The seismic sea wave that rolled onto the shores of the Hawaiian Islands on April 1, 1946, presented a challenge that has been accepted. The challenge was this—detect seismic sea waves and warn the Hawaiian Islands before they strike.
Seismic sea waves, also called tsunami, have been one of the most destructive killers in recorded history. Records date from 479 B.C. Reports indicate that 100,000 people lost their lives at Awa, Japan, in December 1703, that there was heavy loss of life at Hawaii in April 1868, and that after the eruption at Krakatoa late in August 1883 more than 36,000 people were wiped out by seismic sea waves. These are but three of a large number of major catastrophes which may be repeated in the future.
The seismic sea wave of April 1, 1946, destroyed Scotch Cap Light, about 93 miles from the origin of the disturbance, and caused the loss of 173 lives and over $25,000,000 property damage in the Hawaiian Islands, more than 2,200 miles from the origin. The base of Scotch Cap Lighthouse was 57 feet above high water. The waves reached heights of more than fifty feet along some parts of the shore of Hawaii. Several boats were reported destroyed by the same seismic sea wave at Iquique, Chile, and a thirty-foot wave was reported from the Juan Fernandez Islands off the coast of South America, more than 7,800 miles from the origin of the seismic sea wave.
In the past the origin of seismic sea waves has been hunted down and convicted after the damage has been done in much the same manner that a criminal is brought to justice, too late to save the victim. Here is a different objective. Track down the killer before he strikes and warn of his approach.
What causes a seismic sea wave, how fast does it travel, and where does it originate?
A seismic sea wave is generated by a violent submarine earthquake that sends out oscillations in all directions from the bottom of the ocean. The resulting waves are technically described as transverse and gravitational and travel at a speed that varies with the square root of the depth. The formula for determining the velocity of the wave is V=√gd V is the speed of the wave in feet per second, g is the acceleration of gravity (about 32.2), d is the depth of the ocean in feet at the place under consideration. In the deep open sea the waves travel at speeds of 300 to 500 miles per hour, are 75 to 125 miles long and are only a few feet high. These waves pass a ship in deep water without being observed. As the waves approach shoal water their speed decreases, their lengths shorten and their heights increase. Evidence at Hawaii indicates that ordinarily the first rise is not destructive, but that succeeding waves that arrive at ten to forty minute intervals do major damage. Seismic sea waves may be destructive both near the origin of the earthquake and thousands of miles distant.
Earthquakes which spawn destructive seismic sea waves, that may be detected in time to warn the Hawaiian Islands, originate along the Pacific rim of the Western Hemisphere and Asia and among the islands of the West and Southwest Pacific, two thousand or more miles from Honolulu.
How can such a seismic sea wave be detected in time to warn the Hawaiian Islands?
The solution involves the use of seismographs, tide gages, seismic sea wave detectors, a communication system to get information quickly to Honolulu, and the rapid dissemination of a warning to people in the danger areas.
Primary seismological stations are located in Honolulu, T. H., Tucson, Arizona, and College, near Fairbanks, Alaska. Each of the primary stations is equipped with a warning device that sounds an alarm when a major earthquake occurs. There are several secondary seismological stations that can be called upon to furnish supplementary information. These are located at Sitka, Alaska; Berkeley and Pasadena, Calif.; and Tokyo, Japan. Their function is to detect and locate earthquakes.
Seismic sea wave detectors are located at Dutch Harbor, Alaska; Hilo, Hawaii; and Palmyra and Midway Islands in the Pacific. They form a second separate and positive line of defense. These detectors, without seismological intelligence, are designed to screen out both short period wind waves and long period tide waves and tune in seismic sea waves. A detector sounds an alarm that gives a local warning. An earthquake might not be detected by seismographs and the first hint of a seismic sea wave may be an alarm from a sea wave detector.
Tide gage records show the local heights of the wave, the time between crests and, with other records, indicate the time the wave will strike the Hawaiian Islands. A seismic sea wave produces a signature on a tide gage record and a positive clue that a killer is loose. There are 18 tide gages in the Pacific strategically located to intercept these waves.
Sea wave detectors and tide gages are located in sheltered harbors, protected from the direct force of the sea, but subject to the oscillations set up by seismic sea waves.
All of this information is scattered and latent and must be transmitted to a central location to be fully useful. Communication breathes life into these data.
The central laboratory where data are assembled and processed is the Coast and Geodetic Survey Honolulu Magnetic Observatory. Telephone, radio and radioteletype combine to make up a network of communication from Honolulu to all the seismological, tide and sea wave warning stations. Here’s how it works:
An earthquake of considerable magnitude occurs at an unknown location. The earthquake shock is transmitted through the ground, registered, and a warning sounded by seismographs at Honolulu, Tucson and College. The earthquake wave that sets off the alarm travels at about 180 miles a minute. The seismograph records are developed and analyzed by observers at each station. Information is immediately transmitted to Honolulu from Tucson and College by the most rapid communication available. The messages have the highest peacetime priority. Except for necessary delays in relaying a communication at junction points, the message proceeds at a rate of 186,000 miles a second, or for practical purposes instantaneously. The information thus collected is analyzed and plotted. The end result is the location of the epicenter of the earthquake.
Suppose the location is found to be under the ocean. An alert is sent to responsible Army, Navy, civilian and territorial officials to stand by for a possible warning. Next, a seismic sea wave travel time chart is consulted to determine when a sea wave, if one develops, would reach tide or warning stations near the origin of the earthquake. Dispatches are sent to tide stations to stand by and report conditions at their stations.
Suppose the sea wave detector sounds an alarm or the tide gage record shows a sea wave at the expected time of arrival. The observer immediately prepares a brief message for immediate transmission to Honolulu. The observer then stands by, watches the tide gage record and submits more detailed information to Honolulu as conditions warrant.
Honolulu Magnetic Observatory will issue a warning that will be carried by radio, telegraph and airplanes equipped with both radio and loud speakers to danger areas. Sirens used for wartime air warnings will be used to sound local alarms. Police, military and civilian organizations will use their resources to evacuate persons and movable property along the vulnerable shores to higher ground. Ships in a danger area will be ordered to the safety of the open sea.
In the event that tide observers note no unusual conditions at their stations, they report this information and stand by to send further reports if required.
Tests demonstrate that an earthquake likely to cause a seismic sea wave can be located with reasonable accuracy within 1 ½ hours after it occurs. Reports from tide stations that definitely establish whether or not a seismic sea wave has developed can be furnished to Honolulu in less than fifty minutes after information is requested.
Seismic sea waves that will endanger life and property in the Hawaiian Islands will reach Honolulu more than four hours after the earthquake occurs on the Pacific rim of the Western Hemisphere or Asia.
The observational seismological and tidal equipment in the system is supervised by U. S. Coast and Geodetic Survey personnel, all of whom are primarily engaged on duties other than sea wave warning. The communication system is operated by Navy, Army and other Government and private utilities. The plan lists two or more separate methods of communication between Honolulu and the reporting stations. In promulgating the plan the Office of the Chief of Naval Operations has authorized pertinent naval activities to “initiate action as required to assure the expeditious transmission and delivery via channels stated in the plan or via other methods as dictated by local circumstances of traffic pertaining to the seismic sea wave warning system.”
The warning system was designed primarily for the protection of the Hawaiian Islands; but other Pacific islands will be beneficiaries of these warnings.
The major work required to organize a seismic sea wave warning system for the Hawaiian Islands was concerned with perfecting seismographs and seismic sea wave detectors that sound a local alarm, organizing available communication systems into a network and utilizing military, territorial and civilian personnel and communications to warn people in danger areas that a seismic sea wave is approaching.