72 



s. SEKIYA 



Also from alike treatment of the horizontal motions in 18 earth- 

 quakes recorded at Hongö (hard ground), we get : — 



Maximum horizontal motion 0*37 m.m. 



Complete period of maximum horizontal motion ... 0*76 «ec. 

 Duration of horizontal motion 74 sec. 



The values of the horizontal motion given in the first tîibulated 

 set of averao'es are laroer than those in the second and third. The 

 reason is, clearly, that shocks containing vertical motion are generally 

 larger than those witlKnit, so that the sec(jnd and third sets, Avhich 

 include shocks both with and without vertical motion, naturally give 

 smaller averages. 



Although the second and the third sets, referring as they do 

 to ditferent shocks, are not strictly comparable, they nevertheless 

 show in a general way that in hard ground the motion is smaller, 

 the period quicker, and the duration shorter, than in soft soil. Their 

 ratios are 1 to 2, 1 to 1'3 and 1 to 1'5 respectively. 



In the above sets of averages the records of the somewhat des- 

 tructive earthquake of January 15th, 1887, were not included as that 

 Avas much larger than the ordinary shocks we are dealing with. For 

 the sake of comparison the characteristics of that shock as registered 

 at Hitotsubashi are now" given. 



Maximum horizontal motion 21 ni.m. 



Complete period of maximum horizontal motion ... 2* 5 sec. 

 Duration of horizontal motion G min. o-l sec. 



(Principal motion 2 min.) 



Maximum vertical motion 1*8 min. 



Complete period of maximum vertical motion 0*9 sec. 



Duration of vertical motion 98 sec. 



For details see No. So. In the earthquake of October loth, 1884, 



