192 F. OMORI AND K. H IE AT A : 



the preliminary tremor, the principal portion, and the end portion, 

 with respect to both the ripples and the slow undulations. 



The period of the maximum H.M. varied, for slow undula- 

 tions, between 0.53 and 1.7 seconds ; and, for ripples, between 

 0.12 and 0.15 second. The corresponding period in the vertical 

 motion varied, for slow undulations, between 0.53 and 1.7 

 seconds, the vertical and the horizontal components having, in 

 most cases, the same period. 



With regard to ripples, the period, which was identical in 

 all the three components of an earthquake, became generally a 

 little longer in the principal portion than in the preliminary 

 tremor. Thus, for the latter epoch, the mean value of the 

 average periods which varied between 0.04 and 0.087 second, 

 was 0.08 second ; while that of the average periods for the 

 former epoch, which varied between 0.055 and 0.12 second, was 

 0.10 second. For the end portion, the mean value of the average 

 periods of ripples, which varied between 0.06 and 0.10 second, 

 was 0.08 second, namely the same as in the case of the pre- 

 liminary tremor. The average period of ripples in the principal 

 portion, which are characterized by their having larger ampli- 

 tude, is thus seen to be longer than that in the two other 

 portions of the earthquake motion, the difference amounting to 

 0.02 second. 



For the principal portion, the mean value of the average 

 periods of horizontal slow undulations, which varied between 

 0.56 and 1.7 "seconds, was 1.3 seconds in the (EW) and 1.0 

 second in the (NS) ; while the mean of the corresponding 

 quantities for the end portion, which varied between 0.9 and 

 1.7 seconds, was 1.3 seconds in (EW), and 0.94 second in the 

 (NS). Thus, for the horizontal slow undulations, the average 



