Generation of the Rayleigy Waves from an Internal Source of Multiplet-Type and the Mechanism of Shallow Earthquake

[摘要] In the investigations of some shallow earthquakes made on the previous paper under the same title was remarked the occurrence of the remarkable azimuthal distribution with reference to the maximum amplitude of vertical component in the case of shallow earthquakes. In the present paper these observed facts are taken up to be discussed.Supposing the source to be represented as of a multiplet-type, the refferenced fact may not be explained by the nodal-cone-type, provided that the axis of doublet be horizontal.On the contrary, the azimuthal distribution of amplitude with reference to S waves which are originated from doublet source with its axis being horizontal shows its reconciliation with what already mentioned. This may be shown rigorously applying Prof. Sezawa's and Prof. Sakai's method.At first, the displacement at free surface z=0 due to the dilatational point source, is expressed by equations (1).Hereupon, putting cosω=iα/h into (1), we get equation (2).These just coincide with the formulae, taken n=0 in Prof. Sezawa's solution. Then, putting cosω=iβ/z into (3), the displacements due to distorsional point source are given by (4).So that, by similar way, the author has acquired the following term as the vertical displacement at free surface z=0 due to distorsional doublet, when the inclination of axis equals to zero.Comparing the ws, ω=π/2 with the vertical displacement due to a dilatational simple source, we getAccordingly it may easily be ascertained that ws, ω=π/2 are larger than wp when γ is small. Therefore, we can consider that the maximum amplitudes in vertical component for the azimuthal angles of ω=π/2 and 3π/2 are due to by distorsional source of multiplet-type.For the other azimuth, for example ω=0 and π, we get the relations as follows:- where one of double signs of inequality must be properly taken according to the values of and ξ (depth of focus).

[学科分类] 大气科学