Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/659

EARTHQUAKES 607 , to rest by a series of more or less rhythmical surgings of their origin at the rate of almost 2 3 km. per second. continuing over 1 or 2 hours. Although the maximum This velocity increases as the wave path plunges downdisplacement has a definite direction, the successive vibra- wards, attaining in the central regions a velocity of 16 to tions are frequently performed in many different azimuths. 17 kms., whilst the highest average velocity which is The predominating direction at a given station in certain across a diameter lies between 10 and 12 kms. per second. instances is apparently at right angles to the strike of The large surface waves radiating from an origin to a the neighbouring strata, this being the direction of easiest distant place have velocities lying between 1 '6 and 4 kms. yielding. per second, and it has been observed that when the higher Earthquake motion as recorded at stations several thou- velocity has been noted this refers to an observation at a sands of miles distant from its origin exhibits characteristics station very remote from the origin. One explanation of strikingly different from those just described. The pre- this is the assumption that only very large waves indicatcursors now show periods of from 1 to 5 seconds, whilst ing a large initial disturbance are capable of travelling to the largest movements corresponding to the shocks may great distances, and, as pointed out by Mr R. D. Oldham, have periods of from 20 to 40 seconds. The interval of large waves under the influence of gravity will travel faster Velocity. time by which- the first tremors have outraced than small waves. These waves (which may be gravitathe maximum movement has also become tional or distortional) are recorded as slow tiltings of the greater. Within a few hundreds of miles from an origin ground measured by angles of 0'5 to 10 or 15 seconds of arc, this interval increases steadily, the velocity of propagation or as horizontal displacements of 0'5 or several millimetres. of the first movements being about 2 km. per second, Their calculated lengths have reached 50 kms. (31 miles). whilst that of the latter may be taken at about 1 -6 km. per In the section of this article relating to the cause of second. Beyond this distance the velocity of transmission earthquakes a little has been said about their frequency of the first movements rapidly increases, and for great or the number of times these phenomena are redistances, as for example from Japan to England, it is peated during a given interval of time. It has Frequeac^ higher than we should expect for waves of compression pass- been shown that all countries are very often moved by ing through steel or glass. This observation precludes the earthquakes which have originated at great distances. idea that these preliminary tremors have travelled through Great Britain, for example, is crossed about 100 times the heterogeneous crust of the earth, and since the average a year by earthquake waves having durations of from velocity of their transmission increases with the length of 3 minutes to 3 hours, whilst the vibratory motions which the path along which they have travelled, and we but originate in that country are not only small but of rare rarely obtain certain evidence that a seismograph has been occurrence. In the earlier stages of the world’s history, disturbed by waves which have reached it by travelling in because the contraction of its nucleus was more rapid than opposite directions round the world, we are led to the it is at present, it is commonly inferred that phenomena conclusion that earthquake precursors pass through our accompanying bradyseismical activity must have been earth and not round its surface. The following table more pronounced and have shown themselves upon a relating to earthquakes which originated off the coast of grander scale than they do at the present time. Now, Borneo on 20th and 27th September 1897, is illustrative although the records of our rocks only carry us back over of the velocities here considered :— a certain portion of this history, they certainly represent an interval of time sufficiently long to furnish some evidence of such enfeeblement if it ever existed. So far from Distance Velocity Average from in kms. depth of Localities. origin per sec. if „ . , chord in this being the case, however, we meet with distinct eviin degrees. on chord. dences in the later chapters of geological history of plutonic kms. awakenings much more violent than those recorded at its commencement. During Palaeozoic times many mountain Nicolaieff 81° 8-1 8-0 Potsdam 92° 8-4 ranges were formed, and accompanying these orogenic pro9T Catania, Ischia, Rocca di cesses there was marked volcanic activity. In the sucPapa, Rome 96° 9-0 9-5 ceeding Secondary period plutonic forces were quiescent, Isle of Wight 103° 9-8 10-2 but during the formation of the early Tertiaries, when some of the largest mountain ranges were created, they The chords referred to here are those joining the earth- awoke with a vigour greater than had ever been previously quake origins and distant observing stations, and it will exhibited. At this period it is not improbable that Scotbe noted that one-quarter of the square root of the average land was as remarkable for its volcanoes and its earthdepths at which these run closely corresponds to observed quakes as Japan is at the present day. If the statement average velocities if wave paths followed chords. This relating to the general decrease in bradyseismical changes increase of velocity with average depth shows that the referred merely to their frequency, and omitted reference paths followed through the earth must be curved with to. their magnitude, the views of the geologist and physicist their convexity towards the centre of the earth. These might harmonize. One explanation for this divergence of observations do not directly tell us to what extent a true opinion may rest on the fact that too little attention has wave path is deflected from the direction of a chord, but been directed to all the conditions which accompany the they suggest as an extremely plausible assumption that adaptation of the earth’s crust to its shrinking nucleus. the square of the speed is a linear function of the depth As the latter grows smaller the puckerings and foldings of below the surface of the earth. With this assumption Dr the former should grow larger. Each succeeding geological C. G. Knott shows that the square of the speed (v2) can epoch should be characterized by mountain formations be expressed linearly in terms of the average depth of more stupendous than those which preceded them, whilst the chord d, thus : -v2 = 2-9 + '026 d, the units being miles the. fracturing, dislocation, caving-in of ill - supported and seconds. The formula applies with fair accuracy to regions, and creation of lines of freedom for the exhibimoderate and high values of d, but it gives too high a tion of volcanic activity which would accompany these value for short chords. It follows that the square of the changes, would grow in magnitude. The written records speed increases 0‘9 per cent, per mile of descent in the of many countries reflect but on a smaller scale the crystalearth. The conclusion we arrive at is that the preliminary lized records in their hills. In 1844, at Comrie, in Perthtremors which pass through the earth do so in the vicinity shire, as many as twelve earthquakes were recorded in a