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ANNUAIv REPORT SMITHSONIAIT INSTITUTION, 19 31 



per second and their ordinary period 5 to 7 seconds. The second 

 preliminary, designated S for secundiis, is a transverse wave vrith 

 vibrations at right angles to direction of progress. The velocity 

 near the surface is about 4.5 kilometers per second and the period is 

 11 to 13 seconds. The path is approximately that of the P wave. 



Reflections of these waves may occur at the earth's surface at the 

 halfway point or even at two points, respectively, one-third and two- 

 thirds of the way with sufficient energy to be recorded. The first 

 case is designated as PRi, SRi, and the latter PRa and SRo- 



The longitudinal waves which pass along the surface are more 

 complex. Their speed is 3 to 4 kilometers per second, according to 



Figure 2. — Paths of earthquake waves and types of seismogram at different distances 



from an epicenter 



There may be one or more reflections of the different waves at the surface and these 

 reflected waves reach the seismograph station later than the unreflected waves. Such 

 phases may he recognized on some seismograms, their prominence depending upon 

 the distance to the epicenter, path over which the waves (ravel, component, etc. 

 Insert : In some cases the phases on different components of the seismograph are so 

 well defined that the direction of the epicenter may be estimated from the ratio of 

 the amplitudes recorded by two instruments operating at right angles to each other. 



conditions. The velocity for a path entirely beneath the Pacific 

 Ocean is about 20 per cent greater than under the continents. The 

 directions of vibrations are more varied than for the preliminary 

 weaves. The periods vary greatly and may be very large, as 40 

 seconds or even a minute. 



While much more might be said, these statements give the essential 

 facts. It is not, however, the whole story. When the distance 

 exceeds 10,000 kilometers, the waves pass through the central metallic 

 core of the earth, and this modifies the paths so that the system of 

 phases that has been described is replaced by another. However, 

 since my purpose is to discuss the central and surrounding region of 

 an earthquake, I will not go into this. 



