Preliminary Note on the Propagation of the Earthquake Wave of near Origin in Japan

[摘要] For the purpose of determining the epicentre of the destructive earthquake which occurred on Jan. 15th, 1924 in Sagami district, the author of the present paper has drawn a series of isochronal lines using the results of seismic observations made at the meteorological stations in our country. The isochronal lines thus obtained is shown by full lines in Fig. 1. The general form of the lines are coaxial ellipses, their minor axes coinciding with the tangent at the epicentre to the arc of the Japanese Islands and their major axes being perpendicular to the tangent.This means that the velocity of condensational seismic wave differs with the direction of propagation relative to the general feature of our islands.Nextly, the author has drawn a series of lines which connect the stations having the equal duration of preliminary tremor. For simplicity, we call them, iso-PL-lines. The general feature of these curves show also a series of coaxial ellipses, their major axes coinciding with the tangent at the epicentre drawn to the arc of the Japanese Islands and their minor axes being perpendicular to it (dotted lines in Fig. 1).In other words, the elliptical isochronal lines are nearly perpendicular to the elliptical iso-PL-lines. This striking results are varified by a few more examples, two of which are shown in Figs. 2 and 3. To explain this phenomena, isochronal lines for L-phase were drawn in the same earthquake and is shown in Fig. 4. This curves are composed of a number of concentric circles with the epicentrum as its centre. From this fact, we may conclude that the L-phase wave travels with equal velocity in all directions from the origin of earthquake.According to this fact, which is also borne out by a few other semidestructive earthquakes, it becomes certain that the velocity (V1) of condensational seismic wave propagating in the direction perpendicular to the tangent at the epicentre drawn to the arc of the Japanese Islands is greater than that (V2) of the wave propagating along it.Nextly, the mean velocities of condensational wave V1 and V2 estimated from isochronal lines areAgain, the mean velocity of L-wave is estimated from the isochronal lines for L-phase (U1) and U2=4.1km/sec is obtained. Then, if we accept the fact that L-phase coincides with S-phase in a near earthquake, the value of U may be taken as the velocity of transverse seismic wave. So, we may calculate from V1, V2, and U, the elastic constants of the portion of the upper layer of the earth crust under consideration from tbe following formulae: where σ1, and σ2, represent Poisson's ratio of the upper layer of the earth crust in the direction perpendicular and parallel to the tangent drawn to the arc of Honsyu respectively. Values of σ1 and σ2 thus calculated are as follows:This irregularity of elastic property of the earth crust of our great island may perhaps be explained by the continental stress which acts radially on this islands, but confirmation of this conclusion is left to my further studies.

[学科分类] 大气科学