Abstract
Geodetic and seismological characteristics due to a fault movement have been studied on the basis of a simple model. It is intended to examine whether or not such a model is acceptable as a possible representation of earthquake origins. The model is proposed from the standpoint similar to that of REID's hypothesis and the sudden occurrence of a fracture fault in the upper part of the earth's crust is taken as the immediate cause of earthquake shocks as well as of crustal deformations.
First, we deal with statical deformation of the earth's crust caused by stress change (shear) on the fault plane, which is assumed to be developed from the surface vertically down to a certain depth. It has been found that by this model, the general decrease of horizontal displacement of triangulation points with distance from the fault is explained reasonably well. From the comparison of the theory with the observed geodetic data, probable conditions at the actual earthquake fault can be surmised. In the case of the Gomura fault (Tango earthquake), for instance, the depth (H) is estimated at 15km, while the stress change on the fault plane (Yx)0 and the total amount of strain energy (Efault), at 3×107c.g.s. and 4×1022ergs, respectively (length of the fault being assumed to be 30km).
Discussion is also made about the characteristics of seismic waves which are generated by the supposed fault movement. The analysis based on a two-dimensional model has shown that the push-pull distribution of the initial P-wave is of the quadrant type, one of the nodal lines coinciding with the fault. The maximum of the spectral intensity (P-wave) falls on the component, whose wave-length (referred to S-wave) is approximately 1.2 times as large as the fault's length.
These results also agree satisfactorily well with the observed seismological data. The deduced conditions at the fault are also acceptable judging from the mechanical strength of the crust which is already known. The model adopted here affording satisfactory explanations of various aspects of the Tango earthquake, we might say that the physical conditions at the seismic origin do not differ much from those of the model. Possibility of applying the similar model to other cases is also discussed.
