Sensible earthquakes are very numerous, not less than 30,000 is the estimated number per annum; of course, the great majority of these are very light. While any part of the earth's surface may be visited by earthquakes, there is a very great difference between different regions in regard to their seismicity, i.e. the frequency and violence of the shocks which affect them. The main seismic regions, when platted upon a map, are found to be arranged in two great-circle belts, one of which encloses the Pacific Ocean and the other girdles the whole earth. The latter includes the Mediterranean region, the Azores, the bed of the Atlantic westward from the Azores to the West Indies, those islands themselves, Central America, Hawaii, Japan, China, India, Afghanistan, Persia, and Asia Minor.

Fig. 5. - Earthquake regions of the Western Hemisphere, (de Montessus de Ballore).

It must not be supposed that these belts are uninterrupted zones of seismic activity; they are rather seismic tracts separated by other tracts of low seismicity. For example, the eastern Aleutian Islands and the Alaskan coast form a region of frequent and sometimes very violent quakes, while the coast between Alaska and California is not often shaken. California is an earthquake region, as is also southern Mexico, and Central America has a very high degree of seismicity, but there is a long interval before the earthquake region of Ecuador is reached. Though the belts are thus discontinuous, it is nevertheless significant that the separate seismic tracts are arranged in belts.

The regions most subject to earthquakes are those which have the steepest slopes and are associated with the great lines of corrugation of the earth's surface. A sea-bottom steeply descending from the shore is apt to be unstable, especially if high mountains arise near the coast, while a low-lying coastal plain and adjoining gently sloping sea-bottom are usually stable.

Beside the main seismic regions above enumerated, there are many others where the shocks, though not infrequent, are seldom violent. Examples of such regions are New England, Switzerland, Austria, and South Germany.

Although earthquakes are commonly perceptible upon the land, the most frequent seats of disturbance are in the bed of the sea. These submarine quakes occur at all depths of water, and their frequency and violence are independent of the distance from volcanoes. In the sea there are regions quite free from quakes and others of a high degree of seismicity, but quakes also occur in an isolated and scattered manner.

In the Atlantic there are two remarkable seismic belts, one, already mentioned as part of the great earthquake zone, extending westward from the mouth of the Tagus in Portugal, the other nearly equatorial and reaching from the north shore of the Gulf of Guinea toward Brazil. In this second belt the sea-floor has precipitous slopes.

Submarine cables are frequently interrupted at the same points. Thus, the cable from the Lipari Islands to Sicily has been broken five times at the same point; on October 4, 1884, three parallel cables, about 10 miles apart, were simultaneously broken at the base of the steep continental slope, 330 miles east of St. John, N.B. Many similar instances might be given.