In some earthquakes, a prominent wave group of long period accompanied by the T-phase of seismic waves was observed on the records taken by the Wiechert seismographs at the coastal stations, Shionomisaki, Shimizu, Wajima etc.
The period of the long wa v es is 7.5-15 sec., while that of the T-phase is 0.2-1.4sec. The T-phase of seismic waves may be the free waves whose pressure amplitudes are concentrated in the sound channel, so that it may be dispersive. The long waves seem to be the oceanic Rayleigh waves, corresponding probably to the rider waves of Pekeris.
The wave train of long period under consideration is so prominent that it seems the T-phase and the long waves should be connected with each other in a certain way.
The present writer show e d that a kind of coupling between the dispersive short waves and the dispersive long waves was possible, if the group velocities of the two waves were the same and the condition
cT/
c-U =
c'T'/
vc'-U'' where
U'= U',
was satisfied.
For an example, it is shown that the T-phase with a period 0.5 sec. and the oceanic Rayleigh waves with a period 10 sec. may be connected with each other, almost satisfying the condition cited above.
In the case of the Sanriku earthquake of 1933, only the long waves, which may be the rider waves, were observed at Choshi and Kakioka.
View full abstract