Based on the long wave approximation, a formula to compute the tsunami wave form on a flat bottom generated by a broad crustal deformation is introduced. The energy transfer from the sea bottom to the water is examined in relation to the duration of the bottom movement. If the duration is less than several minutes, the deformation may be considered to be abrupt as far as the tsunami is concerned. However, if the movement is completed in a few second, the energy transferred to the compressional water waves might be larger than the tsunami energy. The two-dimensional propagation of tsunami generated by a simple source of rectangular shape is explained by the superposition of various elementary waves. If the deformation of the bottom is a simple uplift or depression, the wave decay with distance near the source deviates from that due to geometrical spreading from a point source: in the direction of the major axis, the decay is faster, indicating a strong effect of diffraction, and in the direction of the minor axis, the decay is slower, indicating the approach to the one dimensional propagation. The variation of the directivity coefficient is conspicuous only for azimuths less than π/4 from the direction of the minor axis. For long distances from the source, the directivity of energy radiation is proportional to the square of the lateral dimension of the source with respect to the direction of the observing point. The nondispersive and dispersive wave forms are compared for long distances from the source, and it is concluded that the long wave approximation may be applicable for pa≳4, pa being a parameter related to the dimension of the source, distance and the depth of water.|長波近似をもちい,海底での大規模な地殻変動によって一様水深の海で発生する津波の波形を求める式を提出し,それを用いて,(1)地殻変動によって海水に伝えられるエネルギーが,変動継続時間によってどう変るか,(2)津波の2次元伝播による波形変化,及びエネルギー伝播の方向性,(3)津波伝播における重力波分散の影響,を調べた.
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
48
号
5
ページ
835 - 869
発行年
1970-12-05
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
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45. Tsunami Source, Energy and the Directivity of Wave Radiation
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