Sample F21 from L. Don Leet and Florence J. Leet, editors, The World of Geology. New York: McGraw-Hill Book Company, Inc., 1961. Pp. 140-145 "Giant Waves" by Joseph Bernstein Pp. 147-148 [Introduction to C] P. 149 "The Restless Earth and its Interior" by L. Don Leet A part of the XML version of the Brown Corpus2,009 words 1 symbolF21

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L. Don Leet and Florence J. Leet, editors, The World of Geology. New York: McGraw-Hill Book Company, Inc., 1961.

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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 .