180 OCEANOGRAPHY 



tions were equipped with special pressure ^'ages to record long-period 

 seismic seawaves; and priority communication systems were organized 

 by which all data could be forwarded to the nerve center of this system 

 at the Honolulu INIagnetic Observatory of the Coast and Geodetic 

 Survey. 



Constant vigil is maintained at the major seismological stations 

 comprising this net and, on tlie average, 20 earthquakes occurring 

 at various places around the world are recorded each day. Most of 

 these are small in magnitude and of interest only to seismologists and 

 geophysicists; however, occasionally this quiet is shattered by violent 

 activity when some adjustment deep within the earth releases vast 

 amounts of energy. Within minutes after an earthquate occurs, de- 

 pending on the distance from the focal point, seismographs will begin 

 wild oscillations which can last for hours. 



Now, Mr. Chairman, I would like at this stage to show you the 

 seismogram from the seismograph in our Washington area which is in 

 the Commerc« Building. It shows you how these waves are recorded. 

 We have indicated time on the gram. It takes the seismogram 1 hour 

 to go across here and thus 3 or 4 days" records can be recorded on this 

 seismogram. 



This is Green\vicli civil time at 1900 and this would be zero hour 

 each day. 



Tliis is where the record for May 22 starts. This is the first earth- 

 quake that came in from Peru and, incidentally, the gain was set w^ay 

 clown because otherwise instrument would hit the stops and oscillate 

 back and forth too much. 



As you look down 2 or 3 days later, this oscillator indicates another 

 quake and this point indicates the time when your major tsunami 

 came in. In this case it was hitting the stops, vibrating back and 

 forth very strongly. 



Tliat is the record we get at various seismographs. 



I might state that diti'erent types of waves come in. There is a P 

 wave and S wave and, because we know how fast those waves travel 

 through the eartli, we can determine how far an earthquake is from 

 a particular seismograph. 



This is just a copy of the record. 



Mr. Miller. How long does it take these to be transported through 

 the earth? This thing took place in Chile, say, at zero hour. How 

 long was it before you got it up here ? 



Captain Earle. Before we got word of that? 



Mr. Miller. No. How long was it before it showed up on your 

 seismograpli ? 



Captain Earle. Those waves travel between 5 and 20 kilometers 

 per second, very fast. In other words, in 12 to 20 minutes it reached 

 all the seismographs around the Pacific. We knew that a major earth- 

 quake occurred. We did not know whether it created a tsunami but 

 we knew there was a major earthquake. 



As soon as the tide stations sent their records to Honolulu, they 

 knew there was a major tsunami. 



Mr. Miller. The vibration of the earth set up by this great release 

 of energy in Chile runs through the earth much more quickl}' than 

 the speed of sound in water. 



