Abstract
In the preceding paper, (Journ. of the Meteor. Soc. of Japan, Ser. 2, Vol. III, No.3), by drawing a serie of isochronal lines using the results of seismic observations made at the various meteorological stations of our country, the author has found that the velocity (V1) of condensational seismic wave propagating in the direction perpendicular to the arc. of the Japanese Island is greater than that (V2) of the wave propagating along it. These velocities of condensational wave V1 and V2 cstimated from isochronal lines are 9.6 and 6.5km. per sec. respectively. But the data used in the preceding paper are confined to the results of observations of earthquakes which have their epicentres along the Pacific coast.
In the present paper, he dealt with five semidestructive carthquakes which occurred on the Japan Sea, side. Of these five quakes, the destructive shock of North Tazima occurred on May 23, 1925 is most notable one. Using these data and with the method deseribed in the preceding paper he has drawn the following conclusions:-
(1) In carthquakes having their epicentres in the Pacific Ocean, velocity of seismic condensational wave propagating along the arc of our island is smaller than propagating perpendicular to it. But in all the earthquakes having their epicentre in the Japan Sea, the difference of the velocity in these two directions is smaller than that in the Pacific quakes. (Figs. 1, 2 and 3)
(2) Velocity of the condensational seismic wave propagating along the coast of the Japan Sea is greater than that propagating along the coast of the Pacific Ocean.
From these two results. we may conclude that the Pacific side of the Japanese Island is stretched and the Japan Sea side is compressed.
(3) According to the results of observation of Tams and Angenheister, the velocity of the Seismic surface wave is greater when it propagates along the ocean bed than that along the continent by 25 percent. But it seems to me that the same conclusion may hold good even in the case of the condensational wave passing through upper strata of the earth's crust. This result may in accord with the thcory of isostasy as well as with Wegener's theory of continental drift.
Moreover, from above results it may be considered that the depth of the discontinuity surface which is generally estimated to be 60km. is different according as it lies under the continent or under the ocean. Namely, under the ocean bed the discontinuity surface is shallower than that under the colltinent, but the confirmation of this conclusion is left to further studies.
(4) From the results, that our great island is stretched at the Pacific coast and consequently compressed on the Japan Sea side, it follows that there may exist many large cracks perpendicular to the are of the island and the Kii Straits running from north to southward between Sikoku and Kii Peninsula is one of these cracks. When condensational seismie wave passes along the straits there oceurs retardation of velocity owing to its less elastic constant. This phenomenon is also observed when condensational wave passes along the Inland Sea from east to westward by the earthquakes which occurred at the coast of the Pacific Ocean.
(5) The so-called “Lauf-zeit”curve for the condensational wave, consists of the three sets of curves for the following three kinds of waves:-
P: Mohoroviêié wave,
Pa: P-wave passing along the are of Japanese Island,
Pp: P-wave passing perpendicularly to the are of the Island. (Fig. 4 and 5)
(6) Seismic long wave (L) may positively be composed of S-wave as was stated by A. Mohoroviêié and afterwards confirmed by Mr. T. Isikawa of this Observatory. This fact is also verified in my papers.
