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SYMPOSIUM ON WAVE ANALYSIS AND WAVE 

 PREDICTION : WAVES AND TSUNAMIS 



A SEISMIC SEA WAVE WARNING SYSTEM FOR THE PACIFIC 



By W. B. Zerbe, U.S. Coast and Geodetic Survey 



The heavy loss of life and property in the Hawaiian Islands that 

 accompanied the seismic sea wave of 1st April, 1946, brought to the 

 attention of scientists and engineers the need for the development of a 

 system for quickly detecting and reporting these waves so that the 

 Hawaiian Islands and other areas might be warned in sufficient time 

 to be spared a repetition of such a disaster. 



Although damaging waves have occurred in the past, progress in 

 rapid communication and in instrumental development had not reached 

 a point where a warning system was feasible. Soon after the earthquake 

 of April, 1946, the Coast and Geodetic Survey undertook the develop- 

 ment of the instruments and procedures required for the successful 

 operation of such a system. These instruments, a visible recording 

 seismograph with an alarm and a seismic sea wave detector, are now in 

 operation, and sufficient progress has been made in the operation of the 

 warning system to warrant reporting it to the scientific world. 



It was obviously desirable that the occurrence of a major earthquake 

 should be known at once. The records from photographic recording 

 seismographs, which are in geperal use, are not visible until the^- are 

 removed and developed. Those that produce a visible record would 

 have to be watched continuously, which is impracticable. A seismograph 

 that would in some way signal an alarm was therefore indicated. This 

 need has been met by an electronic recorder that not only produces 

 a visible record, but also sounds an alarm. The alarm is sounded when 

 the amplitude of vibration of the light-beam is large enough to cause 

 the beam to strike photo-electric cells placed near the ends of its 'swing. 

 The alarm can be either audible or visual and it can be located in the 

 station, the observer's quarters, or elsewhere. (This is a development 

 of the electronic recorder described by F. Keller in the October, 1946, 

 issue of Transactions of the American Geophysical Union.) 



A recent development contains interesting added possibilities 

 regarding the detection, location, and identification of submarine 

 ■earthquakes. This involves the newly-discovered fact that these 

 earthquakes sometimes generate low-frequency sound-waves peculiarly 

 adapted for long-distance transmission through conducting la^-ers of 

 the sea-water. These waves pass into the ground structure of oceanic 

 islands and can be recorded on special seismographs shortl}' after the 

 higher-speed all-ground waves arrive. 



The complete warning system, as at present developed, consists of 

 four Coast Survey seismograph stations, the reports from which will 

 definitely fix the epicentre of the producing earthquake ; a network 

 of tide stations in the Pacific, some of which have seismic sea-wave 

 detectors ; a seismic sea-wave travel-time chart ; and a high priority 

 communication system. 



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