SCIENTIFIC VIEW

Evening Post, Volume CXI, Issue 114, 16 May 1931, Page 22

SCIENTIFIC VIEW

The. recent serious earthquakes raise anew in non-scientific minds - the questions of what is an earthquake and what causes it? .',"'.'•: ... . ,'-.

' The story is : really.told by the name, "earthquake.''' Tito phenomenon is nothing more O r less ;thau. a quaking of tho. earth or a part' of. it.. .Theoretical?' ly, at least, vibration, will , suffice from the .^remojir. caused' by a.passing railway train, to , violent. j disturbances, that shake,down, cities^ ..open earth ere.vices, and send mountainous. "tidal" .waves to. sweep, over coastal lowlands.

The earth waves i/for! earthquakes start, in.some.region, down' in the. earth and spread out in every, direction. One .'of the. simplest, .types, of waves 'that-'.can be obseryed,. those from a-pebble dropped .into! ,a ; . pool,' forms growing circles on the surface, of .the water. But the -waves ..inline earth,:.being in a solid, attempt to form spheres that constantly grow outward. Soon, however, because of the differences: in texture of different, rock materials, the form is not a sphere at all, but a,very irregular curved solid 'instead,, more nearly, that of a potato. ': '-.;: i ". ,

The two principal, types :of /earth waves, those that. travel like, the pulsations in, a: bobbing, spring,, straight out; and those that travel like .ripples and the waves in a-flag, with a side-, ■wise motion, move, at different ;speedsl The pulsing .waves.are. the swifter, and,' of course, the place where they mako themselves felt first and most - strongly is the surface area : directly above their starting-point./ This is technically the "epicentre"'or "epieentruin." These pulsing waves around the epicentre alternately push up the surface of the earth and everything oh it, and let them fall back; ■- This: is done very rapidly, and though the actual rise tpd. fall may be only a fraction of an inch on rocky surfaces or a very few inches on soft soil/ the violence:may: be lilco that i from innumerable tremendous blows, and sufficient to shake down buildings. V ■ .-^ ■-..■;..--<.•• i^ Tho "sidewise" waves-follow along almost immediatelya.ftcr::the first pulsing waves,' and the •two, '-with. perhaps "twisting waves" as Hvelp'tlien operate together!'. : The ground^iriithe .epieentral area, therefore;-;'1 ' gets thrusts and: falls ; and;;at]theCsame. tme '■is, jerked 'sidewise. and'■back, -for ,tiny distances in every direction. At points some distance from the epi-

AN- EARTHQUAKE'S CAUSE

centre the pulsing waves strike the surface at a slant and so accentuate the effects of' tho. "sidewise" waves and create new surface undulations.. The .shaking* !at; ,the epicentre, -tod, throws -the. carth.'s.' surface there into undulatory waves like,, ripples' on a pond, and these also travel outward in widening, circles. 'Thes'ti surface waves are those of greatest up and,down motion but least speed from place; tovplace,'; They register the heaviest vibrations on. the seismographs of the world. .. ... It ds generally"' believed that earth- : quake waves, flow from" a."'point where one of the 'three forces illustrated is suddenly released as a break; a blow, or an explosion. Breaks are credited with the responsibility for most, earthquakes! It is believed -that because of slow contraction or shrinking which may result -ironi the cooling of/the, outer portion tof 'the ■ earth,ior ,of pressufß'-irom. deposited silt, Strains :are set up"'below:the Surface-like' those in the bending sti^k7K^.EventuaJUy the strain reaches the, breaking point,N.and there is 'a snapping "of the\'rocks whielr sends violent w.aves to the surface,;.causing .earthquakes. . , . '";■'/.,.:> The blow type of earthquake is-;prp-. bably less important. Such a blow might result from the. falling tin Jof ,;:g,;gi'eat cavern, but probably would cause only a local .shiver. A mo,re ; t in)pp.rl;aiit cause of a .blow,' might' be: a'slipping 'of one mass of rock 'over.; another. ; wiljli; as collision at the end of'tub elide.'.. Slueh a blow often follows ■■'■■a'.;:bfe'ak, the^Hwo acting/together. . . 5. ; .-,. Earthquakes in a.ctive volcanic regions may rotten- result, from explosions of gapes far'llpneath'the J3urface|.but;'it is possible.that;ev§nJin.the.nejgh^purhood of volcanoes the 'maj6rity,.jof' the quakes,arisei. from breaks'injthe rocks.' -V."■.:■.■• '■:.'.' '■,"'' '.-■'■' '.-.' ''■ >V.- ■ Tho second important question ■in regard 'to an earthquak©' is: ■ What ;sl;art,s thGiwaves?. ■ '/'" '■',/:, '■■■'' '": "'v..';'i No i one • has' ever', see'riv an '.earthquake wave start outward. :;from its ; , centre, and it is'safe enough' to say that no onoi ever: will. .",, But: scienelp 'has built certain ' hypotheses' which' 'are:'.. pretty generally accepted. ' ':','"". . 'Hold a dry stick as big as your thumb in your hands and bend ii until it breaks. At the snap, waves will travel to your hands and usually give you a painful sensation. •. v j. ~>: ~ ;