Oscillations of the Water in the Tôkyô Bay due to the Great Earthquake on September 1, 1923

Abstract

In their investigation of the tunami due to the greate Kwantô earthquake occurred on the September 1 st. 1923, Prof. T. Terada and S. Yamaguti pointed out that the curious oscillations of sea level happened on that occasion at some places on the coast around the Tokyo Bay. At Yokosuka, situated SW-ern coast of the Bay, were observed the oscillations of sea level having the period of about 120 min, at Sibaura situated NW-ern coast of the Bay, that of 75 min.
Mareograms at some places illustrated in the paper of cited above are reproduced in the Fig.1.
Whether the oscillation at Yokosuka may be due to the seiche of the Tokyo Bay having a a period of about 120 min or not was not certainly ascertained by these authors, because that no oscillations having such a period was observed at Sibaura and Hukagawa.
Recently by means of comparison of the mareogram at Yokosuka with that at Tiba, T. Suzuki discovered that the oscillation at Tiba has also the period of almost 120 min which is nearly equal to that of Yokosuka and the phases nearly opposite to those at Yokosuka.
According to these facts, these oscillations were considered by him to be the proper oscillations of the Tokyo Bay. But his discussion was mainly directed to the phenomenon of the land upheaval which took place at the great earthquake and he considered that this had to be the cause of oscillation of the Tokyo Bay, so that the discussion with reference to the sea level change occurred at Sibaura and Hukagawa were scarcely treated in his papers.
In the present paper is mainly treated of these oscillations.
At first, if we suppose the oscillation at Sibaura be due to the secondary or lateral one of the Bay, it may be also expected in the observation at Tiba situated on the NE-ern coast and opposite to Sibaura. But it was not the case as the mareogram at Tiba does not show obviously any oscillation having the similar period. Accordingly we must give up this consideration.
On the other hand, the possibility of another oscillation of sea water has come to us. Let us look at the chart about the Tokyo Bay shown in Fig. 3 and let our attention give to the form of its coastal line about Sibaura and Hukagawa, and we may see that it curves inwardly like the form of a cup and embrace within it an inlet or a little bay refered as the Sinagawa Bay. While the vertical displacement of the earthsurface which took place at the great earthquake and the local distribution of whose magnitude is shown in Fig. 3 caused the tilting of the sea bottom of the Sinagawa Bay. These conditions seem to lead the oscillation like secondary one in the Sinagawa Bay.
For a rough estimation, let the Sinagawa Bay be a rectangular one with uniform depth, length and breath, magnitude of which are measured roughly from the chart to be 4m, 8km and 12km respectively.
Then we have its period equal to 75min, provided that the mouth correction be without consideration. Thus roughly estimated period agrees well with what observed at Sibaura and Hukagawa.
But the calculated amplitude of sea level change at the head of the Bay amounts to about 20cm which is far different from 2m that was observed, and if the tilting of the sea bottom be assumed to take place at the same moment as the earthquake, its first heighest of sea level change at the head of the Bay is reckoned to occur at 37min after the earthquake, which is far different from 91min that was observed. Consequently, we are obliged to abandon the idea of secondary oscillation of Sinagawa Bay as it contradicts the facts.
At last, only one case remains for us, in which the forced oscillation of Sinagawa Bay was caused by that of Tôkyô Bay. If it be so, we may reasonably expect the occurrence of the phase of forced one in the mareogram at Sibaura. And it is shown to be the case by Fig. 2 which expresses the cansidarable coincidence of phases observed at Tiba with corresponding phases at Sibaura.