VOLCANIC EARTHQUAKES 103 
Japan to a height of 8,140 feet above the sea. After a prolonged 
period of rest, it has been subject for about six years to a series 
of strong explosions, the first of which occurred on February 
13, 1908. 
Observations with horizontal pendulum seismographs were 
instituted by Professor Omori at two stations on the southwest 
flank of the volcano, Yuno-taira and Ashino-taira, at heights of 
6,306 and 4,422 feet above the sea. Owing to weather conditions, 
the observations at the former station were confined to the summer 
months. 
The seismograms obtained at these places showed that there 
were two distinct types of earthquakes, some being independent of 
any outburst of the volcano, while others were invariably the 
results of explosions. 
The two types of earthquakes are characterized by several 
marked differences, of which the following are the more important: 
a) The shocks without explosions consisted only of minute 
quick vibrations, while those with explosions consisted of slow 
movements (of as much as 2.6 and 5.3 seconds’ period), on which 
after a few seconds quick vibrations were superposed. 
b) The earthquakes without explosions were distinctly stronger 
than the others, probably because, as Professor Omori suggests, 
a great part of the energy of the explosions is expended in the 
projection of rock fragments and débris. Of 1,485 earthquakes 
without explosions, 21 per cent were sensible at Yuno-taira; of 
8,847 earthquakes with explosions, only 0.3 per cent were sensible. 
Moreover, the strong earthquake of May 26, 1908, which did not 
accompany an explosion and which evidently originated in the 
volcano itself, was felt over an area of 2,400 square miles.’ 
c) The earthquakes without explosions were of shorter duration 
than those with explosions, the averages for the former being 
16.7 seconds in 1911 and 15.0 seconds in 1912, and for the latter 
33.2 seconds in 1911 and 32.7 seconds in 1912. 
d) The two types of earthquakes alternate in frequency. 
During the two years, 1911-12, the maxima of the earthquakes 
‘It may be added that the Usu-san explosions of 1910 were preceded by many 
sensible earthquakes, a few of them strong; but, as a rule, they were unaccompanied 
by sensible shocks. 
