From observations in Japan, it is clearly shown that massive mountain ranges exert a considerable influence upon the extension of seismal disturbances. On one side of a large range of mountains large cities might be laid in ruins, whilst on the other side the disturbance creating this destruction might not be noticed.
Velocity and Acceleration of an Earth Particle.—We now pass on to methods of determining the intensity of an earthquake which are less arbitrary than those which have just been discussed. These methods have already been discussed when speaking of artificial disturbances, where it was shown that the intensity of an earthquake as measured by its destructive effects greatly depended upon the suddenness with which the backward and forward motions of the ground were commenced or ended.
Amongst the earlier investigators of seismic phenomena who observed that there existed a connection between the distance to which bodies had been projected during an earthquake and the suddenness or initial velocity with which the ground had been moved beneath them, was Professor Wenthrop of Cambridge, Massachusetts, who noted that bricks from his chimneys had, by the New England earthquake of 1755, been thrown thirty feet. From this and the known height of the chimney, he calculates that the bricks had been projected with an initial velocity of twenty-one feet per second.[13]
The calculations made by Mallet respecting the maximum velocity of an earth particle at the time of the Neapolitan earthquake in 1857 depended upon the overthrow, projection, and fracture of bodies.
The principles which guided him in making the calculations will be understood from the following illustration.
Fig. 14.
If a column, a b c d, receive a shock or be suddenly moved in the direction of the arrow, the centre of gravity, g, of this column will revolve round the edge, and tend to describe the path g o. If it passes o, the column will fall. The work done in such a case as this is equal to lifting the column through the height o h.
If g a = a, the angle g a h = ϕ, and the weight of the body = w, then the above work equals
wa (1 − cos ϕ).