QUAKE CAUSES

Dominion, Volume 24, Issue 112, 5 February 1931, Page 8

QUAKE CAUSES

Earth’s Crust Changing NO REGIONS FREE Continual Movement Writing in the “Scientific American,” Mr. G. Austin Schroper describes m popular language the causes and varieties of earthquakes. That the outer crust of our old earth is constantly undergoing alteration and modification is a fact patent to the most unskilled of observers, he writes. The mighty forces of nature seem to be forever in a state of .unrest, and they are constantly at work in their task of altering, shifting, and disintegrating the strongest of rocks and the mightiest of mountain ranges. When millions of tons or hundreds of thousands of cubic miles of rock are moved from one portion of the earth’s crust and deposited upon another portion, adjustment of the crust must ensue to support the gigantic loads thrust upon it. Every beam, bulwark, or foundation must have an ultimate breaking point when the loans upon it exceed its strength. And so we find that the crust of the earth must give way in places when the loads upon it exceeds its its strength. In short, a rupture or break is bound to occur, and the broken segments in turn will adjust themselves to the stresses which are acting iipon them. Such ruptures or fractures in the outer crust along which appreciable movement has occurred are “faults,” and it is to them that we must turn in looking for the most common cause of earthquakes. Movements along faults are constantly in progress. No one region is free from them, and if they happen to occur suddenly or with much friction, they are manifested to our senses, but if the slip be slight or gradual, only the most sensitive of seismographs can detect it. Eternal Unrest. No hour in the day, no day in the year, and no region on earth is free from earthquakes. They are as common as life itself, but by far the largest number are not. apparent to the senses. The idea that any given region is free from earthquake danger is fallacious. Some faults, quite naturally, are undergoing no movement at present, and are known as “dead-faults.” Those which are at present undergoing readjustment are “live-faults.” Likewise, gome faults are undergoing more rapid readjustment, or are subject to greater stresses, and hence are more pregnant sources of violent shocks. . „ The great San Andreas nft of California has been studied more in detail, probably, than any other earthquake fault in the world. It has been traced for over 600 miles and forms the boundary between two great crustal blocks, one moving south and the other nortn relative to each other. It was the San Andreas rift which was the source of the California ’quake of 1906. In connection with this temblor, it underwent displacement for over 290 miles. Fences and roads crossing it were observed to have broken and moved apart as much as 20 feet. The “Rumble.’ Awe-inspiring stories without number have been related by the survivors of great earthquakes, of the fearful sounds aecomptipying the shock, which were scarcely less terrifying than the quake itself. . Tales of unbelievable groaning and grinding noises, of whole forests obliterated, of new streams and waterfalls formed where none before existed, of gaping fissures and dried-up wells, are commonplace and, stranger still, in the aggregate they are true. . . w For purposes of. study, most ot the sounds accompanying an earthquake are" referred to- the Davison sound scale, which is given below: (1) Sounds resembling wagons, carriages, engines, or trains passing, generally very rapidly, on hard ground, over a bridge ,or through a tunnel: the dragging of heavy boxes over the floor. (2) Thunder: a loud clap or heavy peal, but most often distant thunder. (3) Sounds resembling . a moaning, roaring, round, strong wind, a rising wind, or a heavy wind pressure against the house, a chimney on fire, and so on. (4) Sounds of great loads of brick, coal, or rock being dumped. . (5) The fall of heavy bodies, banging of a door, pounding of the surf on the shore. * . (6) Distant blasting, explosions, firing of artillery. The Backwash of the Sea. So-called “tidal waves” are another phenomenon accompanying earthquakes, especially in those of submarine origin. These waves have no relation to the tides, but are caused by the elastic rebound of submarine rock masses. In strong submarine earthquakes, the effect on ships is much as though they have struck on a rock or grated over a reef. Seismologists call these vast sea-waves tsunamis, from the Japanese, or when they are observed in bays or inlets, as seiches, especially when they are stationary waves. They may be ns much as 109 or 200 miles from crest to crest. 40 feet high at the point of origin, and they may sweep across the seas with a speed of ‘from 300 to 500 miles per hour. Such tsunamis may be unperceived in the open ocean, but upon reaching the land they mny pile up as grent waves and sweep far inland with great loss of life. Such were the waves that played havoc witli Lisbon in 1755. Japan in 1854 and Peru in 1868.