320 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
The total number of earthquakes is very great. On an average 
there are 10 earthquakes each year which are regarded as national 
disasters. Another hundred, classed as destructive earthquakes, do 
considerable damage, while a thousand do at least some damage. Ten 
thousand others are strong enough to cause alarm, and a hundred 
thousand are felt by human beings every year. In addition to that, 
a great many more earthquakes are instrumentally detected. 
The energy released in an extreme earthquake is more than one 
(American) billion times that in the smallest generally felt earth- 
quake. In the extreme earthquake the energy is about the same as 
that in a major hurricane; but whereas the latter energy takes some 
hours or more to spend itself and covers a great area, the energy 
in an earthquake issues in the space of a few seconds at most from a 
confined region below the earth’s surface whose linear dimensions 
do not ordinarily exceed a few miles. The energy in the extreme 
earthquake is nearly a hundred thousand times that in a normal atom 
bomb explosion, and perhaps ten to one hundred times that in a 
typical hydrogen bomb explosion. 
Seismologists, who study earthquakes by physical means, have long 
realized that any real understanding of the forces that bring about 
earthquakes must be preceded by a thorough detailed study of the 
structure of the earth below the surface. And it happens that earth- 
quakes themselves provide the principal means of unraveling this 
structure. The greater part of seismologists’ efforts is, in fact, de- 
voted to analyzing physical data on earthquakes with a view to 
charting out the earth’s interior to the highest precision possible. 
When an earthquake occurs, it sends waves down into the earth’s 
interior, and the shapes and speeds of the waves are influenced by the 
nether regions they traverse. The waves, on emerging again at the 
surface, are recorded by seismographs in the thousand or so seis- 
mological observatories that are spread over the globe in nearly all 
countries. Every year there are many earthquakes large enough to 
send sizable waves right through the earth’s interior, including the 
center. In deciphering the “seismograms,” i.e., the records of the 
waves taken at the surface, the seismologist is in effect X-raying the 
earth, for seismic waves, mathematically speaking, are very similar to 
light waves. 
The immediate cause of the larger earthquakes is known to be the 
release of elastic strain energy which has accumulated in sizable 
volumes of material in the earth below the surface, sometimes over 
a long period beforehand. There comes a stage when the material 
is strained to breaking point, and the place where fracture starts be- 
comes the focus of an earthquake. The point of the earth’s surface 
above the focus is called the epicenter; it is usually in the vicinity 
of the epicenter that the main damage is done. If the epicenter is 
