In the open ocean the waves produced by the Aleutian quake were only about a foot or two high and would not be noticed from vessels. Their length, however, was enormous, with a distance of about 90 miles between succeeding crests. It took the waves less than five hours to reach the Hawaiian chain, 2300 miles distant, so they must have moved at an average speed of about 470 miles per hour. Along eastern Pacific shores, they were recorded as far into the Southern Hemisphere as Valparaiso, Chile, the distance of 8066 miles from the epicenter being covered by the waves in about 18 hours.
This particular occurrence of seismic sea waves had one result that distinguished it from all its predecessors. It set people to thinking that perhaps we now know enough about such waves and how they behave that a warning system could be devised which would rob them of the terror of the unexpected. Seismologists and specialists on waves and tides co-operated, and now such a system has been established to protect the Hawaiian Islands. A network of stations equipped with special instruments is scattered over the Pacific from Kodiak to Pago Pago and from Balboa to Palau. There are two phases of the warning system. One is based on a new audible alarm at seismograph stations operated by the United States Coast and Geodetic Survey, which calls instant attention to the fact that an earthquake has occurred. If it is found that the epicenter of the quake is under the ocean and so might produce seismic sea waves, a warning is sent to observers at selected tide stations to watch their gauges for evidence of the passage of the racing tsunamis. (Even a very small seismic sea wave can be identified by its peculiar period, and though it may be small at one place, it may reach dangerous heights at another.) When seismologists in Honolulu are notified that an undersea earthquake has occurred and that its waves have actually been recorded at certain stations, they can calculate when the waves will arrive at any point between the epicenter of the quake and the Hawaiian Islands. They can then issue warnings for the evacuation of beaches and waterfront areas. And so, for the first time in history, there is an organized effort to prevent these ominous waves from racing undetected over the empty spaces of the Pacific, to roar up suddenly on some inhabited shore.[20]
The storm waves that sometimes rise over low-lying coast lands in hurricane zones belong in the class of wind waves, but unlike the waves of ordinary winds and storms, they are accompanied by a rise of the general water level, called a storm tide. The rise of water is often so sudden that it leaves no possibility of escape. Such storm waves claim about three-fourths of the lives lost by tropical hurricanes. The most notable disasters from storm waves in the United States have been those at Galveston, Texas, on 8 September, 1900, on the lower Florida Keys on 2 and 3 September, 1935, and the catastrophic rise of water accompanying the New England hurricane of 21 September, 1938. The most fearful destruction by hurricane waves within historic time occurred in the Bay of Bengal on 7 October, 1737, when 20,000 boats were destroyed and 300,000 people drowned.[21]
There are other great waves, usually called ‘rollers,’ that periodically rise on certain coasts and batter them for days with damaging surf. These, too, are wind waves, but they are related to changes in barometric pressure over the ocean, perhaps several thousand miles distant from the beaches on which the waves eventually arrive. Low-pressure areas—like the one south of Iceland—are notorious storm breeders, their winds lashing the sea into great waves. After the waves leave the storm area they tend to become lower and longer and after perhaps thousands of miles of travel across the sea they become transformed into the undulations known as a ground swell. These swells are so regular and so low that often they are unnoticed as they pass through the short, choppy, new-formed waves of other areas. But when a swell approaches a coast and feels beneath it the gradually shoaling bottom, it begins to ‘peak up’ into a high, steep wave; within the surf zone the steepening becomes abruptly accentuated, a crest forms, breaks, and a great mass of water plunges downward.
Winter swell on the west coast of North America is the product of storms that travel south of the Aleutians into the Gulf of Alaska. Swell reaching this same coast during the summer has been traced back to its origin in the Southern Hemisphere belt of the ‘roaring forties,’ several thousand miles south of the equator. Because of the direction of the prevailing winds, the American east coast and the Gulf of Mexico do not receive the swell from far distant storms.
The coast of Morocco has always been particularly at the mercy of swell, for there is no protected harbor from the Strait of Gibraltar southward for some 500 miles. The rollers that visit the Atlantic islands of Ascension, St. Helena, South Trinidad, and Fernando de Noronha are historic. Apparently the same sort of waves occur on the South American coast near Rio de Janeiro, where they are known as resacas; others of kindred nature, having run their course from storms in the west-wind belt of the South Pacific, attack the shores of the Paumotos Islands; still others have been responsible for the well-known ‘surf days’ that plague the Pacific coast of South America. According to Robert Cushman Murphy, it was formerly the custom of shipmasters in the guano trade to demand a special allowance for a certain number of days during which the loading of their vessels would be interrupted by the swell. On such surf days ‘mighty rollers come pouring over the sea wall, and have been known to carry away forty-ton freight cars, to uproot concrete piers, and to twist iron rails like wire.’
The slow progression of swell from its place of origin made it possible for the Moroccan Protectorate to establish a service for the prediction of the state of the sea. This was done in 1921, after long and troublesome experience with wrecked vessels and wharves. Daily telegraphic reports of the condition of the sea give advance notice of troublesome surf days. Warned of the approach of swells, ships in port may seek safety in the open sea. Before this service was established, the port of Casablanca had once been paralyzed for seven months, and St. Helena had seen the wreckage of practically all the ships in her harbor on one or more occasions. Modern wave-recording instruments like those now being tested in England and the United States will soon provide even greater security for all such shores.
It is always the unseen that most deeply stirs our imagination, and so it is with waves. The largest and most awe-inspiring waves of the ocean are invisible; they move on their mysterious courses far down in the hidden depths of the sea, rolling ponderously and unceasingly. For many years it was known that the vessels of Arctic expeditions often became almost trapped and made headway only with difficulty in what was called ‘dead water’—now recognized as internal waves at the boundary between a thin surface layer of fresh water and the underlying salt water. In the early 1900’s several Scandinavian hydrographers called attention to the existence of submarine waves, but another generation was to elapse before science had the instruments to study them thoroughly.
Now, even though mystery still surrounds the causes of these great waves that rise and fall, far below the surface, their ocean-wide occurrence is well established. Down in deep water they toss submarines about, just as their surface counterparts set ships to rolling. They seem to break against the Gulf Stream and other strong currents in a deep-sea version of the dramatic meeting of surface waves and opposing tidal currents. Probably internal waves occur wherever there is a boundary between layers of dissimilar water, just as the waves we see occur at the boundary between air and sea. But these are waves such as never moved at the surface of the ocean. The water masses involved are unthinkably great, some of the waves begin as high as 300 feet.
20
From the time of its establishment up to 1960, the warning system has issued eight alerts warning residents of the Hawaiian Islands of the approach of seismic waves. On three of these occasions, waves of major proportions have in fact struck the islands. None have been so large or so destructive, however, as those of May 23, 1960, which spread out across the Pacific from their place of origin in violent earthquakes on the coast of Chile. Without such warning the loss of life would almost certainly have been enormous. As soon as the seismograph at the Honolulu Observatory recorded the first of the Chilean quakes the system went into operation. Reports from the scattered tide stations gave ample notice that a seismic wave had formed and was spreading out across the Pacific. By early news bulletins and later by an official “sea wave warning” the Observatory alerted residents of the area and predicted the time the wave would arrive and the areas to be affected. These predictions proved to be accurate within reasonable limits, and although property damage was heavy, loss of life was limited to the few who disregarded the warnings. Sea wave activity was reported as far west as New Zealand and as far north as Alaska. The Japanese coasts were struck by heavy waves. Although the United States warning system does not now include other nations, officials at Honolulu sent to Japan warnings of the wave which, unfortunately, were disregarded.
The warning system now (in 1960) consists of eight seismograph stations at points on both eastern and western shores of the Pacific and on certain islands, and of twenty widely scattered wave stations, four of which are equipped with automatic wave detectors. The Coast and Geodetic Survey feels that additional wave-reporting tide stations would improve the effectiveness of the system. Its principal defect now, however, is the fact that it is not possible to predict the height of a wave as it reaches any particular shore, and therefore the same alert must be issued for all approaching seismic waves. Research on methods of forecasting wave height is therefore needed. Even with its present limitations, however, the system has filled so great a need that there is strong international interest in extending it to other parts of the world.
21
The flood of ocean waters that overwhelmed the coast of the Netherlands on February 1, 1953, deserves a place in the history of great storm waves. A winter gale that formed west of Iceland swept across the Atlantic and into the North Sea. All its force was ultimately brought to bear on the first land mass to obstruct the course of its center—the southwestern corner of Holland. The storm-driven waves and tides battered against the dikes in such bitter violence that these ancient defenses were breached in a hundred places, through which the flood rushed in to inundate farms and villages. The storm struck on Saturday, January 31, and by midday of Sunday one-eighth of Holland was under water. The toll included about half a million acres of Holland’s best agricultural land—ravaged by water and permeated with salt—thousands of buildings, hundreds of thousands of live stock, and an estimated 1400 people. In all the long history of Holland’s struggle against the sea, there has been no comparable assault by ocean waters.