PHYSICS OF EARTHQUAKES

Pahiatua Herald, Volume XIV, Issue 3310, 1 May 1909, Page 2

PHYSICS OF EARTHQUAKES

“The Physics of Earthquake Phenomena,” is the title of an interesting hook by Dr. C. G. Knott, who as Professor of Physics at Tokyo spent eight years as a colleague of the great seismologist, John Milne. He reminds us that in 12 years of strenuous work the Seismologies! Society of Japan published 10 volumes of transactions. It is a good instance of Japanese intelligence that as long ago as 1887 Milne was able to discuss in detail the comparatively limited region of Tokyo itself liy means of a system of postcards distributed to about 100 residents, who sent in records whenever a shock was felt. Dr. Knott gives an account of instrumental seismology. Here again Japanese inventors have produced some of the best forms of apparatus. We owe great improvements to Tanakadaie and Omori. The latter has studied the periodicity of Japanese earthquakes very carefully. Professor Knott discusses possible causes of jxm’iodicity at some length, and enumerates tidal action of the sun and moon barometric pressure varying by reason of some outside cause, loading of continents with snow in winter, and so on ; but no very clear result seems to emerge. The chapter devoted to elasticity brings into prominence the work ot two more Japanese seismologists, Nagaoka and Kusakabe. By direct torsion experiments the former brought out many interesting facts, one of which is that the geologically oldest rocks are the most rigid hence an elastic disturbance will be propagated with greater velocity through the more deeply-seated rocks of the eartli s crust. The essential modernity of earthquake-study is shown by the fact that it was not until 1889 that a Japanese earthquake was indicated by an instrument in Europe (at Potsdam), and then only by the -accident that Paschwitz had set up a delicate form of horizontal pendulum, with which he hoped to detect the gravitation influence of the moon. Moreover, the first seismological observatory set up outside recognised earthquake regions was that installed by Milne at the Isle of Wight, when lie left Japan in 1893. There are now fifty or more such stations recording -earthquakes the man in the street never feels. These incidentally show slow daily oscillations of the ground, due to more or less obscure causes. Thus it is possible to show that a valley opens slightly under the influence of the sun’s heat and becomes more steep at night through greater loading of the sides with moisture! An earthquake, of course, starts various tyi>cs of wave in the earth’s substance. The evidence shows that- large waves are generated and sent round the immediate interior of the earth’s crust, like sound round a whispering gallery with a speed of a little less than two miles a second; besides this, tremors pass right 'through the earth (across a diameter), with a velocity of about six miles a s<>cond for the first set of tremors, and four miles a second for the phase that follows. It was to be hoped that a study of the propagation of earthwaves would give us some clue to the state of the interior of the earth ; but the facts are very complex, and it is j possible to reconcile them to many hypotheses Thus Oklliam thinks there | is a soft core as the nucleus of the | earth ; then a hard .shell, and then the crust. On the other hand, Fisher, in | the light of tlie same evidence, main- | tains his view that beneath a crust 18 or 20 miles thick there is lpolten magma with gas in solution, .and that the internal fluidity of the earth can . still he relied on, although air at a depth of 20 miles would have a den.s- ---| ity comparable to that of rock.