywhere can be found scars of earlier breaks once deeply buried.
Today's earthquakes are most numerous in belts where the earth's restlessness is presently concentrated, but scars of the past show that there is no part of the earth that has not had them.

The effects of earthquakes on civilization have been widely publicized, even overemphasized.
The role of an earthquake in starting the destruction of whole cities is tremendously frightening, but fire may actually be the principal agent in a particular disaster.
Superstition has often blended with fact to color reports.

We have learned from earthquakes much of what we now know about the earth's interior, for they send waves through the earth which emerge with information about the materials through which they have traveled.
These waves have shown that 1,800 miles below the surface a liquid core begins, and that it, in turn, has a solid inner core.

Earthquakes originate as far as 400 miles below the surface, but they do not occur at greater depths.
Two unsolved mysteries are based on these facts.
(1) As far down as 400 miles below the surface the material should be hot enough to be plastic and adjust itself to twisting forces by sluggish flow rather than by breaking, as rigid surface rocks do.
(2) If earthquakes do occur at such depths, why not deeper?

Knowledge gained from studying earthquake waves has been applied in various fields.
In the search for oil and gas, we make similar waves under controlled conditions with dynamite and learn from them where there are buried rock structures favorable to the accumulation of these resources.
We have also developed techniques for recognizing and locating underground nuclear tests through the waves in the ground which they generate.

The following discussion of this subject has been adapted from the book Causes Of Catastrophe by L. Don Leet.

The restless earth and its interior

At twelve minutes after five on the morning of Wednesday, April 18, 1906, San Francisco was shaken by a severe earthquake.
A sharp tremor was followed by a jerky roll.