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CHAPTER XXI 
EARTHQUAKES AND PRINCIPLES OF ANTI- 
EARTHQUAKE CONSTRUCTION 
Tue NATURE oF EARTHQUAKE ActioN—The earth's crust 
is in constant tremor. The earth travels along its orbit 
3000 miles a minute, and places near the Equator revolve 
around its axis 1000 miles an hour. As the crust is irregular 
in structure it is always quivering like a badly mounted 
Aywheel. The surface is affected by continual variations 
in temperature, in the weight of the atmosphere, in the 
distribution of snow, rain, and tidal water, by blows on hill- 
sides from the wind, the hammering of surf on the coast, 
and the slip of material down hills and oceanic slopes. The 
crust is violently jerked when blocks sink owing to the loss 
of support, or slide over one another under the lateral pres- 
sure of the contracting crust. All these agencies produce 
waves of distortion, which range from tremors perceptible 
only by the most sensitive instruments, to shocks which 
devastate a province and slay a hundred thousand people. 
The Greeks knew that an earthquake spreads radially from 
its place of origin, which Aristotle called the centrum. The 
term is convenient though the centrum may be a large area 
and not a point. Above the centrum is the epicentrum or 
epifocal area, where the vibration emerges up the seismic 
vertical. Around the epifocal area the angle of emergence 
decreases outward, and is recorded by cracks in masonry 
or plaster, which tend to be normal to the earthquake path. 
Lines representing the angles of emergence converge to the 
centrum. 
The epicentrum may be determined from the times at 
which an earthquake was felt at different. localities. Homo- 
sersts are lines joining places shaken at the same time. 
Isoseists are lines through places where the shock was of 
equal severity. The meigoseismic area is that of the greatest