The Elements of Geology

by William Harmon Norton

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.

The India earthquake of 1897. No earthquake of history has produced greater geological effects than that which shook northeastern India in 1897. It laid in ruins a region thrice the size of the state of New York. In places not a masonry building was left standing and hard-wood trees were snapped across. Foothills of the Himalayas were stripped of soil and forests from base to summit by landslides. Streams which before were busily cutting down their rocky beds were now overloaded with waste from the slides. They were compelled to cease eroding their beds while they spread their valleys deep with sand, over which they now flow in broad and shallow channels. The incoherent deposits of the alluvial plains were riddled with rents through which ground water was forced out in such quantities as to flood considerable areas.

Certain other effects often attributed to the earthquake are rather the manifestations of the dislocation to which the earthquake was due. Permanent changes of level were effected. Some hills were found to have been lifted twenty feet, while others were lowered, and resurveys proved that the entire region had been compressed horizontally from north to south. Displacements occurred along several fault lines. One of these, with a throw of twenty-five feet at the surface and a length of twelve miles, crossed a river repeatedly, causing a series of waterfalls and lakes. All these disturbances are best explained by the theory that the shock was due to a slip along a deep and hidden thrust plane, accompanied by other movements of the strata along minor faults connected with it, some of which reached the surface.

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.

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.