74 s. SEKIYA 



28 shocks already specified as showing vertical motion, 14 showed a 

 horizontal motion of more than 1 m.m., while in the other 14 cases 

 the horizontal motion was less than 1 m.m. 



Again when we analyze the 14 cases which had vertical motion 

 with less than 1 m.m., we see that in 9 shocks tlie o])serving station 

 was in, ov comparatively near to the centre of the disturbed districts, 

 and in the remaininsf 5 cases it Avas at a considerable distance 

 from the origin. In other words vertical motion generally appeared 

 when the orisfin of the disturbance was near tlie observin"* station. 

 In such a case the vertical motion might have come directly through 

 the earth-crust from the origin, and not in the form of free surface 

 waves. It must be, however, noted that there Avere 4 cases in which 

 the observing station w-as comparatively near the centre of dis- 

 tiu'bance, Ijut in which vertical motion did not (jccur. Tbey were 

 all small shocks, with maximum motions less than 1 m.m. 



In earthquakes showing l)oth horizontal and vertical motions, 

 feeble but cpiick-period tremors of both types simultaneously preceded 

 the principal movements. The more decided and pronounced motion 

 usually appeared first in the horizontal component, and then came 

 the large vertical movements. For the diagrams oï motion see 

 Plate IX and l^late XI, Vol. I of this Journal. 



Tremors, or rpiick-period minor waves generally j)recede earth- 

 quake proper and their probable connection with sound phenomena 

 has been discussed in tliis country by Profs. Ewing^, Milne'^ and 

 Knott.^ The vertical motions Avhich have just been considered 

 possess in great measure the chnractcristics of these minor tremors. 



It is a well-known fact that the movement of the grcjund at the 

 time of earthquakes is very complex, and that the ground moves in 



1 Memoirs on Earthquake Measurements, p. 11. 



2 Earthquake Notes — Sound Phenomena, Trans : Seis : Soc : Vol. XII. 



3 Earthquakes and Earthquake Sound, etc., to be published in Trans: Seis: Soc: Vol. XIII. 



