Savannah Morning News

EARTHQUAKES.

Something About the Stability of the Earth.

From Scribner's for March.

The notion that the ground is naturally steadfast is an error—an error which arises from the incapacity of our senses to appreciate any but the most palpable and, at the same time, most exceptional of its movements. The idea of terra firma belongs with the ancient belief that the earth was the centre of the universe. It is, indeed, by their mobility that the continents survive the unceasing assaults of down-wearing which the rivers and glaciers bring about, the ocean waves, and the continuous upward, from age to age, at a rate which compensates for their erosion. Were it not that the continents grow upward, from age to age, at a rate which compensates for their erosion, there would be no lands fit for a theatre of life: if they had grown too slowly, their natural enemies, the waves and rain, would have kept them to the ocean level; if too fast, they would lift new surfaces into the regions of eternal cold. As it is, the incessant growth has been so well measured to the needs that for a hundred million years, more or less, the lands have afforded the stage for prosperous life.

This upward growth, when measured in terms of human experience, is exceedingly slow; it probably does not exceed, on the average, one foot in three or four thousand years. The rate varies in times and places. Under varying conditions, as when a glacial sheet is imposed on the continent—as it was, in the immediate past, on the northern part of North America—a wide area of the ice-laden land sank beneath the sea, to recover its level when the depressing burden was removed. Still the tendency of the continents is to elevation, and even the temporary sinking of one portion of their area is probably, in all cases, compensated by uplifts on another part by which new realms are won from the sea.

OSCILLATIONS OF THE EARTH'S CRUST.

First among these oscillations of the earth we may notice the slow up or down movements which are probably of the same general nature and of the same origin as the movements which build the continents, only much more rapid; so rapid, indeed, that they may be observed from decade to decade, or, at least, from century to century. In this class we include the down-sinking of the coast of New Jersey, the uprising of the Northern part of Scandinavia, or the oscillation of the shore on the coast of the Bay of Naples. These movements which, though in a geological sense rapid, rarely change the level of the land more than a foot or two in a century, appear to be divided into three distinct classes as follows: First, those which are due to the imposition of a heavy weight upon the earth's surface or to the removal of such a weight. A good case of this is the deep depression of the Northern part of North America, where the glacial sheet came upon it, and its rapid re-elevation when the ice melted away. Next, those which are due to the formation of a great fault or break through the rocks as they are shoved about by the compressive forces which build mountain chains. And, finally those which are due to the movements of volcanic gases and the lava which they propel toward the crater, whence, in time, they are to be discharged.

Of these slow movements the most interesting, because the best known, is that which is shown by the ruins of the temple of Jupiter Serapis, near Naples. We see by the evidence of these ruins that the temple has sunk down since the Christian era, so that the marine animals bored into the marble columns at the height of more than twenty feet above the present level of the sea; it then rose up to its original level, and is now again sinking at the rate of one inch in three or four years. A similar movement connected with the process of mountain building has been observed at Subiaco, about forty miles to the north of Rome. A hundred years or so ago the church of Jenne was invisible from Subiaco, while now it is in plain view over the summit of the intervening mountain. This change can only be explained by an alteration in the height of the mountain arches of this district.

THE DISTRIBUTION OF EARTHQUAKES.

In the present condition of the earth's crust, so far as the brief historic record goes to show, earthquakes of an intensity menacing to man are limited to certain regions which probably do not, altogether, include more than one-fourth of the area of the lands, though shocks of a less degree of violence appear to be common to every part of the surface of the continents.

The regions of recurrent shocks of considerable violence are so irregularly distributed that they cannot be adequately noted in this brief essay. They include, in Europe, Iceland, Portugal, Spain, and Southern Italy; the region of the Lower Danube, and some of the islands of the Grecian Archipelago. In Asia, the larger part of Asia Minor, several limited areas in Hindustan, the greater part of the eastern littoral region of Asia, and the islands of the Japanese and Malayan Archipelagos are subjected to destructive shocks. In Africa there is, save in Egypt, little architecture to suffer from earthquake disturbance, and even little history to record it. Egypt seems to have been, on the whole, singularly exempt from great earthquakes, while the western portion of the Mediterranean face of the continent shares the disturbances from which the Spanish peninsula has repeatedly suffered. The vast Australian and Polynesian district of the Pacific affords a number of regions of great earthquake activity, of which New Zealand is the only one where we have anything like good observations for even a few score years. It may be said, however, that the greater part of this vast area seems to be more exempt from these indications of activity in the crust than any other equally extensive part of the earth's surface.

We now come to the twin continents, North and South America. The obvious resemblances in the physical configuration of these continents lead us to expect a likeness in their conditions of stability. This resemblance in a certain measure exists. The western shores of both of these continents, the seaward face of the great Cordilleran range of mountains, is the seat of the most frequent and, on the whole, the most energetic disturbances which occur within their limits, while the Eastern shore of each is comparatively little assailed by shocks. The northern, or Venezuelan, district of South America, which is apparently the seat of an active mountain growth, of which there is no parallel in the northern continent, is a district of recurrent shocks of great violence, such as have never been observed in high latitudes on its own continent.

On the other hand, the region from the mouth of the Amazon to the La Plata river, which corresponds to our seaboard Atlantic states, and the provinces of Canada, enjoy an immunity from disturbances probably not exceeded by any other equally extensive area occupied by the Aryan race, while the corresponding region in North America is much less fortunate.

INDICATIONS OF A QUIET EARTH.

In endeavoring to determine the degree to which the different parts of North America have been subjected to devastating earthquake shocks, or to those which would prove disastrous in a country occupied by the complicated society, we find ourselves met with the difficulty which arises from the brevity of our historic records, concerning the greater part of this continent. It is true that in the peninsula of Yucatan and to the southward, the longest record is only of about half that duration, and comprises nearly the whole of Mexico, and the peninsula is a district which has been under record for 500 years; for the continent our longest record is only of about half that duration, and these concern only a little strip of country along the Atlantic coast of the continent; for the remainder the information is for a brief term of a single century.