The centre of the system is the Coast and Geodetic Survey seismograph 

 station at Honolulu. Here all seismograph reports will be received, 

 epicentres determined, alerts issued, and wave reports evaluated. Here 

 it will be determined whether or not an alarm is justified. Onl}^ a small 

 number of the quakes that occur in the ocean produce seismic sea waves, 

 and not all sea waves are large enough to be dangerous. AH must be 

 treated with respect, however, by those involved in the system. 

 Whenever the possibility of a sea wave exists, those charged with 

 broadcasting warnings to the public and to military and other esta- 

 blishments will be notified to " stand by " so that no time will be lost 

 in issuing a warning should it be found necessary. 



The sequence of events that would set the system in operation 

 will differ at different times. Ideally an earthquake would be detected 

 by the seismograph station as soon as it occurred and be reported to the 

 central station where the position of the epicentre would be determined. 

 If the central station is the first to detect the quake, reports will be 

 requested from the other stations. If the epicentre is found to be under 

 the Pacific Oean where it could generate a sea wave, selected tide stations, 

 favourably situated to intercept the wave before it reached the 

 Hawaiian Islands, would be alerted to watch their gauges for evidence 

 of a wave and report their findings. From the seismic sea-wave 

 travel-time chart the central station can determine quickly- what time 

 to expect the wave in the Hawaiian Islands, and consequently how much 

 time can be used in obtaining tide station reports. Though other places 

 than tide stations may at times be asked for reports, tide stations are 

 in a more favourable position to report usable information, for waves 

 -can be observed on the tide record that maj^ otherwise go unnoticed. 

 The fact that a wave is small at one place does not mean that it will 

 not be large enough to be dangerous at another. 



To take care of the possibility that a quake might remain temporarily 

 undetected and that the first hint of a seismic sea wave might be the 

 arrival of the wave itself, the Coast and Geodetic Survey developed a 

 seismic sea-wave detector. This is a simple device with no moving parts 

 except a column of mercury which moves to close an electric circuit. It 

 makes use of the change of air pressure in a vertical pipe brought about 

 when the rise and fall of the sea has the twelve to twenty minute period 

 characteristic of a seismic sea wave. The detector screens out the rapid 

 rise and fall of wind waves and the slow rise and faU of the tide, but 

 automatically signals an alarm at a tide station upon the arrival of a 

 seismic sea wave. Since most of the tide stations on Pacific islands are 

 at military posts or communication centres where a twenty-four hour 

 watch is maintained, an observer is always available to hear or see 

 the signal. When the signal is observed the observer will check the tide 

 record before alerting the Honolulu centre. Since the detector can be 

 actuated by the first part of the wave motion a few minutes before 

 the arrival of the destructive part, it can also be used to sound an 

 alarm locally for the community or post in which it is located. (The 

 instrument is described in greater detail in the January, 1948, issue of 

 the Coast and Geodetic Siirvey Journal.) 



Detectors have been installed at Honolulu, Hilo, and Midwaj", and 



another wll soon be in operation at Palmyra Island. Others will be 



installed in the Aleutians. Places wdth large ranges of tide are being 



-avoided because the rate of rise and fall of the tide must not approach 



loo closely to that of a possible seismic sea wave. 



361 



