There is often, however, a true succession of original shocks caused by a succession of slips or ruptures at the source. Sometimes these are exceedingly persistent, running through days, weeks, or even months. In such cases a slow faulting is probably in progress, and little slips and stops follow in close succession. In one instance as many as 600 shocks in ten days have been reported.[226]
Gaseous emanations.—Vapors and gas frequently issue from earthquake rents, and are popularly made to serve as causes, but they are usually merely the earth gases that are permitted to escape by the rending of the ground, or are forced out by readjustment of the shaken beds. Like other subterranean gases, they are often sulphurous, and they are sometimes hot, especially in volcanic regions. Where the shocks are connected with eruptions, the gases may be truly volcanic.
Distribution of earthquakes.—Over large portions of the globe, severe earthquakes are exceedingly rare, but in certain regions they are unfortunately frequent. For the most part, these are volcanic districts, but this is by no means a universal relation. Earthquakes and volcanoes are only in part associates. In general, it may be said that earthquakes are frequent where geologic changes are in rapid progress, as along belts of young mountains, where the stresses are not yet adjusted, or at the mouths of great streams, where deltas are accumulating, or about volcanoes, where temperatures and strains are changing, or on the great slopes, particularly the submarine slopes, where readjustments in response to inequalities of surface stress are in progress. Not a few, however, occur where the special occasion is not at all obvious.
The Geologic Effects of Earthquakes.
Earthquakes are of much less importance, geologically, than many gentler movements and activities. Disastrous as they sometimes are to human affairs, they leave few distinct and readily identifiable marks which are more than temporary.
Fracturing of rock.—During the passage of notable earthquake waves, the solid rock is probably often fractured (see [p. 509]), though where it is covered by deep soil the fractures are rarely observable at the surface. Elsewhere the crevices are readily seen, especially if they gape. In a few instances surface-rock has been seen to be thoroughly shattered after the passage of an earthquake, as in the Concepcion earthquake of 1835.[227] Joints which were before closed are often opened during an earthquake. Thus in northern Arizona, not far from Canyon Diablo, there is a crevice traceable for a considerable distance, which is said to have been opened during an earthquake. Locally, it gaps several feet. Other notable earthquake fissures have been recorded in India,[228] Japan, and New Zealand. During an earthquake which shook the South Island of New Zealand in 1848, “a fissure was formed averaging 18 inches in width, and traceable for a distance of 60 miles, parallel to the axis of the adjacent mountain chain.”[229] The development of fractures or the opening of joints is sometimes accompanied by faulting. This was the case in Japan during the earthquake of October 28, 1891, when the surface on one side of a fissure, which could be traced for 40 miles, sank 2 to 20 feet. In this case there was also notable horizontal displacement, the east wall of the fissure being thrust locally as much as 13 feet to the north.[230]
Changes of surface.—Circular surface openings or basins are sometimes developed during earthquakes. This was the case during the Charleston earthquake of 1886,[231] and similar effects have been noted elsewhere. These openings often serve as avenues of escape for ground-water, gases, and vapor. They are commonly supposed to be the result of the collapse of caverns, or other subterranean openings, the collapse often causing the forcible ejection of water. Such openings are likely to be formed only where the surface material is incoherent. Sandstone dikes ([p. 514]) may perhaps be associated in origin with earthquakes.
Earthquakes are likely to dislodge masses of rock in unstable positions, as on slopes or cliffs. They may also occasion slumps and landslides.[232]
Effects on drainage.—The fracturing of the rock may interfere with the movement of ground-water. After new cracks are developed, or old ones opened or closed, the movement of ground-water adapts itself to the new conditions. It follows that springs sometimes cease to flow after an earthquake, while new ones break out where there had been none before. The character of the water of springs is sometimes changed, presumably because it comes from different sources after the earthquake. Joints may be so widened as to intercept rivulets, and the waters thus intercepted may cause the further enlargement of the opening. Illustrations of this sort are furnished by the earthquakes of the Mississippi valley (Lat. 36° to 38°) in 1811–12. Where faults accompany earthquakes, they occasion ponds or falls where they cross streams. Illustrations of both were furnished by the Chedrang River of India after the earthquake of 1897.[233]
Effects on standing water.—Some of the most destructive effects of earthquakes are felt along the borders of the sea. Thus the great sea-wave of the Lisbon earthquake (1755) and that of the earthquake which affected the coasts of Ecuador and Peru in 1868 are examples. Such waves have been known to advance on the land as walls of water 60 feet in height. They are most destructive along low coasts, for here the water may sweep much more extensively over the land. The great loss of life during an earthquake has usually been the direct result of the great waves. Lakes are also affected by earthquakes, their waters sometimes rising and falling for several hours after the initial disturbance, but lake-waves are much feebler than those of the sea, and are not often destructive.