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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



radius. If the circles for all the stations be drawn, their point of 

 intersection makes the x which marks the spot (see fig. 3). 



A well-defined earthquake can be located no matter where it may 

 have occun-ed. Thus, routine statistical seismology is piling up 

 accurate records of where earthquakes occur and when. Further- 

 more, we are now able to deduce the depth at which the shocks 

 originate. 



The point within the earth from which the earthquake energy is 

 liberated is called the focus; the point vertically above it on the 



FiGDKE 3. — Intersection of distance circles indicating the epicenter of the Timiskamini 

 earthquake of November 1, 1935. (Plotted on stereographic projection.) 



earth's surface is called the epicenter. Now if the focus be at or 

 near the surface, we have what may be called, for want of a better 

 name, a normal earthquake. To simplify our discussion we shall 

 from this point confine our attention to the P wave — the first impulse 

 on the record. Obviously, the time-distance graph of a normal earth- 

 quake begins at the zero-zero point — at the origin of coordinates. 

 Suppose another earthquake should happen with the focus at a depth 

 of, say, 100 miles. All distances for our time-distance graph are 

 measured from the epicenter. The epicenter is thus at zero distance. 

 The time required for the impulse P to reach the epicenter at distance 

 zero is in this case more than nothing and the time-distance graph 

 must begin a certain distance up on the time axis, that is, it must 



