102 CHARLES DAVISON 
Thus, both eruptions were preceded by a marked increase in 
seismic frequency, followed by a marked decrease, the maxima 
occurring 24 hours before the first outburst of the Usu-san and 
about 13 and 6 hours before that of the Sakura-jima.* 
Observations were made with a portable horizontal tromometer 
erected by Professor Omori at Nishi-Monbets (5 miles from the 
Usu-san) from July 30 to August 6, and at the West-Kohan School 
(Sobets) at the foot of the East Maru-yama from August 6 to Io. 
At this place, which is close to the nearest craterlet, series of 
well-defined, small, quick, unfelt vibrations, called micro-tremors 
by Professor Omori, were registered, which were entirely absent 
from the records at Nishi-Monbets. The mean range of motion was 
in every case less than one-tenth of a millimeter; but the principal 
periods of the tremors (.53, 1.08, 1.59, 2.14 seconds) were prac- 
tically identical with those of earthquake vibrations recorded at 
Nishi-Monbets (.53, 1.01, 1.58, 2.43 seconds). It would seem, 
then, that the micro-tremors are in reality true earthquake 
vibrations, but so weak that they cannot be recorded more than 
a few miles from the origin. Professor Omori notices that the 
shortest of the foregoing periods is approximately one-half, one- 
third, and one-quarter of the other periods. He also shows that 
moderate explosions from even the nearest craterlet were not as 
a rule accompanied by marked micro-tremors; whereas violent 
explosions from that and other craterlets were usually accompanied, 
and often preceded by several minutes, by well-pronounced 
micro-tremors. ‘The tremors, however, were not confined to the 
epochs of explosions. They sometimes occurred when the smoke 
ejections from the different craterlets were insignificant and even 
when they had completely ceased. At such times, as Professor 
Omori suggests, eruptions were perhaps prevented by the tem- 
porary stoppage of the craterlets.’ 
Asama-yama.—The Asama-yama, one of the greatest of Japa- 
nese volcanoes, rises from the plateau of the central island of 
1 Professor Omori’s observations give precision to a fact which has long been 
known. “It is,” says G. P. Scrope in his Considerations on Volcanos (1825), “‘a 
remark common to the observations made on almost all volcanic eruptions, that 
local earthquakes always precede the emission of lava currents, and cease while the 
lava is flowing, to recommence when it has stopped” (p. 155). 
2 Op. cit., Vol. V (1911), pp. 31-38. 
