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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 hillsides
 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 pressure
 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. Homosersts
 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