82
The East Anglian Earthquake.
amplitude and period depending upon the intensity of the origi-
nating impulse. Nearly all the calculations formerly made
with respect to the maximum velocity of displacement as
deduced from the projection of bodies were based on the sup-
position that the movement at the surface was of the nature
of a sudden jerk, and are thus, as pointed out by Prof.
Ewing,16 devoid of value. The old view of earthquake
motion might hold good in the case of an instantaneous
impulse originating at a point in a homogeneous solid of uni-
form elasticity in all directions,16 but such conditions are
15 'Earthquake Movement,' Tokio, 1883, p. 75.
16 A disturbance originating in such a solid gives rise, as is well known
to students of dynamics, to two sets of waves, the first or normal vibra-
tions causing the particles to move to and fro in the direction of the line
of propagation, and the other or transverse vibrations causing the
particles to oscillate at right angles to the direction of propagation. The
first or normal wave in most cases travels faster than the second or wave
of distortion. The general formula for the velocity of transmission of a
wave is, according to Ewing,
where M is the modulus of elasticity for the particular strain of which
the wave consists, and p the density of the solid. The further the two
sets of waves travel from their origin, the more complete is their sepa-
ration, so that at stations near the focus the effects produced are
generally the confused results of the action of both kinds of waves. On
arriving obliquely at the surface, where the particles are also free to
move in an outward direction, the course of the normal wave becomes
changed, and the direction in which a surface particle moves is in no case
a rectilinear continuation of the line of direction of the original impulse.
In the case of earthquakes of sufficient intensity to produce structural
damage it is generally very difficult, and in the absence of instrumental
registers is in fact almost impossible, to distinguish between the effects
produced by the two kinds of waves, since one component of the
distortional wave may coincide in direction with the normal wave. In all
earthquakes, moreover, the effects are vastly complicated by the hetero-
geneous character of the strata through which the wave passes, and the
old method of determining the depth of origin by the so-called " angle
of emergence " has but little to commend it scientifically in the light of
recent investigations. Cf. W. Hopkins, " On the Geological Theories of
Elevation and Earthquakes," Brit. Assoc. Rep. Oxford, 1847, pp. 74—88,
and Prof. Ewing, loc cit., pp. 1—13. Also a paper by Messrs. Milne and
Gray, Phil. Mag., Nov. 1881, p. 369.