EARTHQUAKES—BULLEN 321 
at sea, then, depending on the characteristics of the fracture at the 
focus, there may be formed great seismic sea waves, or tsunami, which 
can cause havoc along low-lying shores of adjacent U- and V-shaped 
bays. There is as yet no universally accepted explanation of the pri- 
mary causes that give rise to this accumulation of strain energy 
in the earth’s interior, although nearly all theories relate the develop- 
ment of strain to the earth’s thermal history. 
TYPES OF EARTHQUAKE WAVES 
When a seismogram is scrutinized, it is found, on the first level of 
examination, to show the presence of three broad classes of seismic 
waves. The first identification of these waves was made by Oldham 
in England in 1897, nearly 70 years after the relevant mathematical 
wave theory had been formulated. 
One of the three classes consists of waves that travel over the earth’s 
surface and do not penetrate to great depths. On seismograms taken 
at considerable epicentral distances, these waves appear as the largest 
(except when the depth of focus is appreciable) ; this is because the 
surface waves, which spread out in only two dimensions, diminish 
more slowly with distance than the other waves which spread out 
through the three dimensions of the earth’s interior. The surface 
waves are dispersed as they travel (the speed of any one group of 
surface waves depends on the wavelength) and the degree of dis- 
persion for a given epicentral distance throws important light on 
the structure of the outermost 25 miles or so of the earth. The 
present article, however, will be mainly concerned with the other 
classes of waves, called bodily waves, for these are the ones which 
supply information on the earth’s deeper interior. 
The two classes of bodily seismic waves are called the primary or 
P waves, and the secondary or S waves, respectively. Both P and S 
waves travel faster than the surface waves, and they contribute to 
the earlier part of the record on the seismogram. The P waves are 
longitudinal like waves in sound, while the S waves cause the particles 
of the earth to move transversely to the direction of wave advance. 
For points not too close to the focus and epicenter, earthquake energy 
can for many purposes be regarded as traveling outward along seis- 
mic rays, analogous to rays of light. For most of their length both 
P and S rays are curved, with their concavity upward, although a 
limited number of surfaces of discontinuity inside the earth exist at 
which the rays are bent suddenly downward. Thus rays which start 
off downward at the focus, but not too steeply, emerge at relatively 
short distances from the epicenter, while the steeper rays emerge 
at greater distances, even as far away as the antipodal point, or 
anticenter. 
