38 ; MODERN SETSMOLOGY 
“Theory of Elasticity ”) that the results are quoted here without 
proof. 
If the independent variables are x, y, z, the Cartesian co- 
ordinates of a point, and ¢ the time, and the dependent 
variables are uz, v, w, the components of displacement of a 
particle at x, y, z, then the equations are 
0? OO 
(p UF —- WV); Uv; WwW) = C222 wy x) a 
ou ov ow 
Py wy - nya 
= the density 
and ) and yp are Ue defining the elastic properties of the 
medium. 
If +0 we get 
where 6= 
(? oa O=(A+ 2m) VE 
while if 9=o0 we have 
2 
(p = uv?) (u, Vv, W)=O 
Ou ee Ow 
7 yt oe 
We thus find that the motion can be analysed into two types: 
(1) the longitudinal type @ + Oo in which the velocity of propaga- 
tion is V;=( + 2p)/p? and the displacement is in the direc- 
tion of propagation, and (2) the transversal type @=o0 in which 
the velocity of propagation is V,= ?/p? and the displacement 
is at right angles to the direction of propagation. 
The components of stress at any point are in the usual 
notation 
with 
CRANEIZ) Sn Agrton (=, =) Gm, 
ae 
amon (eis 
sateen CO) 
Although the effects of an earthquake observed at a distant 
station may persist even for several hours, we have cumulative 
