THE SEISMOGRAM AND ITS ELONGATION, 

EARLIER EXPLANATIONS. 
On examining the seismograms, we notice that many of them can readily be divided 
into a number of well-defined parts. The movement begins as a slight vibration, known 
as the first preliminary tremors or the first phase; after an interval, dependent upon the 
distance of the station from the origin, there is a marked strengthening of the motion; 
this is called the second preliminary tremors or second phase ; very soon the motion becomes 
quite irregular. After a second interval, also dependent upon the distance of the station, 
the irregularities gradually die down, giving place to waves of long period, 25 to 50 seconds, 
which may have a large amplitude; at many stations the largest earth-amplitudes occur 
during this phase. The time when these waves take on a fairly regular form can usually 
be identified with some accuracy and is therefore taken as the time of arrival of the 
regular waves. It is a little later than the long waves of Professor Omori and a little 
earlier than the large waves of Professor Milne. I have adopted this point, as I found it 
in general more easily identifiable, in the various seismograms, than those just men- 
tioned; tho in some seismograms it is difficult to determine accurately where the regular 
waves begin. In a few cases it is not clear that there are any regular waves at all. This 
phase does not last long, but it is quickly followed by waves of shorter period, 15 to 20 
seconds, during which the pointer is apt to record its greatest amplitude, and which has, 
therefore, been called the large waves or principal part; it dies down with more or less 
irregularity until quiet is restored. This may require several hours, tho the earthquake 
at the origin may have lasted less than a minute. 
A number of hypotheses have been advanced to account for the increasing duration of 
the disturbance as the distance of the station from the origin is greater. In the first 
place, it is the general belief, first suggested by Prof. R. D. Oldham,’ that the first pre- 
liminary tremors are due to longitudinal waves, the second preliminary tremors to trans- 
verse waves, these two being propagated thru the body of the earth; and that the long 
waves and principal part are due to waves transmitted along the surface; altho some 
seismologists think that all waves are transmitted around the earth at or near the surface. 
A part of the record, near its end, is, in some cases, due to surface waves which have past 
around the earth and have approached the station from the antipodes. 
As longitudinal waves advance more rapidly than transverse the interval between them 
naturally increases with the distance of propagation. This is the most satisfactory 
explanation of the increasing interval between the two phases, but according to it we 
should have two groups of waves separated from each other by a period of quiet; whereas, 
in reality, we have a continuous disturbance; and, moreover, observation does not con- 
firm the idea that the first and second preliminary tremors consist solely of longitudinal 
and transverse waves, respectively. 
It has also been suggested that repeated reflections from the earth’s surface would 
cause a succession of impulses; but in this case also they would be discontinuous. Still, 
it is most probable that some of the sudden strengthenings of the movement are due to 
the arrival of these reflected waves. 
1 On the Propagation of Earthquake Motion to Great Distances. Phil. Trans. R. S. 1900-1901, vol. 
194, pp. 135-174. 
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