PRELIMINARY TREMOR OF EARTHQUAKE MOTION. 155 



in § 6/ for No. 11, § 5, and for the Fukuoka Earthquake of the 

 12th August, 1898, as observed in Tokyo, § 4. The curve, p q, 

 drawn through tl^e mean positions, is approximately a straight 

 line, whose equ n is, by calculation, found to be 



7..51 7/ = a- -24.9 km. (1) 



The straight line, p q, cuts the abscissa axis at a point p 

 (a:=^24.9 kilometres) instead of passing through the origin of 

 coordinates. This may be due to the probable fact that the 

 earthquake waves get weakened as they spread out from the 

 origin, the very earliest portion becoming at great distances 

 finally too small to affect the seismograph. For great earth- 

 quakes, whose origin is near to the observing station, say 

 under 100 kilometres, such a dissipative effect would not much 

 affect the duration of the preliminary tremor. These cases 

 must be treated separately, taking into account, of course, the 

 depths of the earthquake foci. 



The duration of the preliminary tremor does not depend 

 on the magnitude of the earthquake, which may be considered 

 as represented by the extent of the area of disturbance. Thus, 

 no difference of this kind is to be seen between the great Mino- 

 Owari Earthquake and its after shocks, (See § 2). Again, the 

 mean radii of propagation oi' the sensible shakings of the Mino- 

 Owari Earthquake and the Rikuchu-Ugo Earthquake of the 

 31st August, 1891, (See No. 1 and No. 3, Table, § 6), were 

 respectively 600 and 520 kilometres : the former earthquake was 

 thus a little greater than the latter, but the durations of their 

 preliminary tremors, observed in Tokyo, were respectively 37 

 and 56 seconds. 



1. The focal depth is assumed to be negligible in comparison with the distance between 

 the ol»serving station and the origin of disturbance. 



