38 The East Anglian Earthquake.
In the present state of Seismological science, and in the
absence of instrumental records, it would be rash to attempt
any specific explanation of this double series of vibrations at
the distant stations. It may be of use, however, to indicate
briefly how such effects may in general be produced.
One cause for the separation of an earthquake disturbance
into two sets of vibrations has already been given, in the
different velocities with which the normal and transverse
waves are propagated (See note 16, p. 32). According to
experiments by Messrs. Milne and Gray19 with artificial
earthquakes the velocities (in hardened mud) of the normal
and transverse vibrations were respectively 438 feet and 357
feet per second, the ratio of propagation thus being 1.28. Of
course, in harder materials, the velocities would be much
greater. It is possible that the two distinct series of vibra-
tions felt at the distant stations may have been produced by
the successive arrival of the two sets of waves.
On considering the geological evidence, which will be
presented subsequently in detail, it will be seen that the
meso-seismic area consists of beds of clay overlying chalk, the
latter in its turn resting on harder rocks, which at the
greatest known depth appear to belong to the Palaeozoic series.
Practically this amounts to the same thing as though great
thicknesses of comparatively soft materials (clay and chalk)
were resting on a basin-shaped floor of hard rock, which
gradually came up to the surface as it spread towards the
most distant stations. It is evident on theoretical grounds,
and has been proved experimentally, that the velocity of pro-
pagation of an earthquake wave is much greater in hard
elastic materials like the Palaeozoic rocks than in soft forma-
tions like clay or chalk. According to Mallet the velocities
in feet per second are :—
Sand ......... 824.915
Granite (shattered) ... 1306.425
Granite (solid) ... 1664.574
According to the experiments of Milne and Gray the
19 Phil. Mag., Nov. 1881, p. 366.