EARTHQUAKE MEASUREMENT AT MIYAKO. 191 



vvell-clefined for comparatively near shocks, but often very 

 indefinite for distant ones. 



41. The duration of the preliminary tremor, which has 

 been measured in 24 cases, varied between and 26 seconds. 

 As already discussed by one of the present authors,' the duration 

 of the preliminary tremor of an earthquake, y (expressed in 

 seconds), at a given station seems to be related to the distance 

 of the latter from the origin, x (exjoressed in kilometres), by 

 the following linear equation — 



7.51 ?/=a3-24.9 km. (2) 



X being assumed to be sufficiently great, say, greater than 

 100 kilometres, or y greater than 10 seconds. The approximate 

 distances of the epicentres from Miyako have been calculated 

 by means of equation (2) in 11 cases, in which the duration 

 of the preliminary tremor was greater than 10 seconds. These 

 results are given in the 17th column of the Table, vvliile the 

 distances judged from the earthquake maps are given in the 

 16th column. 



It will be observed that the two sets of values given in 

 columns 16 and 17 agree on the whole pretty well with each 

 other. Thus, the mean value of the distances under considera- 

 tion deduced from eight cases, namely. Nos. 2, 3, 12, 13, 14, 

 17, 19 and 23, is according to equation (2), 160 km., and 

 according to the method of seismic maps, ^180 km. 



42. The period of vibration. In the Table are given for 

 different earthquakes the complete periods of maximum move- 

 ments, as well as the average complete periods of vibrations in 



1. F. 



mori. Note on the preliminary tremor of earthquake motion. 



