An earthquake is transmitted from the focus through the elastic rocks of the crust, as a wave, or series of waves, of compression and rarefaction, much as a sound wave is transmitted through the elastic medium of the air.  Each earth particle vibrates with exceeding swiftness, but over a very short path.  The swing of a particle in firm rock seldom exceeds one tenth of an inch in ordinary earthquakes, and when it reaches one half an inch and an inch, the movement becomes dangerous and destructive.

The velocity of earthquake waves, like that of all elastic waves, varies with the temperature and elasticity of the medium.  In the deep, hot, elastic rocks they speed faster than in the cold and broken rocks near the surface.  The deeper the point of origin and the more violent the initial shock, the faster and farther do the vibrations run.

Great earthquakes, caused by some sudden displacement or some violent rending of the rocks, shake the entire planet.  Their waves run through the body of the earth at the rate of about three hundred and fifty miles a minute, and more slowly round its circumference, registering their arrival at opposite sides of the globe on the exceedingly delicate instruments of modern earthquake observatories.

Geological effects.  Even great earthquakes seldom produce geological effects of much importance.  Landslides may be shaken down from the sides of mountains and hills, and cracks may be opened in the surface deposits of plains; but the transient shiver, which may overturn cities and destroy thousands of human lives, runs through the crust and leaves it much the same as before.

Earthquakes attending great displacements.  Great earthquakes frequently attend the displacement of large masses of the rocks of the crust.  In 1822 the coast of Chile was suddenly raised three or four feet, and the rise was five or six feet a mile inland.  In 1835 the same region was again upheaved from two to ten feet.  In each instance a destructive earthquake was felt for one thousand miles along the coast.

The great California earthquake of 1906.  A sudden dislocation occurred in 1906 along an ancient fault plane which extends for 300 miles through western California.  The vertical displacement did not exceed four feet, while the horizontal shifting reached a maximum of twenty feet.  Fences, rows of trees, and roads which crossed the fault were broken and offset.  The latitude and longitude of all points over thousands of square miles were changed.  On each side of the fault the earth blocks moved in opposite directions, the block on the east moving southward and that on the west moving northward and to twice the distance.  East and west of the fault the movements lessened with increasing distance from it.