Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/655

 EARTHQUAKES 603 the Cape of Good Hope, Cairo, Beyrout, San Fernando This peculiar association of earthquakes with pronounced in Spain. The principal objects of this extended and topographical configuration and certain geological constill extending system of stations are to determine the ditions evidently indicates that the origin of many of velocity with which motion is propagated over the surface them is connected with rock folding. Inasmuch as certain and through the interior of the earth, to locate the posi- large earthquakes have been accompanied by rock fracture, tions of sub-oceanic earthquake origins, and generally to as for example in 1891, when in Central Japan a fault extend our knowledge respecting the physical nature of some 50 miles in length was created, whilst the origins of the planet on which we live. others have been distinctly traced to the line of an existWe now know that earthquakes are many times more ing fault or its continuation, we may conclude that the frequent than -was previously supposed. In Japan, for majority of earthquakes are spasmodic accelerations in the example, between 1885 and 1892 no fewer than secular movements which are creating (and in some inoZTrtb^ 8331 were recorded—^at is to say, on the stances possibly obliterating) the more prominent features quakes. average there were during that time more than of the earth’s surface. These secular movements, which 1000 disturbances per year. Although many include upheavals, subsidences, horizontal displacements, of these did not cause a sensible shaking over areas all of which are explained on the assumption of a crust exceeding a few hundred square miles, many of them were seeking support on a nucleus gradually contracting by loss sufficiently intense to propagate vibrations round and of heat, are collectively referred to as bradyseismical through the globe. If we pick out the well-marked earth- (/SpaSvs = slo’w) movements. To these may be added quake districts of the world, and give to each of them a movements directly attributable to the influence of gravity. seismicity or earthquake frequency per unit area one-third Sub-oceanic districts in a state of seismic strain may be so of that in Japan, the conclusion arrived at is that con- far loaded by the accumulation of sediments that gentle siderable areas of our planet are on the average shaken bending may be accompanied by sudden yieldings. This every half-hour. possibly accounts for the frequency of earthquakes off the The knowledge which we now possess respecting the mouth of the Tonegawa on the eastern side of Japan. localities where earthquakes are frequent and the forms of The distortions so frequently observed in fossils and the foci from which they have spread, enables pebbles, the varying thickness of contorted strata, and the US to s ea and earth P ^ definitely respecting the originat- “ creep ” in coal mines, together with other phenomena, quakes. ” causes of many of these phenomena. It indicate that rocks may flow. Observations of this nature is found, for example, that although in many lead to the supposition that high plateau-like regions may countries there may be displays of volcanic and seismic be gradually subsiding under the influence of their own activity taking place almost side by side, it is only rarely weight, and that the process of settlement may from time that there is direct relationship between the two. Now to time be spasmodic in its character. Whether the and then, however, before a volcano breaks into eruption earthquakes which originate round the submerged basal there may be a few ineffectual efforts to form a vent, each frontiers of the continents bounding the Pacific are ever of which is accompanied by no more than a slight local attributable to such activities, it is impossible to say. All shaking of the ground. This is true even for the largest that we know with certainty is that they are sometimes and most violent eruptions, when mountains have with accompanied by such a vast displacement of material that practically a single effort blown off their heads and the ocean has been set into a state of oscillation for periods shoulders. Thus the earthquake which accompanied the of 24 hours, that in some instances there have been eruption of Bandaisan, in Central Japan, in 1888 was marked changes in depth, and that enormous sub-oceanic felt only over a radius of 25 miles. The analyses of landslips have occurred. These phenomena are, however, the seismic registers of Japan clearly indicate that com- equally well explained on the assumption of sudden faultparatively few shakings originate near to the volcanoes of ing accompanied by violent shaking, which would dislodge the country, the majority of them, like those of many steeply-inclined beds of material beneath the ocean as it other countries, coming from regions where volcanic rocks does upon the land. are absent. The greatest number spread inland from the Although the proximate cause of earthquake motion is Pacific sea-board, the movement becoming more and more traced to sudden yieldings in the crust of the earth feeble as it approaches the backbone of the country, which brought about by some form of bradyseismical Two ty es is drilled with numerous volcanic vents. What is true for action, the existence of at least two distinct 0f earthJapan is generally true for the western coasts of North types of seismic motion indicates that the quake and South America. mechanical conditions accompanying the frac- motionSpeaking broadly, earthquakes are most frequent along turing of rocks are not always identical. Ninety or ninetythe steeper flexures in the earth’s surface, and in those five per cent, of the earthquakes which can be recorded regions where there is geological evidence to show consist of elastic or quasi-elastic vibrations. The reearth1 0t are s^ow secular movements in the earth’s crust mainder, including the large earthquakes, not only exhibit quakes. P°ssibly yet in progress. With a unit distance the elastic movements, but are accompanied by surface of 2 degrees, or 120 geographical miles, we find undulations which are propagated most certainly for some that the slopes running eastwards from the highlands of hundreds of miles round their origin, and then as horiJapan and westwards from the Andean ridges down into zontal movements sweep over the whole surface of the the Pacific vary from 1 in 20 to 1 in 30, and it is on the globe. The former of these may accompany the formation faces or near to the bottom of these slopes that seismic of a new fault or the sudden renewal of movement along efforts are frequent. The slopes running from Australia, an old one; they are cracking or rending effects, withEastern America, and Western Europe into the neighbour- out any great displacement. The latter are probably ing oceans vary between 1 in 70 and 1 in 250, and in fracturings accompanied by vertical and horizontal disthese regions earthquakes are of rare occurrence. The placements of masses of the earth’s crust sufficiently great seismic activity met with in the Himalayas and the Alps to set up the observed surface undulations. These shocks finds its best explanation in the fact that these mountains are so frequently followed a few minutes later by disare geologically recent, and there are no reasons to doubt turbances, which from their similarity to the movements that the forces which brought their folds into existence which have preceded them may be called earthquake echoes, are yet in action. that we are led to the speculation that we are here dealing