EARTHQUAKE MEASUREMENT AT MIYAKO. 179 



The initial movements ceased almost completely a short 

 time before the appearance of the principal portion, whose 

 maximum 2a was 0.2 mm., and whose average period was 0.084 

 second in each component. It is here to be remarked that in 

 cases of small shocks like the present, there often exists no 

 prominent maximum motion, a great number of vibrations 

 having nearly the same amplitude. 



21. Eqke. No. 16. The 23rd August, 1897 ; 9.32.55. In- 

 tensity, weah. The date is possibly erroneous, there being in the 

 Central Meteorological Observatory no report relating to this 

 earthquake. The duration of the H.M. was 100 seconds, and 

 that of the V.M., 70 seconds. 



The preliminary tremor, whose duration was 11 seconds, 

 consisted of fine ripples of an average period of 0.090 second in 

 the (EW), 0.083 second in the (NS), and 0.086 second in the 

 (V). It is to be remarked that these ripples were greatest in the 

 (NS) and smallest in the (V), the maximum 2a being 0.24 mm. 

 in the (EW), 0.4 mm. in the (NS), and 0.05 mm. in the (V). 



The principal portion, which was well marked off by greater 

 amplitude and larger period, lasted for 9.3 seconds, the motion 

 being especially active during 4 seconds at the middle of this 

 epoch. The average period of these principal vibrations, whose 

 general direction was WSW — ENE, was 0.11 second in each 

 horizontal component ; the maximum motion being 



(H.M.) 2a = 2.1mm. T =0.14 second, 

 (V.M.) 2a =0.3 mm. 



The average period of the ripples occurring immediately 

 after the most active portion of the shock was 0.10 second in 

 the (EW), 0.09 second in the (NS), and 0.10 second in the (V). 



