96 



RAITT 



[chap. 6 



Table III 

 Eastern North Pacific 



Ewing, 1959). The discrepancy is removed by postulating Layer 2. Another 

 piece of evidence comes from the crustal shear waves occasionally observed as 

 second arrivals. It was observed by Ewing, Sutton and Officer (1954), by Katz 

 and Ewing (1956) and by Gaskell, Hill and Swallow (1958) that the travel 

 times of these shear waves were greater than expected if there was no Layer 2 

 and the conversion of compressional to shear waves took place at the base of 

 the sediments. This discrej^ancy is removed if Layer 2 exists and the conversion 

 takes place at the boundary between Layer 1 and Layer 2. 



Unfortunately, these indirect indications of the presence of Layer 2 do not 

 measure its velocity or thickness and are often ignored in the structural inter- 

 pretation. For example, some of the stations of Table I which have no Layer 2 

 in their interpretation are reported by Ewing and Ewing (1959) to have strong 

 indirect evidence for its presence. 



The same reasons which make its detection difficult also make the measure- 

 ment of the velocity of Layer 2 inaccurate. The measured values range between 

 the rough limits of 4 to 6 km/sec, and it is not certain how much of this 

 variability is error of measurement. Most of this error results from the short 

 distance over which Layer 2 can be seen as a first arrival. Hence the velocity is 

 very sensitive to local variation in sediment thickness. As the topography of 

 the base of the sediment may be rough, even where the ocean floor is smooth, 

 this gives a source of error very difficult to control. 



There is some internal evidence that there are real differences in the velocities 

 of Layer 2. For example, in the Pacific Ocean, southeast of Hawaii, the average 

 value of 5.99 km/sec was systematically greater than the average value of 



