PRELIMINARY TREMOR OF EARTHQUAKE MOTION. 157 



Meteorological Observatory, but otherwise nowhere observed or 

 reported. The duration of the preliminary tremor, as given by 

 the Horizontal Pendulum diagram was 23 seconds, which, ac- 

 cording to equation (1), would correspond to a distance of about 

 200 kilometres between the observing station and the probably 

 suboceanic origin of the earthquake. In cases like this, it is 

 impossible in any other way, to estimate the distance of the 

 origin of disturbance. 



14. The earthquake of the 7th October, 1898; 11.0.46 a.m. 

 The earthquake shook moderately Oshirria, Rikuoku, Rikuchu 

 and Iburi. The duration of the preliminary tremor in Tokyo, 

 as recorded by my Horizontal Pendulum was 82 seconds, while 

 that at Miyako was 19.5 seconds. According to equation (1), 

 the distances of the origin from Tokyo and Miyako would 

 respectively be about 640 and 170 kilometres. The position of 

 the origin determined by the intersection of the circles described 

 about Tokyo and Miyako with radii respectively equal to these 

 distances is indicated in fig. 3, it being nearly identical with 

 that to be inferred from the isoseismal lines. 



15. The fact that the duration at a given station of the 

 preliminary tremor of an earthquake is nearly proportional 

 to the distance of its origin from the station can le explained 

 by assuming two sets of waves, which, originating simultaneously 

 at the centre of disturbance, are propagated with different 

 velocities. If Vi denote the velocity of propagation of the 

 small preliminary tremors, and if V2 denote that of the prin- 

 cipal waves of an earthquake, we find 



JL 1 g;-24.9 



v. ~ Vi "" 7,51 a; ' ^^ 



