CHAPTERS VIII 
DETERMINATION OF EPICENTRE AND FOCUS. 
THE first question that arises when a seismogram indicates 
the occurrence of an earthquake is—where did the earthquake 
occur? 
We have hitherto regarded the earthquake as occurring at 
a point called the focus. Strictly the primary shock may 
have extended throughout a considerable region, so that in 
speaking of the focus we assume some average point from 
which the maximum effect appeared to proceed. Again we 
have seen that the focus may be at some depth and not at a 
point on the surface. For distances over 1000 km., however, 
it is quite accurate enough to regard the shock as occurring at 
a point on the surface known as the epicentre. Several 
definitions of epicentre, based on different physical ideas, may 
be given. It may, for example, be defined as the surface point 
first affected by the shock, or the surface point where maxi- 
mum effect is produced. For our immediate purpose it is 
sufficient to define the epicentre as the extremity of the Earth’s 
radius that passes through the focus. Until quite recently the 
method available for obtaining the epicentre was empirical, 
and based on the time curves for P and S as a function of the 
epicentral distance 4, obtained from observations of former 
earthquakes with well-defined epicentres. The most accurate 
of these are the curves obtained by Zoppritz. We shall return 
to the manner in which the primary time curves are to be 
obtained and meanwhile suppose that the table of values of 
S-P in seconds for each 10 km. as interpolated by Zeissig is 
available (published by the Imp. Acad. of Sciences, St. Peters- 
burg). 
If then P and Sare clearly defined on the record the interval 
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