May io, 1906] 



NATURE 



43 



an Antarctic region which lies to the south-west of New 

 Zealand. 



Generally it would appear that regions of instability are 

 to be found along the margins of continents or tablelands 

 which rise suddenly to considerable heights above oceanic 

 or other plains. 



At the present lime we may, therefore, say that mega- 

 seismic disturbances do not occur anywhere, but only in 

 districts with similar contours, .^re we dealing with 

 primitive troughs and ridges which are simply altering 

 their dimensions under the continued influence of secular 

 contraction, or do these reliefs of seismic strain represent 

 isostatic adjustments which denudation and sedimentation 

 demand ? 



These and other activities may be looked to as primal 

 causes leading up to displays of pronounced seismic activity. 

 Their frequency, however, may be dominated by influences 

 which at certain seasons or times cause an increase or 

 decrease in seismic strain. 



In the wide variations in position and rapidity of flow 

 of ocean currents and in measured oscillations of sea-level 

 which appear to be seasonal in their recurrence, we see 

 influences which may give rise to seismic frequency in 

 districts that possess a high degree of seismic sensibility. 

 Other causes affecting large areas, and also possibly the 

 frequency of small or after-shocks in different seismic dis- 

 tricts, have by Knott and others been sought for in the 

 loads due to the accumulation of snow, and in the seasonal 

 fluctuations in the direction of barometric gradients. It 

 does not seem likely, however, that stresses due to such 

 influences have any marked effect upon the frequency of 

 those reliefs of seismic strain which shake the world. 



The data which we possess bearing upon this question 

 are as yet far too meagre to admit of satisfactory analysis. 

 It is, nevertheless, interesting to note the direction in which 

 they point. In the six years ending in 1904 we find that 

 off the west coast of North America fifty-one large earth- 

 quakes originated during the winter months (October to 

 May) and thirty-five during the summer months. Off the 

 east coast of Asia, north of the equator, the numbers for 

 these seasons were forty-nine and forty-three. These 

 numbers added together show that for the North Pacific, 

 as a whole, 100 disturbances took place in winter and 

 seventy-eight in summer, while in the Central Asian or 

 Himalayan region the corresponding numbers are twenty- 

 five and twentv-seven. Beneath an ocean, therefore, some 

 indication has been obtained of seasonal seismic frequency, 

 while on a continental surface no such frequency has yet 

 been indicated. 



If we take a chart showing the varying position of our 

 earth's North Pole in relation to its mean position, we see 

 that the secular movement of the pole is by no means 

 always uniform. Although it may at times follow a path 

 about Its mean position which is appro.ximatelv circular, 

 at other times there are comparatively sharp changes in 

 direction of motion which may even become retrograde. 

 If now on a chart of this description we mark the time- 

 positions of very large earthquakes, we find that they 

 cluster round the sharper bends of the pole path. 



In a period of nearly thirteen years (1802 to 1004) I find 

 records for at least 750 world-shaking earthquakes, which 

 may be referred to three periods continuous with each 

 other, and each two-tenths of a year or seventy-three days' 

 duration. The first period occurs when the pole movement 

 followed an approximatelv straight line or curve of large 

 radius, the second equal period when it was undergoing 

 deflection or following a path of short radius, and the third 

 when the movement was similar to that of the first period. 

 The numbers of earthquakes in each of these periods 

 taken in the order named were 211, 307, and 232, that is 

 to say, during the period when the change In direction of 

 motion has been comparatively rapid, the relief of seismic 

 strain has not only been marked, but it has been localised 

 along the junctions of land blocks and land plains where 

 we should expect to find that the stress due to change in 

 direction of motion was at a maximum. Until the magni- 

 tude of these induced stresses has been estimated. It would 

 be premature to assume that the frequency under consider- 

 ation is directlv due to change in direction of pole move- 

 ment, it being quite as likely that both phenomena may 

 result from a general cause. 



NO. 1906, VOL. 74] 



A world-shaking earthquake, wherever its motion is pro- 

 nounced, gives rise to movements which may extend over 

 three or four hours. They come to a close as a series of 

 pulsations, each lasting a few minutes, and separated from 

 each other oy approximately equal intervals of rest. The 

 expiring efforts of an earthquake present something more 

 akin to musical reverberation than to intermittent and 

 irregular settlement of disjointed material. 



If mstead of studying the life-history of an earthquake 

 as recorded at a given station, we compare the seismograms 

 it has yielded at different distances from its origin, we learn 

 something of the manner in which its energy has been 

 radiated and dissipated. An earthquake which in the 

 vicinity of its origin has a duration of sixty minutes may 

 appear at its antipodes ninety or 100 minutes later as a 

 feeble movement with a duration of only four or five 

 Tiiinutes. From the time this movement has taken to travel 

 the half circumference of the globe the inference may be 

 drawn that the surviving phase of such an earthquake is 

 that of the large waves. The compresslonal and distor- 

 tional precursors, together with the rhythmical succession 

 of followers, are no longer visible on seismograms. The 

 importance of this knowledge to those who are engaged in 

 the analysis of earthquake registers is apparent. 



The paucity of available data renders it premature to 



>30 



+0.30 +0.20 +010 000 -o.io -0.20 -0.30 



Fig. I shows, after Th. Albrecht, the path of the North Pole from 

 1892 to 1894 inclusive. Each year is divided into tenths or periods 

 of 36'5 days. Numerals indicate the number of large earthquakes 

 which occurred in each of these divisions, commencing with the 

 third tenth of 1892. 



make deductions respecting possible alternation in seismic 

 frequency in various localities. But if, instead of confining 

 our attention to a relationship between earthquakes, we 

 consider the question of the relief of volcanic strain, many 

 Illustrations may be adduced which indicate a close con- 

 nection between such activities. For example, all the 

 known volcanic eruptions which have occurred in the 

 .Vntilles, from the first which took place in 1692, have been 

 heralded or closely accompanied by large earthquakes in 

 that region, but more frequently by like disturbances in 

 neighbouring rock-folds, particularly that of the Cordilleras. 

 This was notably the case in 1902. On April 19 of that 

 year an unusually large earthquake devastated cities in 

 Guatemala. Small local shocks were felt in the West 

 Indies, and on .April 25 it was noticed that steam 

 was escaping from the crater of the Mont Pelee, in 

 Martinique. These activities continued to increase until 

 May 8, when they terminated with terrific explosions, sub- 

 marine disturbances, and the devastation of great portions 

 of the islands of Martinique and St. Vincent. 



The last illustration of hypogene relationship between 

 these regions occurred on January 31 of the present year. 

 On that date a heavy earthquake originated off the mouth 

 of the Esmeralda River, in Colombia. Sea-waves inundated 

 the coast, islands sank, and a volcano erupted. The news- 



