Miscellaneous Concluding Observations. 185
Without asserting that the present shock is actually
connected with the extension of any of these faults in the
Chalk, it may be pointed out, as a fact of possibly great
significance, that the line connecting the Deptford and
Tiptree Heath disturbances, if prolonged, runs parallel with,
or even joins on to, the main axis of the East Anglian earth-
quake, which, as already stated (p. 92), extends in a N.E. to
S.W. direction from Wivenhoe to Peldon.92
VII. Miscellaneous Concluding Observations.
The Angle of Emergence.
It has been made sufficiently clear in the preceding pages
of this report that none of the methods by which modern
seismologists are striving to arrive at a more exact knowledge
of the nature of earthquake-motion are applicable in the case
of the present disturbance. When going over the area of
structural damage it soon became evident that the angles
formed by the cracks in fractured buildings gave no reliable
data for determining the angle of emergence by Mallet's
method, even if we had entertained any ideas as to the trust-
worthiness of this method. It must be admitted, however,
that the method in question is by no means satisfactory,
being based entirely upon the supposition that the fracturing
is caused by the normal wave only, a belief which, to say the
least, requires verification. The damage caused to a building
by an earthquake cannot be ascribed to such a simple impulse
as is required by Mallet's treatment, but is rather the result
of the racking and straining to which the structure is sub-
jected on account of the want of synchronism in the vibrating
periods of its different parts. This principle of "relative
vibrational periods" is well illustrated by Milne in the
following extract:—
92 The conclusions arrived at respecting the propagation of the shock
along the older rocks would not be affected should it be hereafter
established that the disturbance actually originated in the Cretaceous
beds. It must be borne in mind that an earthquake-centre is a focus
from which vibrations are propagated outwards in all directions.