-"•^-^ •^ -'-',1m 



12 



(if a ]iaitii'lc (nica.suroil from tlie position occupied hcfoi-c (lie distiirliancc lK'ü,an) 

 liotli ill iiiai;iiitii(lc and dircitidii, in (i>iijiiiiiti<iu witli tlic tiinc. To iliis end it 

 is usual, and for meist ]iiiriiiisi's desiralde, to treat each displacement as the re- 

 sultant (tf three rectanjjjiilar components, one vertical and two iiori/.oiital. If the 

 I'clation of the displacements alonjr three axes to the time lie reci)rde<l, we possess 

 coinjilcte information as to the movement of the <;roiin>l at the ohserviiii; station, 

 i'y comhinini;' the rccoids for three axes we inav of course dediK'c (lie ahsojiite 

 direction of motion of a ])article, its velocity, and its rate of acceleration at aii\- 

 instant <lnriiiii' the earthipiaki' ; while hy comparison of the records taken at om- 

 station with <'orrespoinlinii- reeor<ls lal<<n at oilier stations it i^ (t heorelicallv, and 

 in a few eases practie;dh') pussilile to ilelcriniiie the \elocitv and <lii(ction of 

 ]>rop;iL;;i(ioM of the (■.■irthi|Makc waves. 



!} 12. Mitli'lii (it lilt siirfiirc iif lllf siii/ iliii- lo (I iiiiniKll ii'iirr. 

 At any point well within a iniirovni silii'atliii;' mass the dire.-lion of motion 

 of a jiarticje diiriiii;- llic passage ol a normal wave coineidi's with the line ol 

 transit of the wave (^ 1). I!iit mIicii a \\a\<' of c'ompressieni and <lilatation aji- 

 proMciies a lioinidaiN' of the inedinni in whieii it travels, in anv other <lircet Ion 

 than perpcndieiilar to the lioiindary, thi' movenieni of particles there will no 

 loiin'cr coincide with the line of transit of (he wave. If ilic iieiw-hl)i)uiin^ siili- 

 stani'c, ii-oni whieh the dm iliinn in ipiestion is separated l.y (he iionndary, «lilfcrs 

 Iroin it in density or ehisticity, the portion^ of the mii^inal mcilinni w liieli lie near 

 the Iionndary will he eidier more or less free to expand :ind contract la(i'ially 

 ilniinn' the |iass;ie'e of the wave (h:in oilier poilions. To lake the important |iiac- 

 tieal ease, a p.ortioii of (he soil at or close to ihe snrfaee of the eai ill is more free to 

 expand l:itei;dl\' w lien it is compressed hv an oliliniu'lv incident normal wa\ e. than 

 olliir |()i(ions Ivine' in tli(" same wave front, and heiii'c the stiaiii at (he siirfic" 

 cansed hy a |p1ani' wave (or spheiieally divereen( wave from a disiaiit source) 

 will nol he a simple |oiii;iliidinal si rain, Imi, since a si.ate o|' compression coexists 

 W'i(h a s(ate of displacement away from the orii^in, the displaeeinent of a surface 

 particle dnriui;- the ]>as.<aL;;e of (he phase of eom|)rcssion will he upwards as well 

 as oii(wards alone; (he normal (o (he wave. The direction of (he pai(iile".s motion 

 will (her<fiie lie more neaih \-calie.al than llic line of tr:in>i( of the wave — a fact 

 which ])revents lis from iiiferrine- the direction of transit iVom a kiiowledee of 

 the direction in which a ]iarticle at the surface is initially disjilaced. 





