Toshiaki Shirai

Connexion between the Warping of the Geoid with Seismic and Volcanic Activities

It has often been alleged that in the region where gravity shows large variation, seismic and volcanic activities are predominant. Japan offers an example where reliable gravity data are found, but as the extent of the survey is limited to the main islands, it is too narrow to arrive at a decisive conclusion. A world wide determination of gravity at regular intervals of the earths surface is desirable, but at the present moment it cannot be realized, though much hoped for. Unfortunately there is often lack of trustworthy determinations in lands where such activities are strongly felt. On this account, only a rough sketch of the problem can be treated from the data derived from several thousand measurements of the intensity of gravity scattered all over the world. Recently Ackerl1) calculated the distribution of gravity by expansion in spherical harmonics ; his method of reduction, following the method of Hopfner, is however open to criticism, so that his data were not used. Hirvonen2) used free air correction and calculated the warping of the geoid by extending Stokess theorem. The selection of data may still be criticised, so that his result is only approximate, as in future additional and more reliable observations will provide new materials for carrying out exact evaluations of the warping. Moreover the values of gravity near oceanic coasts are liable to be affected by local peculiarities, so that much weight is to be laid on points in the continents. The use of gravitational potential, instead of its first derivative, seems to be more rational in such discussions. By drawing curves of deviations N of geoid from the normal ellipsoid all over the world from Hirvonens data, for equi-parallels and longitudes, the general result is as follows:1) Great earthquakes occur in places where warping is negative and where the gradient is steep.