Rensselaer Republican, Volume 17, Number 42, Rensselaer, Jasper County, 25 June 1885 — How Earthquakes Are Caused. [ARTICLE]

How Earthquakes Are Caused.

It has befn asked of late whether the hurricanes which folldwed the Spanish earthquakes were not produced by those subterranean disturbances, and all-explaining electricity has been called upon to explain how earth-throes might have caused atmospheric disturbances. I know of no way in which such consequences could have followed from a displacement of the earth’s, crust. To me it seems far more natural to conclude that the hurricanes and earthquakes were alike' produced (the hurricanes chiefly, the earthquakes partially) by the atmospheric compression which preceded the subterranean disturbances. This compression indicated a heaping of air over the disturbed region; the earth’s crust yielded undefr this increase of pressure, combined with the action of other forces, and earthquakes followed; the compressed air swept away to regions of less pressure, and the rarefaction following led in the usual way to the indraught which precedes a cyclonic disturbance in the air. Brit while the action of atmospheric pressure in helpin'g to excite subterranean activities must not be overlooked, the varying. pressure exerted by seas and oceans is a more potent disturbing factor. Atmospheric pressure is distributed in such a way that though the weight of air on anv given area is continually changing, there are no sharply defined lines, at any time, which separate regions of less pressure from regions of greater pressure. It is otherwise with the sea along a shore line. Here we have the sea acting with constantly varying intensity, as its level changes, on the seaward side of the shore line, while on the landward side there are no such variations of pressure. Let us consider what this means. Take a tolerably straight shore line 500 miles in length, and suppose shat along this shore line a region of ocean 100 miles broad rises through a height of three feet under the combined action of the sun and moon. -Raising a tidal wayo. and favflpifig'strong winds urging shoreward. Then we have 50,000 square miles of sea water three feet deep, added as so much deadweight to that part of the earth’s crust which underlies the sea along that shore. Each square mile contains in round numbeis 3,000,000 square yards, or 27,000,000 square feet. The additional weight corresponds, then (as the added layer is three feet deep), to 50,000 times 81,000,000 cubic feet of water, each weighing 64J pounds, or to 116,000,000,000 tons. It is clear that the addition of so enormous a weight as (this to the submerged part of the earth’s crust, outside of the shore line, biay well produce strains too great to he resisted. It must be remembered that the very existence of a precipitous line (as distinguished from one ■where the land above water and the palts submerged form one great slope) indicates the comparative weakness of thel crus t along that coast. It has yiel<Ud on one side to pressure thrusting i* upward above the sea-leVel, -and on tht, other side to the presure of the water forcing it down. It is true the actual ime of yielding may not coincide with the existent shore line. For the act; on 0 f the sea waves may (and generally must) have altered the position the coast Irom that which it occupied When first formed. But it may be taken f or granted that not far from every precipitous shore lino lies a line of weaki^ 33) where the crust has given way in tit p as t, and may give way again. JJh this consideration, undoubtedly, we find a part of the explanation of the observed fact that almost all the great regions of subterranean activity on the earth lie near the sea-shore. But whfie the changes of atmospheric and oceanic pressures are po“tent m the production of earthquakes, anq ar e probably in a great number of their direct occasion, it is, of course > to the subternanean regions themselves that we must look for the forces at work in upheaving the crust of the \p ar th. The forces acting from the outtide aro as the pull on the trigger; the Vnprisoned gases ana vapors generate by internal heat are as the powder whose explosion the missile is ejected. Yet even ireconsidering the earth’s subterranean ifctivities we still have to look outside fcM a part, at least, of the causes of distiM-bance. The air, perhaps, may in tK s respect be neglected, but the water It has been said, indecft and probably with a nearer approach to truth than usual in the case of gentalizations of the sort, “Without water! there can be no volcano,” and a similir rule (not quite so general) appliesl to earthquakes; few probably occur! possibly none, save through the actidp of water in some way or other. All active volcanoes except one (in midi Asia) are by the seashore. .Nearly afi the great earthquakes recorded hy history Jhavo taken place, and have apparently had their center of disturbance near the sea. There can be ve£ y %ttle doubt, indeed, that the di rec t cause of every great subterranean) disturbance is water in the form of sti eam —steam superheated, under gifeat pressure, and therefore possessing much greater expansive power than;gteam a t ordinary temperature.---lUc/^ Proctor, in Harper’s Magazine*