OCEANOGRAPHY 181 



Captain Earle. It is just like a vast explosion and goes out in all 

 directions. They are elastic waves travelino; in all directions out 

 from the center. They go through the earth and around the earth. 



The first two waves, known as the primary and secondary waves, 

 are waves that cut through the earth to the seismograph. For ex- 

 ample, I have another map that I will show you later. 



I think it takes 10 minutes for those waves to reach Tucson, and 

 so forth. 



Mr. Miller. Where is your institution in Hawaii actually located ? 



Captain Earle. It is out near Ewa, right past the Ewa Air Field. 

 We are moving that institution and we are dedicating a new observa- 

 tory on the 2?)d of June. That was necessary because the old observa- 

 tory had been surrounded by many Standard Oil buildings. They 

 were also building a rehneiy near by. It did not aifect seismology 

 as much as it affected our magnetic work at this observatory. Great 

 masses of steel and also pipelines disrupted the study of magnetism 

 so that Ave had to move the observatory. 



The Navy supplied us with an area of 177 acres and we received 

 money to move the observatory. 



Mr. Miller. xVdmiral Karo was kind enough to send us invitations 

 to attend this dedication and, if we could get time out and he would 

 furnish transportation, the committee would be glad to go. 



Captain Earle. I hope the committee will be able to see one of our 

 observatories. 



Mr. Miller. You may proceed. 



Captain Earle. To continue, when a major shock wave reaches a 

 specific installation, a buzzer warns the seismologists that a major 

 earthquake has occurred. They will analyze their records to deter- 

 mine the arrival times of the ground waves and the estimated distance 

 to the epicenter. This information is dispatched to the central station 

 at Honolulu by the quickest means available. The communication 

 channels have been previously established and are kept open at all 

 times so that immediate transmission is accomplished. 



Within a few minutes after the earthquake occurrence, the staff at 

 Honolulu, previously alerted by its own seismograph, begins to plot 

 the location of the earthquake, using data coming in from the net- 

 work. If the epicenter is in a Pacific Ocean area, an advisory is is- 

 sued, warning of a possible tsunami. 



The tsunami travels at speeds up to 500 miles per hour, much slower 

 than the ground waves of the earthquake itself. The tsunami wave 

 is but a few feet high and hundreds of miles long, but when it strikes 

 a coast it may gain great height and become very destructive. 



Positive identification of an actual tsunami is necessary. Two 

 points make the decision difficult. First, few earthquakes "produce 

 tsunamis, and, second, tsunami waves are impossible to recognize at 

 sea because of their low height. Identification is made through the 

 tide stations of the network at which observers scan their tide gage 

 records for a period when the wave might be expected. 



Upon receipt of a positive report from a tide station, a warning- 

 is issued. This is usually completed in less than 3 hours after the 

 earthquake, giving time for evacuation and other precautions at dis- 

 tances of 1,500 miles or more from the quake. Coastal areas at remote 

 places thus have warnings, sometimes of several hours. 



