This sudden slip set up an earthquake lasting sixty-five seconds, followed by minor shocks recurring for many days.  In places the jar shook down the waste on steep hillsides, snapped off or uprooted trees, and rocked houses from their foundations or threw down their walls or chimneys.  The water mains of San Francisco were broken, and the city was thus left defenseless against a conflagration which destroyed $500,000,000 worth of property.  The destructive effects varied with the nature of the ground.  Buildings on firm rock suffered least, while those on deep alluvium were severely shaken by the undulations, like water waves, into which the loose material was thrown.  Well-braced steel structures, even of the largest size, were earthquake proof, and buildings of other materials, when honestly built and intelligently designed to withstand earthquake shocks, usually suffered little injury.  The length of the intervals between severe earthquakes in western California shows that a great dislocation so relieves the stresses of the adjacent earth blocks that scores of years may elapse before the stresses again accumulate and cause another dislocation.

Perhaps the most violent earthquake which ever visited the United States attended the depression, in 1812, of a region seventy-five miles long and thirty miles wide, near New Madrid, Mo.  Much of the area was converted into swamps and some into shallow lakes, while a region twenty miles in diameter was bulged up athwart the channel of the Mississippi.  Slight quakes are still felt in this region from time to time, showing that the strains to which the dislocation was due have not yet been fully relieved.

Earthquakes originating beneath the sea.  Many earthquakes originate beneath the sea, and in a number of examples they seem to have been accompanied, as soundings indicate, by local subsidences of the ocean bottom.  There have been instances where the displacement has been sufficient to set the entire Pacific Ocean pulsating for many hours.  In mid ocean the wave thus produced has a height of only a few feet, while it may be two hundred miles in width.  On shores near the point of origin destructive waves two or three score feet in height roll in, and on coasts thousands of miles distant the expiring undulations may be still able to record themselves on tidal gauges.

Distribution of earthquakes.  Every half hour some considerable area of the earth’s surface is sensibly shaken by an earthquake, but earthquakes are by no means uniformly distributed over the globe.  As we might infer from what we know as to their causes, earthquakes are most frequent in regions now undergoing deformation.  Such are young rising mountain ranges, fault lines where readjustments recur from time to time, and the slopes of suboceanic depressions whose steepness suggests that subsidence may there be in progress.