Earthquakes and Related Sources of 
Evidence on the Earth’s Internal Structure 
By K. E. BULLEN 
Professor of Applied Mathematics 
University of Sydney, Australia 
EARTHQUAKE ENERGY 
WHEN THE suBJECT of earthquakes is mentioned the nonseismologist 
commonly thinks of death and destruction: every year there are a 
number of earthquakes in various parts of the world which com- 
mand newspaper headlines because of the great damage done. Per- 
haps the greatest recent earthquake is the one that occurred in Assam 
on August 15, 1950, causing utter devastation over some thousands of 
square miles, and felt over an area in excess of a million square miles. 
During the past 200 years or so there has been a steady accumula- 
tion of systematic knowledge of earthquake occurrence and effects. 
For example, it is now well known that earthquakes originate largely 
in two main belts. Eighty percent of all earthquake energy comes 
from a belt that passes around the Pacific Ocean and affects countries 
with coastlines bordering on this ocean—for example, New Zealand, 
New Guinea, Japan, the Aleutian Islands, Alaska, California, 
Colombia, Ecuador, and Chile. A second belt passes through the 
Mediterranean region eastward across Asia, and joins the first belt 
in the Kast Indies. The energy released in the second belt amounts 
to 15 percent of the total, leaving only 5 percent for the whole of the 
rest of the world. 
It is also known that 85 percent of the energy comes from centers 
or “foci” within 50 miles or less of the surface, the remaining 15 
percent coming from foci down to a depth of 450 miles. During the 
present century, 1.e., over the whole period of accurate recording of 
earthquakes, there has, in fact, been no earthquake with a focal depth 
exceeding 450 miles. Moreover, all but one of the earthquakes which 
originated near this extreme depth have been confined to the circum- 
pacific belt. The exceptional earthquake originated at a depth of 
nearly 400 miles below Spain in 1954. 
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