42 The East Anglian Earthquake.
tends to confirm the view that the actual amplitude of
vibration at the surface was small in the case of the present
earthquake.
IV. Nature and Amount of the Structural Damage.
The damage to buildings caused by an earthquake does not
depend merely upon the amplitude of vibration at the
surface, but upon the rate of change in the motion of a
surface particle or the acceleration. Thus slow earthquakes
may have a large amplitude34 but may cause no damage, while
short and quick vibrations may dislodge chimneys, fracture
walls, or throw down buildings. It has been estimated,
judging from the effects of earthquakes of which the accele-
ration has been measured, that a rate of change of motion of
about two feet per second would have produced the results
witnessed after the present disturbance. Unfortunately no
data were obtainable in the case of the recent disturbance,
which could be considered sufficiently trustworthy to enable
the maximum velocity of displacement to be calculated, and it
would not have been deemed advisable to have burdened this
report with calculations open to such objections as apply to
the results obtained from the projection or overthrow of
bodies, even if the necessary observations had been made at
the time.
The actual destruction witnessed throughout the area
of damage was, except along the main axis of disturbance,
clearly the secondary effect of the shock, and was chiefly the
result of the overthrow of heavy chimney-stacks, which in
falling crashed through roofs or caused damage in other
ways. Along the main axis the direct effects of the disturb-
ance were traceable in fractured walls and rents in the solid
brickwork of buildings, the latter often of most substantial
construction. It was, in most cases, impossible to disen-
24 An earthquake in Japan, on Oct. 15th, 1834, gave record of a
maximum displacement of the ground (E. and W.) to the extent of 3.7
centimetres (roughly about 11/2 in.). The vibration was too slow to cause
damage, but Prof. Ewing regards this amplitude as excessive.—'Nature,'
April 23rd, 1885, p. 581.