on Earthquakes in Japan, 371 



which the destructive power is to be measured, it being pro- 

 portional to the maximum kinetic energy of the bodies on 

 the earth's surface relative to that surface during the shock. 

 According to this, then, a shock, the vibrations in which are 

 of one-second period, will have only one fourth of the intensity 

 of a shock with vibrations of half a second period, if the 

 amplitudes be the same. We may write intensity proportional 

 to amplitude divided by the square of the period, or 



Marked instances of the effect of period are often to be ob- 

 served by simply referring to our senses for the intensity, and 

 then to our records for the amplitude and period. We find 

 sometimes that what we considered a severe shock has had a 

 very small amplitude of movement, but that it has had a short 

 period. 



Another interesting point to be observed, is the variation of 

 the affected area with intensity, amplitude, and duration. Of 

 these elements it would appear from our observations that dura- 

 tion is the most important. A comparatively small earthquake, 

 if it last for several minutes, will be felt over a large area ; 

 whereas a severe earthquake which consists of only a few bumps 

 will only be felt for a few miles. A very good illustration of 

 this was obtained on December 23, 1880, when an earthquake 

 which was felt at Yedo as a somewhat severe shock, and which 

 lasted between three and four minutes, was felt in several 

 places in Yezo ; whereas a previous earthquake, which was 

 much more destructive, but which lasted less than half the 

 time, was not felt through half the distance. Both these 

 shocks seemed to originate near the same place. 



The geological nature of the rocks near the origin from 

 which some of the earthquakes we have felt in Yedo appeared 

 to emanate, is as follows: — 



Round the northern end of Yedo Bay we meet on all sides 

 with greyish-coloured partially clay-like rock, which on close 

 examination appears to consist of a decomposed volcanic ash. 

 These tuffs are horizontally stratified, and very little faulted or 

 altered. As we go southwards on both sides of the bay, these 

 same rocks gradually become coarser in texture, until finally 

 they are little more than agglomerates consisting of undecom- 

 posed volcanic ashes. Along with this lithological change we 

 observe a physical change to be taking place. The rocks 

 become gradually bent into gentle anticlinal and synclinal 

 folds ; and faults become more numerous. Near to ihe 



