General Characters of the Disturbance. 23
Intensity of the Shock.—The data necessary for calculating
the intensity of an earthquake shock with any approach to
accuracy8 can only be obtained from instrumental records,
and, as the present disturbance was wholly unrecorded by
such means, the intensity as compared with other earth-
quakes can only be roughly estimated by comparing the
respective areas shaken.
Thus, taking the amount of material moved as pro-
portional to the moving force, this quantity of material will
vary as the square of the radius of the area shaken, so that
the intensity of the shock varies as the square of this
radius.0 This method of comparison is of course based on
the supposition that the centrum of the shocks being com-
pared is at the same depth beneath the surface, but as the
true depth of the origin is in most cases unknown, an error
is introduced which renders such calculations of but little
value except as crude approximations.
The great Lisbon earthquake of 1755 had a radius of 600
miles, so that, as compared with the present shock (assuming
the two disturbances to have originated at the same depth),
the intensity was :—
6002 : 1352 = 360,000 : 18,225 = 19.7.
The Lisbon earthquake was thus nearly 20 times the
intensity of the present shock.
Taking Mallet's classification according to extreme radius,
earthquakes are of the 1st, 2nd, or 3rd class, according as
the area shaken has a radius of 540, 180, or 60 miles. The
shock forming the subject of the present Report was thus
intermediate between the 2nd and 3rd classes.
8 The intensity of a shock is proportional to the maximum kinetic
energy of the objects on the earth's surface relatively to that surface, or
symbolically:—
where I is intensity, A amplitude, and T the period of vibration. See
a paper by Messrs. Milne and Gray, ' Phil. Mag.,' Nov. 1884, p. 371; also
Prof. Ewing's record on ' Earthquake Measurement,' Tokio, 1883, p. 9,
for a more elaborate treatment of this subject.
9 Milne, on the Japanese earthquake of Feb. 22nd, 1880. Trans.
Seism. Soc. Japan, vol. i., part ii., p. 53.