The mechanism of the earthquakes

A brief account on the development of the knowledge on the mechanism of the earthquakes since about the year 1930, is given in this report. There are two kinds of prevailing hypotheses as to the earthquake mechanism. In one of them, a couple of two equal and opposite forces with moment is supposed to act at the focus of the earthquake, while in the other one a set of two couples of forces with moment which are perpendicular to each other is assumed. We will call the former the force system of the type I, and the latter that of the type II. A set of two couples of forces of the nature of pressure and tension which are perpendicular to each other, and the radial force proportional to sin(2theta)cos(phi) in spherical coordinates, are equivalent to the force system of the type II respectively. The distribution of the direction and magnitude of the initial motion of the P, S and ScS waves of the near deep and intermediate earthquakes, observed in Japan can be explained by the theories based on the assumption that the force system of the type II acts at the origin in an infinite elastic solid. The directions of the pressure of the stresses causing the earthquakes seem to be directed perpendicularly to the trends of the deep and intermediate earthquake zones in and near Japan. For the investigation of the P, S and surface waves and the deformation of the earth's surface observed in the cases of very shallow earthquakes occurred in Japan, the theories for the force system of the nature of the type II or the radial force proportional to sin(2phi) in cylindrical coordinates, acting at the origin on the surface of a semi-infinite elastic solid, have been applied. The stereographic projection methods which are appropriate for the investigation of the earthquake mechanism based on the observations at the stations distributed over the world, have been proposed and developed by P. Byerly and J . H. Hodgson, and applied to the studies of numerous earthquakes by many seismologists, the force system of the type I being assumed as the basis of the investigation. The plane bisecting the couple of forces is considered to be the fault plane. The relations between the strikes and dips of the fault planes, the motion directions along the fault planes and the geological features, are investigated. According to the theories on the elastic waves, the patterns of the initial motions of the P waves for the force systems of the types I and II, are the same to each other. As the differences of the effects of the force systems are to be noticed in the patterns of the S waves, it is desired that the S waves observed at the distant stations may be investigated further in detail.