ON THE EARTHQUAKE AND VOLCANIC THENOMENA OF JAPAN. 125 



quakes it appearocl that the motion at the bottom of the pit was very 

 much less than that observed on the surface, while for small disturbances 

 the difference between the surface and pit records was too small to be 

 measurable. In 1886 a pit 18 feet in depth was sunk through dry 

 compact earth at the Imperial University in Tokio, at the bottom of 

 which seismometers were established on a brick pavement. These 

 seismometers and others in the Seismological Laboratory a few yards 

 distant when placed side by side gave records which were identical. 

 The work was commenced by Professor S. Sekiya, and continued by myself, 

 and the records obtained have now been subjected to a careful analysis 

 by Mr. F. Omori, a graduate of the University, who has taken from ten 

 to thirty waves in thirty different earthquakes and for each of these 

 waves calculated its amplitude, period, maximum velocity, and maximum 

 acceleration. Of these thirty disturbances, for each of which diagrams 

 were obtained on the sui'face and in the pit, three were strong and 

 twenty-seven were feeble. For each set of calculations referring to 

 a particular earthquake average values were obtained, and the average 

 for these average values was as follows : — 



1. Ratio of Quantities Observed on the Surface to tJiose Observed in the Pit. 



(rt) Feeble Disturbances. 



fE.W. component 

 ■ \N.S. component 

 r E.W. component 

 1 N.S. component 

 /E.W. component 

 LN.S. component 



4. Ratios of maximum accelerations < xt'c ' t 



l_N.S. component 



Average 



1. Ratio of amplitudes 



2. Eatio of Periods . 



3. Ratios of maximum velocities 



l-0\ 

 1-3J 

 0-9 \ 

 1-1 /■ 

 l-2\ 

 13j 

 1-4 t 

 20/ 



1-2. 



10. 



1-3. 



1-7. 



From the above it appears that for small disturbances the motion on 

 the surface is slightly greater than it is in the pit ; further, from an 

 inspection of the diagrams, it is seen that those from the pit are always 

 smoother than those from the surface. In severe earthquakes Mr. Omori 

 points out that this latter character is strongly marked. 



(Jj) Steong Distuebances 



1. Ratio of amplitudes 



2. Ratio of periods . . . , 



3. Ratio of maximum velocities 



4. Ratio of maximum accelerations 



/E.W. component 

 \^N.S. component 

 /E.W. component 

 J^N.S. component 

 /E.W. component 

 \N.S. component 



{E.W. component 

 N.iS. component 



1} 

 1} 



1} 



Average 

 1-4. 



11. 



1-3. 



l-S. 



(c) Ripples Supeeimposed on 



1. Ratio of amplitudes . 



2. Ratio of periods . 



3. Ratio of maximum velocities 



■4. Ratio of maximum accelerations 



AVaves of Strong Distuebances. 



Average 



fE.W. component . . 2-0"\ ^y.c, 



■ 1 N.S. component . . 2-3/ " ''' 



/E.W. component . - 08 1 „ „ 



\ N.S. component . . 0-8/ "'**• 



fE.W. component . . 3-0 "1 „„ 



• t N.S. component . . 2-6/ **■ 



J E.W. component . .5-81 . _ 



(^N.S. component . . 3-5/ 



