Hogben. — Theory of New Zealand Earthquakes. 503 



waves. The most important of these are : a, the beginning 

 of the preliminary tremors ; e, the chief of the large waves ; 

 h, the last marked phase of the series. The position of the 

 corresponding waves on fig. 2 is marked by the letters a, e, h. 

 In figs. 3 and 4 the downstroke on the margin shows the 

 beginning of the preliminary tremors (a) ; the black line right 

 across the seismogram gives the position of c, d, and e ; the 

 last phase (h) is shown in fig. 3 by another downstroke, but 

 cannot be distinguished in fig. 4.* 



As I have stated in a former paper, the times of arrival of 

 these waves (a) at the various seismological stations of the 

 world appear to show that they travel round the earth, and not 

 through its central portion — that is. along arcs parallel to the 

 surface, and not along chords. That is certainly true of the 

 other waves (b-h). 



The following table shows the transit-velocities of the chief 

 waves of the four earthquakes just referred to, in kilometers per 

 second : — 



Earthquake. a Waves. e Waves. h Waves. 



Guatemala .. 15-6 3-2 21 



Tonga .. 13-0 3-3 2-2 



Cheviot .. 12-4 3-5 2-3 



East Coast .. 125 3-5 2-3 



The high transit-speed of the a waves or preliminary tremors 

 can be accounted for only on the assumptions — (1) that the vi- 

 brations originated in rocks under a maximum strain (it will be 

 remarked that the speed is greatest for the earthquake of greatest 

 intensity) ; (2) that their path through the earth's crust was the 

 path of maximum velocity, and therefore through the rocks of 

 highest rigidity and elasticity. 



The velocities of the e waves and h waves for various large 

 earthquakes seem to vary very little, the average being 3-3 and 

 2-2 kilometers per second respectively. Now, from theory based 

 upon the experimental determinations of the rigidity, elasticity, 

 and density of various rocks, the speed of large normal or longi- 

 tudinal vibrations through such rocks as hard granite is estimated 

 to be between 3 - l km. and 3-95 km. per second, and the speed of 

 transverse waves two-thirds of that of normal waves proceeding 

 from the same origin at the same time and along the same path. 



* The fact that the interval between the arrival of the preliminary 

 tremors and of the normal waves is very small when the origin is near, 

 and increases with the distance from the origin, is explained on the assump- 

 tion that they start at or about the same time from the origin, and gradu- 

 ally become separated owing to the d fterence in their transit-speeds. 



