Observations of long-period ocean waves have been carried out at Miyagi-Enoshima on the north-eastern coast of Japan for a long time. For tsunami study, however, it is necessary to have observation points similar to Enoshima at several places in the Pacific coast of Japan, and as a realization of this plan, the Izu-Oshima Tsunami Observatory was established in September 1960. This observatory stands at Senzu, the north-eastern part of Oshima, a volcanic island situated on the Izu-Marianne ridge which separates the Philippine Sea from the Pacific Ocean. The observatory is therefore situated at a favorable position to record tsunamis originating both from the NE and SW parts of Honshu Island of Japan. The observatory includes an observation house made of concrete blocks and a gauge well built on lava of Mt. Mihara projecting into the sea. In the observatory are installed the ERI-IV tsunami recorder, a microbarograph and a microseismograph, as shown in Table 1. Tsunamis have not yet been observed, but spectra of records in different meteorological conditions were obtained to investigate the features of the background noise constantly observable in the records of the tsunami recorder. The spectral analyses were made by the wave analyser specially constructed for studies of the tsunami spectra. The following features were known by the present spectral analyses. 1. When typhoon 6124 approached Oshima island and sea and swell became heavy, the tsunami recorder recorded prominently 1 to 2 minute waves. As heights of this 1-2 minute waves have a good correlation with wind velocities, they may be the ""surf beat"" named by Munk. 2. The spectra of the records were obtained for the following four cases. (1) Oct. 8-9, 1961, when typhoon 6124 was approaching. (2) May 14-15, 1961, when a small depression was passing along the southern coast of Honshu Island. (3) Dec. 25-26, 1961, when a monsoon wind was blowing. (4) May 4-5, 1961, when a depression was passing through the Japan sea. In the first and second cases when wind waves were very rough off the coast of Senzu, the parts of the spectrum of the period shorter than 12 to 18 minutes are about twice as large as the parts of longer period. On the other hand, in the third and fourth cases when the sea off the coast of Senzu was rather calm, the spectra are flat extending over the whole period band. Therefore, it is supposed that the short period components of spectra relate to the activity of the sea just adjoining the coast. In the latter two cases, another remarkable fact is that the sea level fluctuations of the period longer than 60 minutes increased later to the greatest energy. 3. The predominant period of 11 to 12 minutes was recorded in each spectrum, which may be explained as being due to the island seich of this island. 4. In every case stated above, there is a striking fact that all spectra have a minimum at about 40 minutes in period. If we assume the mouth of Sagami Bay to be the line connecting Oshima and Boso peninsula, the natural period of the bay will be 38 minutes. The fact that a spectrum of the tide gauge record at Ito on the coast of Sagami Bay shows a peak of 42 minutes will be in favor of this assumption. If there is such a resonator it is only reasonable that the amplitude of the sea level fluctuations near Oshima is attenuated at the period of the free oscillation of Sagami Bay. The minimum point of the spectrum mentioned above may be the result of this effect. 5. A spectrum of the microbarograph record obtained at Oshima is hardly similar to that of the tsunami recorder, even when the microbarograph has recorded fair and regular waves. This fact shows that the long-period sea level fluctuations have little relation with the microbarometric disturbances at this island.
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
40
号
3
ページ
561 - 573
発行年
1962-11-30
ISSN
00408972
書誌レコードID
AN00162258
フォーマット
application/pdf
日本十進分類法
453
出版者
東京大学地震研究所
出版者別名
Earthquake Research Institute, University of Tokyo
伊豆大島で観測された長周期波の周期分析
ji0403010.pdf
(959.73KB)
