INTERPRETATION OF SEISMOGRAMS 49 
practice it is desirable that the fundamental constants of all 
three instruments should be precisely the same. 
The first phase (undae primae) is initiated by P either as a 
sharp impulse (impetus) or rapid succession of impulses, or by 
a more gradual development (emersio). This lasts a few 
seconds, and is interpreted as the arrival of longitudinal waves. 
In many cases P is more pronounced in Z than in (XY). P 
is succeeded by a series of smaller movements of a very ir- 
regular character, with turning points sharply marked, at inter- 
vals of a few seconds. There is in general a marked absence 
of periodicity or motion of a sinusoidal nature. We do, however, 
sometimes find minute movements with a period of about 
I second, and I have seen an instance (earthquake in Yap) where 
P started with a few waves of small amplitude of a distinctly 
sinusoidal nature. Such cases are, however, rare. 
During the first phase we have some outstanding sharp 
movements. If these happen to be the P’s of subsequent 
shocks they will be confirmed by the later part of the seismo- 
gram. They may, however, be the reflected effects PR, etc., 
corresponding to P. I have already mentioned that with 
earthquakes in the Philippines PR,, which arrives here about 
four minutes after P, is usually much larger than P. 
After the first phase, which lasts for a time depending on 
the distance from focus to station, the seismogram changes 
its type. There is as a rule a large movement denoted by S 
which initiates the second phase (undae secundae). Its in- 
cidence is less sharply marked than P and it is sometimes very 
indistinct. It is clearer in (XY) than in Z. S is interpreted 
as the arrival of the transversal waves. Following S the 
movements are again very irregular. They are larger than 
those occurring between P and S, and occur at longer inter- 
vals. The turning-points are rounded, and occasionally give 
a suggestion of sinusoidal movement. During this phase we 
may have outstanding movements which may be the S’s of 
subsequent shocks or the reflexions of P and S. For dis- 
tances > 11000 km. it becomes difficult to say precisely when 
the second phase starts, and we have explained in the pre- 
ceding chapter how this probably arises. 
4 
