132 



SCIENCE 



[N. S. Vol. XXIX. No. 734 



responding character. If storms are 

 really earthquake-breeders — instead of the 

 traditional calm, sultry, so-called "earth- 

 quake weather" — then the shocks they 

 precipitate can be foretold only so long in 

 advance as the storms themselves are fore- 

 known. 



The most potent of all precipitants of 

 earthquakes is also useless to the fore- 

 caster because its action is unforeseen. It 

 is the earthquake wave emanating from a 

 nearby focus. The response to such an 

 impulse follows the initial shock so closely 

 that the two shocks are combined in a 

 single seismic event— an earthquake with 

 two foci, or a "double-earthquake." 



Prelude.— Hhe forecasting of earth- 

 quakes by means of prelude has nothing 

 in common with other methods, but re- 

 sembles rather the forecasting of the 

 weather for the day by a glance at the sky 

 in the morning. It depends on the recog- 

 nition of premonitory signs, and also, to 

 some extent, on the recognition of the 

 earliest phases of the event itself. 



When a fracture or other parting of the 

 rock takes place, the jar which is com- 

 municated to surrounding portions of the 

 crust is not a simple impulse, but a con- 

 geries of vibrations differing in amplitude 

 and period, and in speed of transmission. 

 At any point of the focus they begin 

 together, but traveling through the rock 

 at different rates, they arrive at any dis- 

 tant point at different times; and the 

 greater the distance the greater their sepa- 

 ration. The strongest of the vibrations, 

 or those said to constitute the principal 

 shock, are not the first to arrive, but are 

 preceded by vibrations which are much 

 weaker, and are known as the preliminary 

 tremors. At a point twenty miles from 

 the origin the preliminary tremors are 

 felt four or five seconds before the princi- 

 pal shock. There are also vibrations too 

 minute to be felt, and not yet recorded by 



the most delicate seismographs, but of 

 such frequency that they faU within the 

 register of the ear and are perceived as 

 sounds, and these usually begin to arrive 

 before the preliminary tremors. The 

 sounds and faint tremors are notes of 

 warning, and to him who not only hears 

 and feels but understands they give com- 

 mand of precious seconds. People who 

 live in earthquake countries and are 

 familiar with these warnings acquire the 

 habit of instantly taking precautionary 

 measures. 



Still earlier than the soimds and tremors 

 with which the earthquake begins, are 

 sometimes sounds, tremors or minor 

 shocks, and it is suspected that phenomena 

 of this sort may betray growing seismic 

 activity and thus constitute premonitory 

 symptoms of the final rupture. Little is 

 known of them in any exact way, because 

 they occur at a time when attention is not 

 directed to such matters; and nearly all 

 records are made from memory after the 

 occurrence of the earthquake. If they are 

 veritable preludes, connected in a system- 

 atic way with the mechanics of the earth- 

 quake, they are probably analogous to the 

 cracklings and crepitations observed in 

 strained beams and strained blocks of rock 

 before collapse occurs. With reference 

 to the possibilities of forecasting, expecta- 

 tion centers especially on faint tremors 

 such as are occasionally perceived a few 

 minutes or a few hours before an earth- 

 quake shock. They are more frequently 

 inferred from the peculiar behavior of 

 animals; and after making much allow- 

 ance for the influence of imagination on 

 the memory of observers, there is stiU 

 reason to think that various animals are 

 affected by vibrations to which man is in- 

 sensible, and that their reported uneasiness 

 before earthquake shocks is real and is oc- 

 casioned by premonitory vibrations. 



The scientific study of preludes belongs 



