394 L. MARTIN ^ALASKAN EARTHQUAKES OF 1899 



Computing these velocities by the above formula, the speed of trans- 

 mission of surface waves is seen to vary between 3.1 and 3.3 kilometers 

 per second, or nearly two (1.95) statute miles per second, a rate agreeing 

 substantially with those reached independently from the several compu- 

 tations cited above. 



DISTURBANCE OF EARTH' 8 SURFACE 



The seismic disturbances traversed the rocks of the earth's crust to all 

 parts of the world from the origin in Yakutat Bay, where Prof. John 

 Milne has estimated that during the faulting one or two cubic miles of 

 rocky materiaP^^ were disturbed and molar disturbances took place.^^^ 

 No seismograph known to have been in operation in September, 1899, 

 failed to record these shocks if that type of instrument was capable of 

 registering them. 



The disturbances may be divided into two classes : (a) those that seis- 

 mologists infer to have gone directly through the earth, and (&) those 

 that follow the earth^s outer crust. These are indistinguishable at dis- 

 tances of less than 650 miles. Beyond that distance — for example, at 

 Victoria — the seismograph records show slight disturbances arriving 

 very soon (preliminary tremors) and great motion after a longer time 

 (principal portion or large waves). The preliminary tremors come 

 directly through the earth, along chords. They are generally thought to 

 be longitudinal, compressional vibrations. They have a shorter distance 

 to go and also move at a faster rate than the large surface waves. On 

 September 3, 1899, these direct waves traversed the chord from Yakutat 

 to Victoria in 3% to 4i/^ minutes, the large waves, moving presumably 

 along the arc, taking 13 minutes and 29 seconds to reach Victoria through 

 the earth's outer crust. 



These large waves of the principal portion vibrate transversely to the 

 line of propagation. They were formerly thought to make the earth's 

 crust actually rise in long, undulating earth-waves. For example, it was 

 estimated^*'^ that the large waves of the Yakutat Bay earthquake of Sep- 

 tember 3 passed through Shide, England, as earth-waves about 28 miles 

 in length and 11 2/5 inches high (45 kilometers by 29 centimeters) 

 The earth-waves of the great shock on September 10 at Shide were com- 

 puted as 74 miles long and 15 2/5 inches high (120 kilometers by 39 



107 Nature, vol. 75, 1907, p. 224. 



108 Seventh Report of Committee on Seismological Investigations of the British Asso- 

 ciation for the Advancement of Science, 1902, p. 62 ; Nature, vol. Ixvii, 1902-1903, p. 69. 



109 J. Milne : Fifth Report on Seismological Investigations of the British Association 

 for the Advancement of Science, 1900, p. 83. 



