Note on the Preliminary Tremor of 

 Earthquake Motion. 



By 

 F. Omori, D. Sc. 



Professor of Seismology, Tokyo Imperial University. 



With Plates XIII—XVI. 



1. It is well known that an earthquake usually begins 

 with movements, whose amplitude of vibrations is much smaller 

 than that of the subsequent motions, which form the principal 

 portion of tlie shock. These small introductory movements, the 

 *' preliminary tremor," whose period is generally very short, 

 are doubtless, in some instances, the cause of the so-called 

 earthquake sound, often heard just before the arrival of the 

 tremblings of the ground. The quickest tremor vibration, which 

 I have ever observed, was one of the after-shocks of the great 

 Mino-Owari earthquake of the 28th October, 1891, instru- 

 mentally registered at the temporary seismological observatory 

 set up in the village of Midori in the Neo- Valley, the most 

 central part of the meizoseismal zone, its complete period being 

 only 0.023 second. In the case of some very distant earth- 

 quakes, however, the preliminary tremor consists entirely of 

 small slow undulations with a period nearly equal to that in 

 the principal portion, 



