The present volume differs from a text-book of seismology in giving brief, though detailed, accounts of individual earthquakes rather than a discussion of the phenomena and distribution of earthquakes in general.
r vast ages of time. With every fault-slip, intense friction is suddenly brought into action by the rubbing of one mass of rock against the other; and, according to the modern view, it is this friction that gives rise to the earthquake waves.
In most earthquakes, the slip takes place at a considerable depth, perhaps not less than one or several miles, and the vertical slip is so small that it dies out before reaching the surface. But, in a few violent earthquakes, such as the Japanese and Indian earthquakes described in this volume, the slip is continued up to the surface and is left visible there as a small cliff or fault-scarp. In these cases, the sudden spring of the crust may increase and complicate the effects of the vibratory shock.
 If a is the amplitude of the vibration and T its period, the maximum velocity is 2*pi*a/T and the maximum acceleration 4*pi^2a/T^2
 See Chapter VIII., on the Hereford and Inverness earthquakes.