EARTHQUAKE MEASUREMENT AT MIYAKO. 171 



also present to a proportionately large amount. The character 

 of motion in each earthquake was very simple, the preliminary 

 tremor being followed abruptly by a large vibration. 



The area of strong earthquake motion was an oblong ellipse, 

 whose major axis, about 320 kilometres in length, was in the 

 direction of NEN— SWS, and the shocks were probably caused 

 by the formation of a long fissure or dislocation at a great depth 

 under the mountain chains forming the back-bone of the northern 

 part of the Main Island. The epicentre, or the central point 

 of the meizoseismal area, was situated at about 130 km. S 60° W 

 of Miyako. (See fig. 5). 



The maximum range of motion of the first shock was 

 5.4 mm. in the (EVV), 2.0 mm. in the (NS), and 0.5 mm. in 

 the (V). 



The maximum motion of the second shock was as follows : 



26.0 mm. towards W, 9.6 mm. towards S, 3.5 mm, down- 

 wards. Eesultant 2a=28.0mm., direction S 65° W, the angle of 

 emergence=7.°2. 



This maximum displacement was preceded by the following 

 next-maximum movements : — 



(Fir.st motion). 4.2 mm. towards W, 3.5 mm. towards S, 0.5 mm. 



downwards. Eesultant 2a = 5.5 mm., direction SW, the angle of 



emergence = 5. °2. 

 (Second Motion). 14.4 mm. towards E, 8.4 mm. towards N, 2.3 mm. 



upwards. Resultant 2a = 17.0 mm., direction N 55° E, the angle 



of emergence = 7. °8. 



As the rotation of the record-receiving drum was unfor- 

 tunately irregular, the periods of particular waves can not be 

 accurately determined. But, judging from the number of the 

 superposed ripples, the period of the principal vibration was 



