EARTHQUAKE BELTS.

Gisborne Times, Volume XXV, Issue 2080, 15 May 1907, Page 4

EARTHQUAKE BELTS.

EFFECT. OF ItOCK MOVEMENTS

During the past year throe populous cities —San Francisco, Valparaiso, ami Kingston—were partially destroyed by earthquakes. The appalling loss of life and property thus brought about within nine months created a widespread fooling of uneasiness. Many begun to wc lor when and whore tho next great orld-tro-ntor would occur. People •. iked nervously of tho earth’s passing through a period of unusual instability; -and sought oagorly for an explanation of what soomod to bo' tho unwonted seismic activity. Tho intorost arousod in the problems of soisinology has prompted Professor It. S. Tarr, of Cornell University, to give in tho North American lloviow a popular exposition of what science lias/to say about tho causes of earthquakes. To begin with, lie is at pains to show that seismologists have no reason to sharotho popular belief that tho earth’s crust is or lias been in a state of abnormal instability. According to records kept by Professor Milne, Chairman of the Soismologioal Committeo oi the British Association, tliero aro about GO world-sliaking earth tremors oacii year. Those tremors aro such as would destroy buildings near the centre of disturbance, and sond out waves which could bo recorded by seismographs all over the world. Tho exceptional feature of tho past year has been, not an increase in tho number of great earthquakes, but the occurrence of throe out of tho sixty near large centres of population. About 57 others, probably of equal intensity, have escaped public attention, oitlior because they did littl 3 damage or because their effects v/ero restricted to regions in which bus slight intorost is ta'kon. Each of tho tlireo cities—San Francisco, Valparaiso, and Kingston—lies in a district known to bo peculiarly liable to earthquakes, and in each ease tho tremor of last year' was only one of a series.

Olio of the most important discoveries that have boon made about earthquakes is that they nearly all take place on two narrow bands encircling tlio oartli, and crossing each other at two points. This discovery is duo to Count de Montossus do Ballore, of the French army, who has made a life study of earthquake distribution. Ho found up to 1903 records of 159,784 earthquakes, and carefully marked on a map their centres of disturbance. No fewer than 94 per cent, lay upon the two belts, while in all the vast area outside them, including most' of the wellsettled parts of North and South Amoriea, Europe, Asia, Africa, and Australia, only 6 per cent, of all recorded shocks have occurred. One belt covers the Mediterranean region, Asia Minor, the Caucasus, tho Himalayas, the East Indies, Central America, and tho West Indies. The second belt almost.' surrounds tho Pacific Ocean. Passing northward from the extremity of South America along the Andes, it crosses tho first belt iu Central America, and then extends up the western coast of North America. Crossing over to Asia by the Aleutian Islands, it goes down through Kamchatka, the Japanese Islands, and the Philippines. After again intersecting tho first belt in the East Indies, it runs on to New Zealand. As Professor Tarr says, tho fact that some parts of the earth are specially subject to shocks, while other and largor areas are practically free from them, is, of courso, definitely related to the causo of earthquakes. Tho question of causo Professor Tarr divides into two parts—first, the immediate explanation of individual shocks, and, secondly, the underlying reason for the conditions by which the immediate cause is ablo to operate. Upon the first much light has been thrown, but to tho second, dealing as it does with the hidden interior of tho earth, no answer has, ho says, as yet been found. Generally speaking, any jar in the earth’s crust will causo an earthquake. Sometimes volcanic action produces the jar, but it is a mistake to attribute all or oven a comparative’y large number of earthquakes to such a causo. “As a matter of fact,” says Professor Tarr, "while what wo may call volcanic earthquakes are common enough, and locally of great violence, they are not usually world-shaking. The reason for this is that the blow which causes the - shock strikes only a small area of tho crust, and while tho jar may bo powerful enough to shake severely the immediate vicinity, it is soon dissipated as its waves spread out from tin. small centre of disturbance. This being the case, when an earthquake of first violence occurs, and. when the seismographs of all the world record it, and especially when it occurs in a noil-volcanic region liko San Francisco and Kingston, the inference is at once drawn that is is probably due to some other cause than volcanic action.”

The other an dmore general cause is believed to be the movement of rocks deep down beneath the earth’s surface along the planes of faults. Such movement is a natural adjustment to accumulated strains,- and the strains, according to Professor Tarr, are intimately connected with mountain growth. The coast ranges about San Francisco arc a growing mountain chain. They are gradually rising. Ihe rocks there are in a state of strain. During the past history of the mountain growth the strata have been broken and forced to move along the planes of breakage. When groat masses of rock suddenly slide over ono another, even for a few feet, the grinding sots the oartli .trembling. At the time geologists were of opinion that the San Francisco earthquake was the effect of a movement along one of the fault planes, and, there" foie, ,the result of normal mountain growth.

Ihis opinion is now confirmed by the report of a' state commission, which has carefully studied the facts, and found that for at least 185 miles there has been a slip, on one side of which the mountains have been moved bodily in relation to the other side. 'ln this instance the movement,’ comments Professor Tarr “was mainly horizontal, instead of vertical, as is normally the ease. In places the shifting amounted to only tw-o or three feet; in others to as much as 20ft. By it roads wero dislocated; fences broken and moved apart, water-pipes separated, and long furrows opened in the ground. The movement of a great block of the earth’s crust, from 2ft to 20ft along a piano nearly 200 miles in length, and extending to an unknow'n depth, but probably thousands of feet, furnishes amplo. explanation of a shock whoso vibrations reached the seismographs in all parts of tlio earth and whose area of destruction extended 400 miles in one direction !o>d 50 miles in another,”