92 



aa the " Discovery " and westwards across Asia and Europe, whilst they do not appear to have reached 

 nearer stations in North America. On the westward route it may be noticed that the path would be 

 sub-continental, whilst in going eastwards it would be sub-oceanic. 



The loudness of the sound made by a gun depends in part upon the direction in which the gun is trained 

 with regard to the observer. In a somewhat similar manner, if we hold the blade of a spade in water and 

 then suddenly move it, the largest waves are forced in the direction of the primary impulse. 



If these analogies may be used to explain why earthquakes from District M are propagated more 

 vigorously in a N.W. direction rather than in any other, one inference is that the fault or faults from 

 which these disturbances spring strike in a N.E. and S.W. direction, that is, they are parallel to the 

 New Zealand axis, and they hade towards the direction of the longest path along which movement is 

 recorded. Similar inferences may be made with regard to the origins of movements in other districts. 

 (See Map, Plate 3.) 



4. Velocity Determinations. 



In a few instances, when accurate data have been obtainable, calculations have been made of the speeds 

 with which earthquake motions have been transmitted in various directions round and through the world. 



Speeds along paths which are continental have been compared with those which are sub-oceanic. For 

 example, for earthquakes with origins off the coast of Eastern Asia, the rate at which waves have been 

 transmitted across Asia and Europe may be compared with the rate at which the same travelled beneath 

 the Pacific Ocean to New Zealand and the " Discovery." The material at my disposal does not show that 

 there is any certain difference in speeds. Certain tables relating to speed strengthen the suggestion that, 

 for particular phases of earthquake motion, velocity is not constant. The large waves, or P 8 , apparently 

 increase in speed in quadrantal regions. Other tables relating to rate of propagation are only of value as 

 indications of the character of motion which has reached distant stations. To this I refer in the next 

 section. 



A knowledge of the time taken by earthquake waves to travel from one seismic region to another 

 occasionally leads to the conclusion that one earthquake may be regarded as the cause of a second 

 disturbance. Illustrations of earthquakes having originated in a district at the times when teleseismic 

 movement reached that district are to be found in earthquakes numbered 4, 8, 45, 48, and 117. 



5. The Surviving Phase of Earthquake Motion. 



With exceptionally large earthquakes we may obtain at very distant stations seismograms which 

 exhibit all three phases of earthquake motion. More frequently, however, at such stations the record is a 

 mere thickening of the photographic trace, a small fraction of a millimetre in amplitude, and with a 

 duration of 3 or 4 minutes. Near to its origin the maximum motion of the same earthquake may have 

 been pronounced, while its total duration may have extended over at least 1 hour. 



The test which has been used to determine the phase of motion to which the surviving tremors 

 represented by a thickening are to be referred has been determinations of the speed with which they have 

 been transmitted from their origin to the station at which they were observed. In a few instances the 

 times of origin and the positions of epifocal districts have been obtained with a fair amount of accuracy, 

 and the results relating to earthquake speeds may be regarded as reliable determinations of the same. 



This, however, is not the case with the majority of velocity tables which have been compiled, the reason 

 being that they have been dependent upon data relating to times of origin and positions of centres which 

 in all probability may in certain instances deviate by 5 in distance and 5 minutes in time from the truth. 



Notwithstanding this, as the velocities of PI, P-2, PS for long arcs are respectively about 12, 6, and 

 3 kms. per second, although the velocities deduced for surviving phases may want in accuracy, they 

 seem to be sufficient to suggest the type of wave to which they belong. The type determined appears to 

 be PS, which at stations comparatively near to the origin is announced as an undulation of the earth's 

 surface.* 



* For list of shocks showing these survivals, see ' Antipodean Recurrences,' p. 292. 



