Water Level Oscillations on the Continental Shelf in the Vicinity of Miyagi-Enoshima.

The spectra of tsunamis observed on a continental shelf are closely related to the spectra of water level fluctuations in ordinary days with the period of 10 to 100 minutes, the so-called back-ground spectra. Therefore, the nature of back-ground oscillations on the shelf in the vicinity of Enoshima Island, Miyagi Prefecture, where a tsunami observatory is located, has been investigated. Theoretical trapped and leaky modes of oscillations were computed for the shelf near Miyagi-Enoshima and also the records of the back-ground oscillations sampled from the data through the year in 1965 were examined by means of the Fourier analysis. The Fourier spectrum averaged through a year coincided fairly well with the theoretical spectrum of the leaky mode for the constant spectral input offshore, particularly in the period band shorter than about 40 minutes. In the periods longer than about 50 minutes, the spectral intensity of the back-ground is much larger than the theoretical spectrum. There is a possibility that the incoming waves which excite back-ground oscillations may be more intensive in the longer period band. Another possibility is the contribution of the trapped waves for this period band. Very often the phase delay time obtained from the phase angle of the Fourier spectrum coincides fairly well with the one derived from the calculated dispersion curve on the assumption that waves start from a localized source with the same phase for all frequency. This suggests the existence of edge waves. However, the coherency analysis of water level variations observed simultaneously at Enoshima and Ofunato, contains an obscurity on the existence of waves propagating along the shelf.陸棚上で観測された津波のスペクトルは,特別な擾乱源のない平常時の海面振動,所謂back groundのスペクトルと密接な関係がある.それ放ここでは,このback ground の海面振動について,計算と実記録の解析の両面より考察を行った