EARTH-TREMORS. 



223 



EARTH-TREMORS. Recent investigations 

 have shown that the crust of the earth is in a 

 constant state of vibration. Besides the vio- 

 lent shiverings called earthquakes, there are 

 vibrations of the same nature, but of micro- 

 scopic extent, which take place constantly over 

 the whole surface. These microseismic tremors 

 have hitherto escaped observation, although 

 they are among the most universal of natural 

 phenomena, because they can only be perceived 

 by the aid of the most delicate instruments. 

 There is another kind of movement affecting 

 the earth's mass which is called oscillation. 

 An increase in the barometric pressure will 

 produce flexures in the surface of the earth 

 which may be measured in inches, but which, 

 like the earth-tremors, have been overlooked 

 by scientific observers for the reason that the 

 period of vibration is so long. 



Earth-tremors are generally due to two 

 causes : to the removal of barometic pressure, 

 or to the passing of the tides. The release of the 

 earth's mass from the superincumbent weight 

 of air or water produces a quivering of the 

 earth. The hugeness of the earth accounts for 

 the fact that variations in the weight of air or 

 water resting upon it are followed by elastic 

 oscillations. Sir William Thomson explains the 

 principle by the illustration of a mass of jelly, 

 which the greater its size, the more it would 

 be depressed by any weight resting upon it, 

 and the more it would vibrate upon the re- 

 moval of the load. 



The first announcement of microseismic move- 

 ments was made by Timoteo Bertelli, in 1870, 

 who declared that he perceived in Florence 

 the earthquakes which occurred in Romagna. 

 Observations of minute earth-movements were 

 instituted in Italy by Professor De Rossi in 

 1874. It was these observations which estab- 

 lished the fact that microseismic storms follow 

 the removal of barometric pressure. It was 

 also noticed that these disturbances occurred 

 sometimes simultaneously over the entire Ital- 

 ian Peninsula. The microseismic vibrations 

 have also been studied in Japan. Local tremors 

 of artificial origin, as when a railroad-train 

 passes, have often interfered with delicate sci- 

 entific observations in populous districts. 



A committee, consisting of George H. Dar- 

 win, with his brother, Horace Darwin, Sir Will- 

 iam Thomson, and others, was appointed by 

 the British Association to determine the extent 

 of the lunar disturbance of gravity. The work 

 was conducted by the Darwins and Sir William 

 Thomson at Cambridge. But it was found that 

 the lunar changes in the plumb-line were ren- 

 dered indistinguishable by the constant trem- 

 bling of the earth. This was the first time 

 that the vibrations from natural causes had 

 been noticed in England. A mountain-mass 

 depresses the yielding earth, and in plumb-line 

 observations the deflection caused by the attrac- 

 tion of the mountain, and the apparent deflec- 

 tion due to the slope produced by the weight 

 of the mountain, are superimposed, and the ap- 



parent displacement of the plumb-line is exag- 

 gerated. The heaped-up air in an anti-cyclone 

 would, in like manner, produce a depression 

 of the earth's surface, and the attraction of the 

 increased mass of air would cause a deviation 

 of a plummet or pendulum. The effect of the 

 ocean-tides is of the same order. The piling 

 up of water against a shore at flood- tide causes 

 the land to sink through a perceptible distance, 

 and the retreat of the tide causes the land to 

 rise, the slope of the flexures making the flood- 

 tide higher and the ebb-tide lower. The ele- 

 vation of the soil between the spring and neap 

 tides varies as much as five inches. The de- 

 flections of the plumb-line due to these several 

 causes are of the same order as that produced 

 by the moon, and are sufficient to eclipse the 

 lunar disturbance of gravity. As far as sixty 

 miles from the coast the deflection due to tidal 

 action is calculated to be as great as the maxi- 

 mum deflection of the plumb-line due to the 

 direct action of the moon. 



Earthquakes are the result of a complexity 

 of causes, and are among the rare and irregular 

 phenomena which can be studied with diffi- 

 culty. The earth tremors and oscillations, on 

 the other hand, are of continual recurrence, 

 and depend upon natural laws which are known 

 for the most part. It is a question whether 

 the frequent disturbances of the earth's mag- 

 netism are not caused by microseismic storms, 

 since it is known that earthquakes produce 

 electric disturbances of great magnitude. 



A series of observations upon earthquakes 

 was instituted in Switzerland in November, 

 1879. From the first report, extending to the 

 end of December, 1880, it appears that there 

 were twenty-six earthquakes felt in Switzer- 

 land during the thirteen months. Many of 

 these were of slight force, and consisted of a 

 single shock. It was often possible to distin- 

 guish between one principal shock and a num- 

 ber of secondary ones, as if the strata which 

 had been shaken violently had cracked and 

 broken. The surface over which the earth- 

 quakes were felt extended from less than three 

 and a third to over three hundred and fifty 

 miles in diameter. Half of them affected only 

 an area of less than twelve square miles, and 

 three were felt over one thousand square miles. 

 The great earthquakes at Chios, in 1881, last- 

 ed six days and consisted of thirty or forty 

 shocks, any one of which was sufficient to de- 

 stroy buildings. The soil seemed to dance 

 up and down with the powerful vibrations. 

 The earthquake in Casamicciola, by which one 

 hundred and fifty lives were destroyed, was 

 entirely local. This one was caused by the 

 washing of the hot springs in the vicinity. 

 The posts established by Professor De Rossi, 

 of the Papal Academy, at Nuovi Lincei, re- 

 corded two hundred and forty-four percepti- 

 ble earthquake-shocks in Italy in 1881. In 

 the Italian stations the form of the motion, its 

 intensity, direction, and speed, are studied. 

 The vibrations from powerful earthquakes ex- 



